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1.
  • Overview of the JET results
  • 2015
  • In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 55:10
  • Journal article (peer-reviewed)
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2.
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3.
  • Abel, I, et al. (author)
  • Overview of the JET results with the ITER-like wall
  • 2013
  • In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:10, s. 104002-
  • Journal article (peer-reviewed)abstract
    • Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma-facing materials. The large reduction in the carbon content (more than a factor ten) led to a much lower Z(eff) (1.2-1.4) during L- and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that the fuel retention rate with the new wall is substantially reduced with respect to the C wall. The re-establishment of the baseline H-mode and hybrid scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type-I ELMy H-mode regimes with H-98,H-y2 close to 1 and beta(N) similar to 1.6 have been achieved using gas injection. ELM frequency is a key factor for the control of the metallic impurity accumulation. Pedestal temperatures tend to be lower with the new wall, leading to reduced confinement, but nitrogen seeding restores high pedestal temperatures and confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on Be wall plasma-facing components due to lower radiation made the routine use of massive gas injection for disruption mitigation essential.
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4.
  • Romanelli, F, et al. (author)
  • Overview of the JET results
  • 2011
  • In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 51:9
  • Journal article (peer-reviewed)abstract
    • Since the last IAEA Conference JET has been in operation for one year with a programmatic focus on the qualification of ITER operating scenarios, the consolidation of ITER design choices and preparation for plasma operation with the ITER-like wall presently being installed in JET. Good progress has been achieved, including stationary ELMy H-mode operation at 4.5 MA. The high confinement hybrid scenario has been extended to high triangularity, lower ρ*and to pulse lengths comparable to the resistive time. The steady-state scenario has also been extended to lower ρ*and ν*and optimized to simultaneously achieve, under stationary conditions, ITER-like values of all other relevant normalized parameters. A dedicated helium campaign has allowed key aspects of plasma control and H-mode operation for the ITER non-activated phase to be evaluated. Effective sawtooth control by fast ions has been demonstrated with3He minority ICRH, a scenario with negligible minority current drive. Edge localized mode (ELM) control studies using external n = 1 and n = 2 perturbation fields have found a resonance effect in ELM frequency for specific q95values. Complete ELM suppression has, however, not been observed, even with an edge Chirikov parameter larger than 1. Pellet ELM pacing has been demonstrated and the minimum pellet size needed to trigger an ELM has been estimated. For both natural and mitigated ELMs a broadening of the divertor ELM-wetted area with increasing ELM size has been found. In disruption studies with massive gas injection up to 50% of the thermal energy could be radiated before, and 20% during, the thermal quench. Halo currents could be reduced by 60% and, using argon/deuterium and neon/deuterium gas mixtures, runaway electron generation could be avoided. Most objectives of the ITER-like ICRH antenna have been demonstrated; matching with closely packed straps, ELM resilience, scattering matrix arc detection and operation at high power density (6.2 MW m-2) and antenna strap voltages (42 kV). Coupling measurements are in very good agreement with TOPICA modelling. © 2011 IAEA, Vienna.
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5.
  • Algaba, Juan-Carlos, et al. (author)
  • Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
  • 2021
  • In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 911:1
  • Research review (peer-reviewed)abstract
    • In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M o˙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
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6.
  • Petroff, E., et al. (author)
  • A polarized fast radio burst at low Galactic latitude
  • 2017
  • In: Monthly notices of the Royal Astronomical Society. - : Oxford Academic. - 0035-8711 .- 1365-2966. ; 469:4, s. 4465-4482
  • Journal article (peer-reviewed)abstract
    • We report on the discovery of a new fast radio burst (FRB), FRB 150215, with the Parkes radio telescope on 2015 February 15. The burst was detected in real time with a dispersion measure (DM) of 1105.6 +/- 0.8 pc cm(-3), a pulse duration of 2.8(-0.5)(+1.2) ms, and a measured peak flux density assuming that the burst was at beam centre of 0.7(-0.1)(+0.2) Jy. The FRB originated at a Galactic longitude and latitude of 24.66 degrees, 5.28 degrees and 25 degrees away from the Galactic Center. The burst was found to be 43 +/- 5 per cent linearly polarized with a rotation measure (RM) in the range -9 < RM < 12 rad m(-2) (95 per cent confidence level), consistent with zero. The burst was followed up with 11 telescopes to search for radio, optical, X-ray, gamma-ray and neutrino emission. Neither transient nor variable emission was found to be associated with the burst and no repeat pulses have been observed in 17.25 h of observing. The sightline to the burst is close to the Galactic plane and the observed physical properties of FRB 150215 demonstrate the existence of sight lines of anomalously low RM for a given electron column density. The Galactic RM foreground may approach a null value due to magnetic field reversals along the line of sight, a decreased total electron column density from the Milky Way, or some combination of these effects. A lower Galactic DM contribution might explain why this burst was detectable whereas previous searches at low latitude have had lower detection rates than those out of the plane.
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7.
  • Adrian-Martinez, S., et al. (author)
  • The First Combined Search For Neutrino Point-Sources In The Southern Hemisphere With The Antares And Icecube Neutrino Telescopes
  • 2016
  • In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 823:1
  • Journal article (peer-reviewed)abstract
    • We present the results of searches for point-like sources of neutrinos based on the first combined analysis of data from both the ANTARES and IceCube neutrino telescopes. The combination of both detectors, which differ in size and location, forms a window in the southern sky where the sensitivity to point sources improves by up to a factor of 2 compared with individual analyses. Using data recorded by ANTARES from 2007 to 2012, and by IceCube from 2008 to 2011, we search for sources of neutrino emission both across the southern sky and from a preselected list of candidate objects. No significant excess over background has been found in these searches, and flux upper limits for the candidate sources are presented for E-2.5 and E-2 power-law spectra with different energy cut-offs.
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8.
  • Abdalla, H., et al. (author)
  • HESS and MAGIC observations of a sudden cessation of a very-high-energy gamma-ray flare in PKS 1510-089 in May 2016
  • 2021
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 648
  • Journal article (peer-reviewed)abstract
    • The flat spectrum radio quasar (FSRQ) PKS 1510-089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E>100 GeV) gamma rays. The VHE gamma -ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE gamma -ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of similar to 20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE gamma -ray spectrum of PKS 1510-089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R-band observations with ATOM revealed a counterpart of the gamma -ray flare, even though the detailed flux evolution differs from the VHE gamma -ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE gamma -ray flare. In the high-energy (HE, E> 100 MeV) gamma -ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE gamma -ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the gamma -ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located similar to 50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE gamma rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.
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9.
  • Abdalla, H., et al. (author)
  • TeV Emission of Galactic Plane Sources with HAWC and HESS
  • 2021
  • In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 917:1
  • Journal article (peer-reviewed)abstract
    • The High Altitude Water Cherenkov (HAWC) observatory and the High Energy Stereoscopic System (H.E.S.S.) are two leading instruments in the ground-based very-high-energy gamma-ray domain. HAWC employs the water Cherenkov detection (WCD) technique, while H.E.S.S. is an array of Imaging Atmospheric Cherenkov Telescopes (IACTs). The two facilities therefore differ in multiple aspects, including their observation strategy, the size of their field of view, and their angular resolution, leading to different analysis approaches. Until now, it has been unclear if the results of observations by both types of instruments are consistent: several of the recently discovered HAWC sources have been followed up by IACTs, resulting in a confirmed detection only in a minority of cases. With this paper, we go further and try to resolve the tensions between previous results by performing a new analysis of the H.E.S.S. Galactic plane survey data, applying an analysis technique comparable between H.E.S.S. and HAWC. Events above 1 TeV are selected for both data sets, the point-spread function of H.E.S.S. is broadened to approach that of HAWC, and a similar background estimation method is used. This is the first detailed comparison of the Galactic plane observed by both instruments. H.E.S.S. can confirm the gamma-ray emission of four HAWC sources among seven previously undetected by IACTs, while the three others have measured fluxes below the sensitivity of the H.E.S.S. data set. Remaining differences in the overall gamma-ray flux can be explained by the systematic uncertainties. Therefore, we confirm a consistent view of the gamma-ray sky between WCD and IACT techniques.
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10.
  • Abdalla, H., et al. (author)
  • Searching for TeV Gamma-Ray Emission from SGR 1935+2154 during Its 2020 X-Ray and Radio Bursting Phase
  • 2021
  • In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 919:2
  • Journal article (peer-reviewed)abstract
    • Magnetar hyperflares are the most plausible explanation for fast radio bursts (FRBs)-enigmatic powerful radio pulses with durations of several milliseconds and high brightness temperatures. The first observational evidence for this scenario was obtained in 2020 April when an FRB was detected from the direction of the Galactic magnetar and soft gamma-ray repeater SGR 1935+2154. The FRB was preceded by two gamma-ray outburst alerts by the BAT instrument aboard the Swift satellite, which triggered follow-up observations by the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observed SGR 1935+2154 for 2 hr on 2020 April 28. The observations are coincident with X-ray bursts from the magnetar detected by INTEGRAL and Fermi-GBM, thus providing the first very high energy gamma-ray observations of a magnetar in a flaring state. High-quality data acquired during these follow-up observations allow us to perform a search for short-time transients. No significant signal at energies E > 0.6 TeV is found, and upper limits on the persistent and transient emission are derived. We here present the analysis of these observations and discuss the obtained results and prospects of the H.E.S.S. follow-up program for soft gamma-ray repeaters.
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11.
  • Abdalla, H., et al. (author)
  • An extreme particle accelerator in the Galactic plane : HESS J1826-130
  • 2020
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 644, s. 1-8
  • Journal article (peer-reviewed)abstract
    • The unidentified very-high-energy (VHE; E > 0.1 TeV) gamma -ray source, HESS J1826-130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steady gamma -ray flux from HESS J1826-130, which appears extended with a half-width of 0.21 degrees +/- 0.02 (stat)degrees stat degrees +/- 0.05 (sys)degrees sys degrees . The source spectrum is best fit with either a power-law function with a spectral index Gamma = 1.78 +/- 0.10(stat) +/- 0.20(sys) and an exponential cut-off at 15.2 (+5.5)(-3.2) -3.2+5.5 TeV, or a broken power-law with Gamma (1) = 1.96 +/- 0.06(stat) +/- 0.20(sys), Gamma (2) = 3.59 +/- 0.69(stat) +/- 0.20(sys) for energies below and above E-br = 11.2 +/- 2.7 TeV, respectively. The VHE flux from HESS J1826-130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula, HESS J1825-137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826-130 VHE emission related to PSR J1826-1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826-130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to greater than or similar to 200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants, molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.
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12.
  • Abdalla, H., et al. (author)
  • Search for Dark Matter Annihilation Signals from Unidentified Fermi-LAT Objects with HESS
  • 2021
  • In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 918:1
  • Journal article (peer-reviewed)abstract
    • Cosmological N-body simulations show that Milky Way-sized galaxies harbor a population of unmerged dark matter (DM) subhalos. These subhalos could shine in gamma-rays and eventually be detected in gamma-ray surveys as unidentified sources. We performed a thorough selection among unidentified Fermi-Large Area Telescope Objects (UFOs) to identify them as possible tera-electron-volt-scale DM subhalo candidates. We search for very-high-energy (E greater than or similar to 100 GeV) gamma-ray emissions using H.E.S.S. observations toward four selected UFOs. Since no significant very-high-energy gamma-ray emission is detected in any data set of the four observed UFOs or in the combined UFO data set, strong constraints are derived on the product of the velocity-weighted annihilation cross section sigma v by the J factor for the DM models. The 95% confidence level observed upper limits derived from combined H.E.S.S. observations reach sigma vJ values of 3.7 x 10(-5) and 8.1 x 10(-6) GeV(2 )cm(-2 )s(-1) in the W (+) W (-) and tau (+) tau (-) channels, respectively, for a 1 TeV DM mass. Focusing on thermal weakly interacting massive particles, the H.E.S.S. constraints restrict the J factors to lie in the range 6.1 x 10(19)-2.0 x 10(21) GeV(2 )cm(-5) and the masses to lie between 0.2 and 6 TeV in the W (+) W (-) channel. For the tau (+) tau (-) channel, the J factors lie in the range 7.0 x 10(19)-7.1 x 10(20) GeV(2 )cm(-5) and the masses lie between 0.2 and 0.5 TeV. Assuming model-dependent predictions from cosmological N-body simulations on the J-factor distribution for Milky Way-sized galaxies, the DM models with masses >0.3 TeV for the UFO emissions can be ruled out at high confidence level.
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13.
  • Abdalla, H., et al. (author)
  • Evidence of 100 TeV gamma-ray emission from HESS J1702-420 : A new PeVatron candidate
  • 2021
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 653
  • Journal article (peer-reviewed)abstract
    • Aims. The identification of PeVatrons, hadronic particle accelerators reaching the knee of the cosmic ray spectrum (few x 10(15) eV), is crucial to understand the origin of cosmic rays in the Galaxy. We provide an update on the unidentified source HESS J1702-420, a promising PeVatron candidate. Methods. We present new observations of HESS J1702-420 made with the High Energy Stereoscopic System (H.E.S.S.), and processed using improved analysis techniques. The analysis configuration was optimized to enhance the collection area at the highest energies. We applied a threedimensional likelihood analysis to model the source region and adjust non thermal radiative spectral models to the gamma-ray data. We also analyzed archival Fermi Large Area Telescope data to constrain the source spectrum at gamma-ray energies >10 GeV. Results. We report the detection of gamma-rays up to 100 TeV from a specific region of HESS J1702-420, which is well described by a new source component called HESS J1702-420A that was separated from the bulk of TeV emission at a 5:4 sigma confidence level. The power law gamma-ray spectrum of HESS J1702-420A extends with an index of Gamma = 1:53 +/- 0:19(stat) +/- 0:20(sys) and without curvature up to the energy band 64 113 TeV, in which it was detected by H.E.S.S. at a 4:0 sigma confidence level. This makes HESS J1702-420A a compelling candidate site for the presence of extremely high energy cosmic rays. With a flux above 2 TeV of (2:08 +/- 0:49(stat) +/- 0:62(sys)) x 10(-13) cm(-2) s(-1) and a radius of (0:06 +/- 0:02(stat) +/- 0:03(sys))degrees, HESS J1702-420A is outshone - below a few tens of TeV - by the companion HESS J1702-420B. The latter has a steep spectral index of = 2:62 +/- 0:10(stat) +/- 0:20(sys) and an elongated shape, and it accounts for most of the low-energy HESS J1702-420 flux. Simple hadronic and leptonic emission models can be well adjusted to the spectra of both components. Remarkably, in a hadronic scenario, the cut-o ff energy of the particle distribution powering HESS J1702-420A is found to be higher than 0:5 PeV at a 95% confidence level. Conclusions. For the first time, H.E.S.S. resolved two components with significantly di fferent morphologies and spectral indices, both detected at >5 sigma confidence level, whose combined emissions result in the source HESS J1702-420. We detected HESS J1702-420A at a 4:0 sigma confidence level in the energy band 64 113 TeV, which brings evidence for the source emission up to 100 TeV. In a hadronic emission scenario, the hard gamma-ray spectrum of HESS J1702-420A implies that the source likely harbors PeV protons, thus becoming one of the most solid PeVatron candidates detected so far in H.E.S.S. data. However, a leptonic origin of the observed TeV emission cannot be ruled out either.
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14.
  • Abdalla, H., et al. (author)
  • LMC N132D : A mature supernova remnant with a power-law gamma-ray spectrum extending beyond 8 TeV
  • 2021
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 655
  • Journal article (peer-reviewed)abstract
    • Context. Supernova remnants (SNRs) are commonly thought to be the dominant sources of Galactic cosmic rays up to the knee of the cosmic-ray spectrum at a few PeV. Imaging Atmospheric Cherenkov Telescopes have revealed young SNRs as very-high-energy (VHE, >100 GeV) gamma-ray sources, but for only a few SNRs the hadronic cosmic-ray origin of their gamma-ray emission is indisputably established. In all these cases, the gamma-ray spectra exhibit a spectral cutoff at energies much below 100 TeV and thus do not reach the PeVatron regime. Aims. The aim of this work was to achieve a firm detection for the oxygen-rich SNR LMC N132D in the VHE gamma-ray domain with an extended set of data, and to clarify the spectral characteristics and the localization of the gamma-ray emission from this exceptionally powerful gamma-ray-emitting SNR. Methods. We analyzed 252 h of High Energy Stereoscopic System (H.E.S.S.) observations towards SNR N132D that were accumulated between December 2004 and March 2016 during a deep survey of the Large Magellanic Cloud, adding 104 h of observations to the previously published data set to ensure a > 5 sigma detection. To broaden the gamma-ray spectral coverage required for modeling the spectral energy distribution, an analysis of Fermi-LAT Pass 8 data was also included. Results. We unambiguously detect N132D at VHE with a significance of 5.7 sigma. We report the results of a detailed analysis of its spectrum and localization based on the extended H.E.S.S. data set. The joint analysis of the extended H.E.S.S and Fermi-LAT data results in a spectral energy distribution in the energy range from 1.7 GeV to 14.8 TeV, which suggests a high luminosity of N132D at GeV and TeV energies. We set a lower limit on a gamma-ray cutoff energy of 8 TeV with a confidence level of 95%. The new gamma-ray spectrum as well as multiwavelength observations of N132D when compared to physical models suggests a hadronic origin of the VHE gamma-ray emission. Conclusions. SNR N132D is a VHE gamma-ray source that shows a spectrum extending to the VHE domain without a spectral cutoff at a few TeV, unlike the younger oxygen-rich SNR Cassiopeia A. The gamma-ray emission is best explained by a dominant hadronic component formed by diffusive shock acceleration. The gamma-ray properties of N132D may be affected by an interaction with a nearby molecular cloud that partially lies inside the 95% confidence region of the source position.
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15.
  • Abdalla, H., et al. (author)
  • Simultaneous observations of the blazar PKS 2155-304 from ultra-violet to TeV energies
  • 2020
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 639, s. 1-13
  • Journal article (peer-reviewed)abstract
    • Here we report the results of the first ever contemporaneous multi-wavelength observation campaign on the BL Lac object PKS 2155-304 involving Swift, NuSTAR, Fermi-LAT, and H.E.S.S. The use of these instruments allows us to cover a broad energy range, which is important for disentangling the different radiative mechanisms. The source, observed from June 2013 to October 2013, was found in a low flux state with respect to previous observations but exhibited highly significant flux variability in the X-rays. The high-energy end of the synchrotron spectrum can be traced up to 40 keV without significant contamination by high-energy emission. A one-zone synchrotron self-Compton model was used to reproduce the broadband flux of the source for all the observations presented here but failed for previous observations made in April 2013. A lepto-hadronic solution was then explored to explain these earlier observational results.
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16.
  • Abdalla, H., et al. (author)
  • Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow
  • 2021
  • In: Science. - : American Association of Advancement in Science. - 0036-8075 .- 1095-9203. ; 372:6546, s. 1081-1085
  • Journal article (peer-reviewed)abstract
    • Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from extragalactic sources followed by fading afterglow emission, are associated with stellar core collapse events. We report the detection of very- high-energy (VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours after the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The low luminosity and redshift of GRB 190829A reduce both internal and external absorption, allowing determination of its intrinsic energy spectrum. Between energies of 0.18 and 3.3 tera-electron volts, this spectrum is described by a power law with photon index of 2.07 +/- 0.09, similar to the x-ray spectrum. The x-ray and VHE gamma- ray light curves also show similar decay profiles. These similar characteristics in the x-ray and gamma-ray bands challenge GRB afterglow emission scenarios.
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17.
  • Abdallah, H., et al. (author)
  • Search for dark matter annihilation in the Wolf-Lundmark-Melotte dwarf irregular galaxy with HESS
  • 2021
  • In: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 103:10
  • Journal article (peer-reviewed)abstract
    • We search for an indirect signal of dark matter through very high-energy gamma rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would produce Standard Model particles in the final state such as gamma rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well-measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the five-telescopes of the high energy stereoscopic system observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation as a function of the dark matter particle mass for various continuum channels, as well as the prompt gamma gamma emission. For the tau+tau- channel, the limits reach a value of about 4 x 10-22 cm3 s-1 for a dark matter particle mass of 1 TeV. For the prompt gamma gamma channel, the upper limit reaches a value of about 5 x 10-24 cm3 s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200, with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.
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18.
  • Abdalla, H., et al. (author)
  • A very-high-energy component deep in the gamma-ray burst afterglow
  • 2019
  • In: Nature. - : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 575:7783, s. 464-467
  • Journal article (peer-reviewed)abstract
    • Gamma-ray bursts (GRBs) are brief flashes of gamma-rays and are considered to be the most energetic explosive phenomena in the Universe(1). The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow-produced by the interaction between the ejected matter and the circumburst medium-slows down, and a gradual decrease in brightness is observed(2). GRBs typically emit most of their energy via.-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments(3). However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive(4). Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow-ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and gamma-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.
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19.
  • Abdalla, H., et al. (author)
  • HESS and Fermi-LAT observations of PSR B1259-63/LS 2883 during its 2014 and 2017 periastron passages
  • 2020
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 633, s. 1-14
  • Journal article (peer-reviewed)abstract
    • Context. PSR B1259-63/LS 2883 is a gamma-ray binary system consisting of a pulsar in an eccentric orbit around a bright Oe stellar-type companion star that features a dense circumstellar disc. The bright broad-band emission observed at phases close to periastron offers a unique opportunity to study particle acceleration and radiation processes in binary systems. Observations at gamma-ray energies constrain these processes through variability and spectral characterisation studies. Aims. The high- and very-high-energy (HE, VHE) gamma-ray emission from PSR B1259-63/LS 2883 around the times of its periastron passage are characterised, in particular, at the time of the HE gamma-ray flares reported to have occurred in 2011, 2014, and 2017. Short-term and average emission characteristics of PSR B1259-63/LS 2883 are determined. Super-orbital variability is searched for in order to investigate possible cycle-to-cycle VHE flux changes due to different properties of the companion star's circumstellar disc and/or the conditions under which the HE gamma-ray flares develop. Methods. Spectra and light curves were derived from observations conducted with the H.E.S.S-II array in 2014 and 2017. Phase-folded light curves are compared with the results obtained in 2004, 2007, and 2011. Fermi-LAT observations from 2010/11, 2014, and 2017 are analysed. Results. A local double-peak profile with asymmetric peaks in the VHE light curve is measured, with a flux minimum at the time of periastron t(p) and two peaks coinciding with the times at which the neutron star crosses the companion's circumstellar disc (similar to t(p) 16 d). A high VHE gamma-ray flux is also observed at the times of the HE gamma-ray flares (similar to t(p) + 30 d) and at phases before the first disc crossing (similar to t(p) - 35 d). The spectral energy range now extends to below 200 GeV and up to similar to 45 TeV. Conclusions. PSR B1259-63/LS 2883 displays periodic flux variability at VHE gamma-rays without clear signatures of super-orbital modulation in the time span covered by the monitoring of the source with the H.E.S.S. telescopes. This flux variability is most probably caused by the changing environmental conditions, particularly at times close to periastron passage at which the neutron star is thought to cross the circumstellar disc of the companion star twice. In contrast, the photon index remains unchanged within uncertainties for about 200 d around periastron. At HE gamma-rays, PSR B1259-63/LS 2883 has now been detected also before and after periastron, close to the disc crossing times. Repetitive flares with distinct variability patterns are detected in this energy range. Such outbursts are not observed at VHEs, although a relatively high emission level is measured. The spectra obtained in both energy regimes displays a similar slope, although a common physical origin either in terms of a related particle population, emission mechanism, or emitter location is ruled out.
  •  
20.
  • Abdalla, H., et al. (author)
  • Probing the Magnetic Field in the GW170817 Outflow Using HESS Observations
  • 2020
  • In: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 894:2, s. 1-5
  • Journal article (peer-reviewed)abstract
    • The detection of the first electromagnetic counterpart to the binary neutron star (BNS) merger remnant GW170817 established the connection between short gamma-ray bursts and BNS mergers. It also confirmed the forging of heavy elements in the ejecta (a so-called kilonova) via the r-process nucleosynthesis. The appearance of nonthermal radio and X-ray emission, as well as the brightening, which lasted more than 100 days, were somewhat unexpected. Current theoretical models attempt to explain this temporal behavior as either originating from a relativistic off-axis jet or a kilonova-like outflow. In either scenario, there is some ambiguity regarding how much energy is transported in the nonthermal electrons versus the magnetic field of the emission region. Combining the Very Large Array (radio) and Chandra (X-ray) measurements with observations in the GeV-TeV domain can help break this ambiguity, almost independently of the assumed origin of the emission. Here we report for the first time on deep H.E.S.S. observations of GW170817/GRB 170817A between 124 and 272 days after the BNS merger with the full H.E.S.S. array of telescopes, as well as on an updated analysis of the prompt (<5 days) observations with the upgraded H.E.S.S. phase-I telescopes. We discuss implications of the H.E.S.S. measurement for the magnetic field in the context of different source scenarios.
  •  
21.
  • Abdalla, H., et al. (author)
  • Detection of very-high-energy gamma-ray emission from the colliding wind binary eta Car with HESS
  • 2020
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635, s. 1-8
  • Journal article (peer-reviewed)abstract
    • Aims. Colliding wind binary systems have long been suspected to be high-energy (HE; 100 MeV < E < 100 GeV) gamma-ray emitters. eta Car is the most prominent member of this object class and is confirmed to emit phase-locked HE gamma rays from hundreds of MeV to 100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE; E >100 GeV) gamma-ray emission from eta Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.).Methods. The region around eta Car was observed with H.E.S.S. between orbital phase p = 0.78-1.10, with a closer sampling at p approximate to 0.95 and p approximate to 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the eta Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions.Results. H.E.S.S. detected VHE gamma-ray emission from the direction of eta Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE gamma rays agree within statistical and systematic errors before and after periastron. The gamma-ray spectrum extends up to at least 400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.
  •  
22.
  • Abdalla, H., et al. (author)
  • Very high energy gamma-ray emission from two blazars of unknown redshift and upper limits on their distance
  • 2020
  • In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 494:4, s. 5590-5602
  • Journal article (peer-reviewed)abstract
    • We report on the detection of very high energy (VHE; E > 100 GeV) gamma-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or preceded by multiwavelength observations with Fermi/LAT, XRT and UVOT onboard the Swift satellite, and ATOM. Based on an extrapolation of the Fermi/LAT spectrum towards the VHE gamma-ray regime, we deduce a 95 per cent confidence level upper limit on the unknown redshift of KUV 00311-1938 of z < 0.98 and of PKS 1440-389 of z < 0.53. When combined with previous spectroscopy results, the redshift of KUV 00311-1938 is constrained to 0.51 <= z < 0.98 and of PKS 1440-389 to 0.14 (sic) z < 0.53.
  •  
23.
  • Abdallah, H., et al. (author)
  • Search for dark matter signals towards a selection of recently detected DES dwarf galaxy satellites of the Milky Way with HESS
  • 2020
  • In: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 102:6, s. 1-20
  • Journal article (peer-reviewed)abstract
    • Dwarf spheroidal galaxy satellites of the Milky Way are prime targets for indirect detection of dark matter with gamma rays due to their proximity, high dark matter content, and absence of nonthermal emission processes. Recently, the Dark Energy Survey (DES) revealed the existence of new ultrafaint dwarf spheroidal galaxies in the southern-hemisphere sky, therefore ideally located for ground-based observations with the imaging atmospheric Cherenkov telescope array H.E.S.S. We present a search for very-high-energy (E greater than or similar to 100 GeV) gamma-ray emission using H.E.S.S. observations carried out recently towards Reticulum II, Tucana II, Tucana III, Tucana IV, and Grus II satellites. No significant very-high-energy gamma-ray excess is found from the observations on any individual object nor in the combined analysis of all the datasets. Using the most recent modeling of the dark matter distribution in the dwarf galaxy halo, we compute for the first time on DES satellites individual and combined constraints from Cherenkov telescope observations on the annihilation cross section of dark matter particles in the form of Weakly Interacting Massive Particles. The combined 95% C.L. observed upper limits reach similar or equal to 1 x 10(-23) cm(3) s(-1) in the W+W- channel and 4 x 10(-26) cm(3) s(-1) in the gamma gamma channels for a dark matter mass of 1.5 TeV. The H.E.S.S. constraints well complement the results from Fermi-LAT, HAWC, MAGIC, and VERITAS and are currently the most stringent in the gamma gamma channels in the multi-GeV/multi-TeV mass range.
  •  
24.
  •  
25.
  • Litnovsky, A., et al. (author)
  • Diagnostic mirrors for ITER : A material choice and the impact of erosion and deposition on their performance
  • 2007
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 363, s. 1395-1402
  • Journal article (peer-reviewed)abstract
    • Metal mirrors will be implemented in about half of the ITER diagnostics. Mirrors in ITER will have to withstand radiation loads, erosion by charge-exchange neutrals, deposition of impurities, particle implantation and neutron irradiation. It is believed that the optical properties of diagnostic mirrors will be primarily influenced by erosion and deposition. A solution is needed for optimal performance of mirrors in ITER throughout the entire lifetime of the machine. A multi-machine research on diagnostic mirrors is currently underway in fusion facilities at several institutions and laboratories worldwide. Among others, dedicated investigations of ITER-candidate mirror materials are ongoing in Tore-Supra, TEXTOR, DIII-D, TCV, T-10 and JET. Laboratory studies are underway at IPP Kharkov (Ukraine), Kurchatov Institute (Russia) and the University of Basel (Switzerland). An overview of current research on diagnostic mirrors along with an outlook on future investigations is the subject of this paper.
  •  
26.
  • Litnovsky, A., et al. (author)
  • Overview of material migration and mixing, fuel retention and cleaning of ITER-like castellated structures in TEXTOR
  • 2011
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 415:1, s. S289-S292
  • Journal article (peer-reviewed)abstract
    • Plasma-facing components (PFCs) in ITER will be castellated by splitting them into small-size blocks to maintain the thermo-mechanical stability. However, there are concerns in particular on retention of codeposited radioactive fuel in the gaps. An R&D program is underway in TEXTOR addressing this acute issue of castellation. Material migration and fuel inventory are investigated using long- and short-term discharge-resolved experiments with castellated structures in TEXTOR. Significant impurity transport to the gaps was detected and results were in part quantitatively reproduced with 3D-GAPS code. Deposits containing up to 70 at.% of tungsten on the gap areas closest to the plasma were detected in recent experiments. Deposition in the gaps accompanied by metal mixing demand for development of effective cleaning techniques. In experiments with ITER-like castellation, the gaps were cleaned from carbonaceous deposits using oxygen plasmas at 350 degrees C. This contribution contains an overview of experimental and modeling results along with recommendations for PFCs in ITER.
  •  
27.
  • Strachan, J. D., et al. (author)
  • Modelling of carbon migration during JET C-13 injection experiments
  • 2008
  • In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 48:10
  • Journal article (peer-reviewed)abstract
    • JET has performed two dedicated carbon migration experiments on the final run day of separate campaigns ( 2001 and 2004) using (CH4)-C-13 methane injected into repeated discharges. The EDGE2D/NIMBUS code modelled the carbon migration in both experiments. This paper describes this modelling and identifies a number of important migration pathways: ( 1) deposition and erosion near the injection location, ( 2) migration through the main chamber SOL, (3) migration through the private flux region (PFR) aided by E x B drifts and ( 4) neutral migration originating near the strike points. In H-Mode, type I ELMs are calculated to influence the migration by enhancing erosion during the ELM peak and increasing the long-range migration immediately following the ELM. The erosion/re-deposition cycle along the outer target leads to a multistep migration of C-13 towards the separatrix which is called 'walking'. This walking created carbon neutrals at the outer strike point and led to 13C deposition in the PFR. Although several migration pathways have been identified, quantitative analyses are hindered by experimental uncertainty in divertor leakage, and the lack of measurements at locations such as gaps and shadowed regions.
  •  
28.
  • Counsell, G., et al. (author)
  • Tritium retention in next step devices and the requirements for mitigation and removal techniques
  • 2006
  • In: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 48:12B, s. B189-B199
  • Journal article (peer-reviewed)abstract
    • Mechanisms underlying the retention of fuel species in tokamaks with carbon plasma-facing components are presented, together with estimates for the corresponding retention of tritium in ITER. The consequential requirement for new and improved schemes to reduce the tritium inventory is highlighted and the results of ongoing studies into a range of techniques are presented, together with estimates of the tritium removal rate in ITER in each case. Finally, an approach involving the integration of many tritium removal techniques into the ITER operational schedule is proposed as a means to extend the period of operations before major intervention is required.
  •  
29.
  • Rubel, Marek, et al. (author)
  • Overview of wall probes for erosion and deposition studies in the TEXTOR tokamak
  • 2017
  • In: Matter and Radiation at Extremes. - : Elsevier B.V.. - 2468-2047 .- 2468-080X. ; 2:3, s. 87-104
  • Journal article (peer-reviewed)abstract
    • An overview of diagnostic tools – test limiters and collector probes – used over the years for material migration studies in the TEXTOR tokamak is presented. Probe transfer systems are shown and their technical capabilities are described. This is accompanied by a brief presentation of selected results and conclusions from the research on material erosion – deposition processes including tests of candidate materials (e.g. W, Mo, carbon-based composites) for plasma-facing components in controlled fusion devices. The use of tracer techniques and methods for analysis of materials retrieved from the tokamak are summarized. The impact of research on the reactor wall technology is addressed.
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30.
  • Sergienko, G., et al. (author)
  • Erosion of a tungsten limiter under high heat flux in TEXTOR
  • 2007
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 363, s. 96-100
  • Journal article (peer-reviewed)abstract
    • Erosion characteristics of a tungsten plate heated up in TEXTOR by the plasma load have been investigated at temperatures extending to the melting point. No enhancement of atomic release exceeding physical sputtering and normal thermal sublimation for temperatures below 3700 K was observed. The liquid tungsten moved fast along the plate in the direction perpendicular to the magnetic field lines. The motion is caused by the Lorentz force due to the thermoelectron current emitted from the hot tungsten surface. The motion of liquid tungsten caused a material loss of 2.85 g during two discharges. The material redistribution due to the melt layer motion is compared with a MEMOS-1.5D simulation.
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31.
  • Sergienko, G., et al. (author)
  • Experience with bulk tungsten test-limiters under high heat loads : melting and melt layer propagation
  • 2007
  • In: Physica Scripta. - 0031-8949 .- 1402-4896. ; T128, s. 81-86
  • Journal article (peer-reviewed)abstract
    • The paper provides an overview of processes and underlying physics governing tungsten melt erosion in the fusion plasma environment. Experiments with three different bulk tungsten test-limiters were performed in TEXTOR: (i) thermally insulated solid plate fixed on a graphite roof-like limiter heated up by the plasma to the melting point, (ii) macro-brush of the ITER-relevant castellated structure and (iii) lamellae structure developed for the JET divertor. The main objectives were to determine the metal surface damage, the formation of the melt layer and its motion in the magnetic field. PHEMOBRID-3D and MEMOS-1.5D numerical codes were used to simulate the experiment with the roof-like test-limiter. Both experiments and simulation showed that the melting of tungsten can lead to a large material redistribution due to thermo-electron emission currents without ejection of molten material to the plasma.
  •  
32.
  • Tsitrone, E., et al. (author)
  • Multi machine scaling of fuel retention in 4 carbon dominated tokamaks
  • 2011
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 415:1, s. S735-S739
  • Journal article (peer-reviewed)abstract
    • In order to benchmark predictions for the in vessel tritium inventory in ITER, a survey of fuel retention measured in 4 carbon dominated tokamaks (TEXTOR, ASDEX Upgrade in the 2002-2003 carbon configuration, Tore Supra and JET) was performed, showing retention rates from similar to 1 g D/h in TEXTOR (L mode, limiter machine) up to similar to 6-12 g D/h in AUG (H mode, divertor machine). A simple scaling used for ITER predictions is applied for comparison with experimental values: (1) estimate of wall fluxes, (2) estimate of the gross carbon erosion, (3) estimate of the net erosion/redeposition assuming a redeposition fraction and (4) estimate of the retention rate using D/C ratio scalings. The validity of each step is discussed, showing that this approach yields the right order of magnitude, but tends to underestimate the experimental values unless a high wall flux, a low local redeposition fraction and/or a high D/C ratio are used.
  •  
33.
  • Weckmann, Armin, et al. (author)
  • Review on global migration, fuel retention and modelling after TEXTOR decommission
  • 2018
  • In: NUCLEAR MATERIALS AND ENERGY. - : ELSEVIER SCIENCE BV. - 2352-1791. ; 17, s. 83-112
  • Research review (peer-reviewed)abstract
    • Before decommissioning of the TEXTOR tokamak in 2013, the machine was conditioned with a comprehensive migration experiment where MoF6 and N-15(2) were injected on the very last operation day. Thereafter, all plasmafacing components (PFCs) were available for extensive studies of both local and global migration of impurities - Mo, W, Inconel alloy constituents, 15 N, F - and fuel retention studies. Measurements were performed on 140 limiter tiles out of 864 throughout the whole machine to map global transport. One fifth of the introduced molybdenum could be found. Wherever possible, the findings are compared to results obtained previously in other machines. This review incorporates both published and unpublished results from this TEXTOR study and combines findings with analytical methods as well as modelling results from two codes, ERO and ASCOT. The main findings are: Both local and global molybdenum transport can be explained by toroidal plasma flow and (sic) x (sic) drift. The suggested transport scheme for molybdenum holds also for other analysed species, namely tungsten from previous experiments and medium-Z metals (Cr-Cu) introduced on various occasions. Analytical interpretation of several deposition profile features is possible with basic geometrical and plasma physics considerations. These are deposition profiles on the collector probe, the lower part of the inner bumper limiter, the poloidal cross-section of the inner bumper limiter, and the poloidal limiter. Any deposition pattern found in this TEXTOR study, including fuel retention, has neither poloidal nor toroidal symmetry, which is often assumed when determining deposition profiles on global scale. Fuel retention is highly inhomogeneous due to local variation of plasma parameters - by auxiliary heating systems and impurity injection - and PFC temperature. Local modelling with ERO yields good qualitative agreement but too high local deposition efficiency. Global modelling with ASCOT shows that the radial electric field and source form have a high impact on global deposition patterns, while toroidal flow has little influence. Some of the experimental findings could be reproduced. Still, qualitative differences between simulated and experimental global deposition patterns remain. The review closes with lessons learnt during this extensive TEXTOR study which might be helpful for future scientific exploitation of other tokamaks to be decommissioned.
  •  
34.
  • Garcia Carrasco, Alvaro, et al. (author)
  • Nitrogen removal from plasma-facing components by ion cyclotron wall conditioning in TEXTOR
  • 2015
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 463, s. 688-692
  • Journal article (other academic/artistic)abstract
    • The efficiency of ion cyclotron wall conditioning (ICWC) in the removal of nitrogen from plasma-facing components in TEXTOR was assessed. In two experiments the wall was loaded with nitrogen and subsequently cleaned by ICWC in deuterium and helium. The retention and removal of nitrogen was studied in-situ by means of mass spectrometry, and ex-situ by surface analysis of a set of graphite, tungsten and TZM plates installed on test limiter systems. N-15 rare isotope was used as a marker. The results from the gas balance showed that about 25% of the retained nitrogen was removed after ICWC cleaning, whereas surface analysis of the plates based on ToF-HIERDA showed an increase of the deposited species after the cleaning. This indicates that during ICWC operation on carbon devices, nitrogen is not only pumped out but also transported to other locations on the wall. Additionally, deuterium surface content was studied before and after ICWC cleaning.
  •  
35.
  • Litnovsky, A., et al. (author)
  • Dust investigations in TEXTOR : Impact of dust on plasma-wall interactions and on plasma performance
  • 2013
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 438:Suppl., s. S126-S132
  • Journal article (peer-reviewed)abstract
    • Dust will have severe impact on ITER performance since the accumulation of tritium in dust represents a safety issue, a possible reaction of dust with air and steam imposes an explosion hazard and the penetration of dust in core plasmas may degrade plasma performance by increasing radiative losses. Investigations were performed in TEXTOR where known amounts of pre-characterized carbon, diamond and tungsten dust were mobilized into plasmas using special dust holders. Mobilization of dust changed a balance between plasma-surface interactions processes, significantly increasing net deposition. Immediately after launch dust was dominating both core and edge plasma parameters. Remarkably, in about 100 ms after the launch, the effect of dust on edge and core plasma parameters was vanished: no increase of carbon and tungsten concentrations in the core plasmas was detected suggesting a prompt transport of dust to the nearby plasma-facing components without further residence in the plasma.
  •  
36.
  • Monier-Garbet, P, et al. (author)
  • Impurity-seeded ELMy H-modes in JET, with high density and reduced heat load
  • 2005
  • In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 45:11, s. 1404-1410
  • Journal article (peer-reviewed)abstract
    • Experiments performed at JET during the past two years show that, in high triangularity H-mode plasmas with I-p = 2.5 MA, n(e)/n(Gr) approximate to 1.0, it is possible to radiate separately up to approximate to 40% of the total injected power on closed flux surfaces in the pedestal region (argon seeding) and up to approximate to 50% of the injected power in the divertor region (nitrogen seeding), while maintaining the confinement improvement factor at the value required for ITER, H98(y, 2) 1.0. The total radiated power fraction achieved in both cases (65-70%) is close to that required for ITER. However, Type I ELMS observed with impurity seeding have the same characteristics as that observed in reference pulses without seeding: decreasing plasma energy loss per ELM with increasing pedestal collisionality. One has to reach the Type III ELM regime to decrease the transient heat load to the divertor to acceptable values for ITER, although at the expense of confinement. The feasibility of an integrated scenario with Type-III ELMS, and q(95) = 2.6 to compensate for the low H factor, has been demonstrated on JET. This scenario would meet ITER requirements at 17 MA provided that the IPB98 scaling for energy content is accurate enough, and provided that a lower dilution is obtained when operating at higher absolute electron density.
  •  
37.
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38.
  • Weckmann, Armin, et al. (author)
  • Local Migration Studies of High-Z Metals in the TEXTOR Tokamak
  • 2016
  • In: Physica Scripta. - : Institute of Physics Publishing (IOPP). - 0031-8949 .- 1402-4896. ; T167
  • Journal article (peer-reviewed)abstract
    • Volatile compounds of tungsten (WF6) and molybdenum (MoF6) were used as tracers of high-Z metal migration in the TEXTOR tokamak in several gas injection experiments when puffing was done through a test limiter. The experiments with W were performed prior major shut-downs, while the MoF6 was followed by the final shutdown in connection with TEXTOR decommissioning. In all cases a set of various surface probes and limiter tiles were retrieved and analysed with electron and ion beam techniques. The focus was on the local deposition in the vicinity of the gas inlet and in the inlet system. Depth profiles in the deposits and metal distribution maps clearly shown that only near the gas inlet significant amounts of Mo are deposited along the scrape-off layer flow and E×B drift directions, which could be reproduced by ERO-code modelling. Correlation between the plasma operation scenario and the deposition patterns is presented.
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39.
  • Weckmann, Armin, et al. (author)
  • Physics affecting heavy impurity migration in tokamaks : Benchmarking test-ion code ASCOT against TEXTOR tracer experiment
  • 2019
  • In: Nuclear Materials and Energy. - : Elsevier Ltd. - 2352-1791. ; 19, s. 307-315
  • Journal article (peer-reviewed)abstract
    • Erosion, transport and deposition of wall impurities are major concerns in future magnetic fusion devices, both from the perspective of the fusion plasma and the machine wall. An extensive study on molybdenum transport and deposition performed in the TEXTOR tokamak yielded a detailed deposition map that is ideal for benchmark deposition studies. A qualitative benchmark is attempted in this article with the ASCOT code. We set up a full 3D model of the TEXTOR tokamak and studied the influence of different physical mechanisms and their strengths on molybdenum deposition patterns on the simulated plasma-facing components: atomic processes, Coulomb collisions, scrape-off layer (SOL) profiles, source distribution, marker starting energy, radial electric field strength, SOL flow and toroidal plasma rotation. The outcome comprises 13 simulations, each with 100,000 markers. The findings are: • Toroidal plasma movement, either within the LCFS or as SOL flow, is negligible. • SOL profile and marker starting energy have modest impact on deposition. • Source distribution has a large impact in combination with radial electric field profiles. • The E⇀×B⇀ drift has the highest impact on the deposition profiles.
  •  
40.
  • Weckmann, Armin, et al. (author)
  • Whole-machine material migration studies in the TEXTOR tokamak with molybdenum
  • 2017
  • In: NUCLEAR MATERIALS AND ENERGY. - : Elsevier. - 2352-1791. ; 12, s. 518-523
  • Journal article (peer-reviewed)abstract
    • MoF6 injection from a localised source into plasma edge in the TEXTOR tokamak was the last experiment before the final shut-down of the TEXTOR machine. During decommissioning all plasma-facing components (PFCs) became available for surface studies. Detailed mapping of Mo deposition was performed in order to determine its migration on global scale. The concentration of Mo on PFC decays exponentially with distance from the source. The decay length is of the order of 0.1 m on the main PFC and 1 m on the receded components. Also the decay lengths modelled with the ERO code are between 0.15-1.3 m, depending on the anomalous cross-field diffusion coefficient. The inner bumper limiter is found to be the major repository for Mo. Material balance measurements show that only up to 22% of the injected Mo was detected on all the PFCs thus indicating that a large fraction of injected Mo may have been pumped out before being deposited.
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41.
  • Emmoth, Birger, et al. (author)
  • In-situ measurements of carbon and deuterium deposition using the fast reciprocating probe in TEXTOR
  • 2009
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 390-91, s. 179-182
  • Journal article (peer-reviewed)abstract
    • Silicon samples were exposed in the scrape-off layer of the TEXTOR plasma using a fast reciprocating probe, with the aim of studying carbon deposition and deuterium retention during Dynamic Ergodic Divertor (DED) operation. Separate samples were exposed for 300 ms at the flat-top phase of neutral beam heated discharges. The exposure conditions were varied on a shot-to-shot basis by external magnetic perturbations generated by the DED in the m/n = 3/1, DC regime, base configuration. Nuclear Reaction Analysis (NRA) was used to characterise collector sample surfaces after their exposure. Enhanced concentrations of both carbon and deuterium (C 3-10 x 10(16) at./cm(2), D 8-60 x 10(15) at./cm(2)) were found. The D/C ratio was less than unity which indicates that most of the carbon and deuterium were co-deposited. Carbon e-folding lengths of about 2 cm were found on both toroidal sides of the probe independent of DED perturbations.
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42.
  • Fortuna-Zalesna, E., et al. (author)
  • Dust Survey Following the Final Shutdown of TEXTOR : Metal Particles and Fuel Retention
  • 2016
  • In: Physica Scripta. - : Institute of Physics Publishing (IOPP). - 0031-8949 .- 1402-4896. ; T167
  • Journal article (peer-reviewed)abstract
    • The work presents results of a broad TEXTOR dust survey in terms of its composition, structure, distribution and fuel content. The dust particles were collected after final shutdown of TEXTOR in December 2013. Fuel retention, as determined by thermal desorption, varied significantly, even by two orders of magnitude, dependent on the dust location in the machine. Dust structure was examined by means of scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, focused ion beam and scanning transmission electron microscopy. Several categories of dust have been identified. Carbon-based stratified and granular deposits were dominating, but the emphasis in studies was on metal dust. They were found in the form of small particles, small spheres, flakes and splashes which formed “comet”-like structures clearly indicating directional effects in the impact on surfaces of plasma-facing components. Nickel-rich alloys from the Inconel liner and iron-based ones from various diagnostic holders were the main components of metal-containing dust, but also molybdenum and tungsten debris were detected. Their origin is discussed.
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43.
  • Kreter, A., et al. (author)
  • Fuel retention in carbon materials under ITER-relevant mixed species plasma conditions
  • 2009
  • In: Physica scripta. T. - : Institute of Physics Publishing (IOPP). - 0281-1847 .- 0031-8949 .- 1402-4896. ; T138, s. 014012-
  • Journal article (peer-reviewed)abstract
    • Samples of CFC NB41 and fine-grain graphite ATJ have been exposed to PISCES plasmas containing (i) pure deuterium, (ii) deuterium and beryllium, (iii) deuterium, beryllium and helium, and (iv) deuterium, beryllium and argon. Thermal desorption spectrometry (TDS) and nuclear reaction analysis (NRA) have been used to measure the amount and distribution of retained deuterium in the samples. For the case of pure deuterium plasma, parametric studies of deuterium retention in NB41 have been done with variations of the incident deuterium fluence (Phi = 1 x 10(25)-5 x 10(26) m(-2)), ion energy (E-i = 20-120 eV) and sample surface temperature (T-s = 370-820 K). It has been found, that for T-s = 470K the retention scales as Phi(0.35). For T-s = 820K the retention saturates at a level of similar to 10(21) Dm(-2). The retention increases with E-i and drops with higher T-s. At T-s = 720 K, the beryllium seeding results in a building of a protective beryllium carbide layer, which appears to prevent the in-bulk diffusion of deuterium, thus reducing the retention. Admixture of Ar and, in the case of low E-i, He leads to a significant reduction of the retention.
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44.
  • Liang, Y., et al. (author)
  • Active control of type-I edge localized modes on JET
  • 2007
  • In: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 49:12B, s. B581-B589
  • Journal article (peer-reviewed)abstract
    • The operational domain for active control of type-I edge localized modes (ELMs) with an n = 1 external magnetic perturbation field induced by the ex-vessel error field correction coils on JET has been developed towards more ITER-relevant regimes with high plasma triangularity, up to 0.45, high normalized beta, up to 3.0, plasma current up to 2.0 MA and q(95) varied between 3.0 and 4.8. The results of ELM mitigation in high triangularity plasmas show that the frequency of type-I ELMs increased by a factor of 4 during the application of the n = 1 fields, while the energy loss per ELM, Delta W/W, decreased from 6% to below the noise level of the diamagnetic measurement (<2%). No reduction of confinement quality (H98Y) during the ELM mitigation phase has been observed. The minimum n = 1 perturbation field amplitude above which the ELMs were mitigated increased with a lower q(95) but always remained below the n = 1 locked mode threshold. The first results of ELM mitigation with n = 2 magnetic perturbations on JET demonstrate that the frequency of ELMs increased from 10 to 35 Hz and a wide operational window of q95 from 4.5 to 3.1 has been found.
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45.
  • Litnovsky, A., et al. (author)
  • Carbon deposition and fuel accumulation in a castellated limiter exposed under erosion-dominated conditions in the SOL of TEXTOR
  • 2005
  • In: 32nd EPS Conference on Plasma Physics 2005, EPS 2005, Held with the 8th International Workshop on Fast Ignition of Fusion Targets. - 9781622763320 ; , s. 361-364
  • Conference paper (peer-reviewed)abstract
    • Castellated limiter with ITER-like geometry was exposed under the erosion-dominated conditions in the SOL of TEXTOR. After the exposure, deposits were found in the gaps, with a negligible amount of deposited material that was detected on top surfaces open to the plasma. The thickest deposits are located on plasma shadowed sides of the gaps with maximum thickness up to 0.5 μm on the plasma closest locations as it was measured with SIMS diagnostic. The deposit thickness decrease exponentially with the depth of gaps with e-folding length of 1.7-2 mm as inferred from EPMA measurements. Material intermixing was found to occur in the deposited layers. Deposits are mixed Mo:C:D:O:B:H layers. Gaps contain at least (5÷10)×1016 of D fuel atoms per cm2. This corresponds roughly to 0.02÷0.04 % of total averaged D fluence impinging on the castellation.
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46.
  • Moeller, S., et al. (author)
  • Fundamental processes of fuel removal by cyclotron frequency range plasmas and integral scenario for fusion application studied with carbon co-deposits
  • 2015
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 463, s. 1109-1112
  • Journal article (peer-reviewed)abstract
    • Plasma impact removal using radio frequency heated plasmas is a candidate method to control the co-deposit related tritium inventory in fusion devices. Plasma parameters evolve according to the balance of input power to losses (transport, radiation, collisions). Material is sputtered by the ion fluxes with impact energies defined by the plasma sheath. H-2, D-2 and O-18(2) plasmas are produced in the carbon limiter tokamak TEXTOR. Pre-characterised a-C:D layers are exposed to study local removal rates, The D-2 plasma exhibits the highest surface release rate of 5.7 +/- 0.9 * 10(19) D/m(2)s. Compared to this the rate of the O-2 plasma is 3-fold smaller due to its 11-fold lower ion flux density. Re-deposition of removed carbon is observed, indicating that pumping and ionisation are limiting the removal in TEXTOR. Presented models can explain the observations and allow tailoring removal discharges. An integral application scenario using ICWC and thermo-chemical removal is presented, allowing to remove 700 g T from a-C:DT co-deposits in 20 h with fusion compatible wall conditions using technical specifications similar to ITER.
  •  
47.
  • Petersson, Per, et al. (author)
  • Overview of nitrogen-15 application as a tracer gas for material migration and retention studies in tokamaks
  • 2014
  • In: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; T159, s. 014042-
  • Journal article (peer-reviewed)abstract
    • Experimental and analytical procedures related to the application of nitrogen-15 isotope for material migration studies have been developed and used for tracer experiments in the TEXTOR and ASDEX-Upgrade tokamaks in order to assess the retention of nitrogen in plasma-facing components made of graphite and tungsten. The surface study was performed by time-of-flight heavy ion elastic recoil detection analysis and by means of nuclear reaction analysis based on the N-15(p, gamma alpha)C-12 process. In both tokamaks nitrogen retention has exceeded 10% of the injected gas. In ASDEX-Upgrade the largest fraction of N-15 has been detected on protruding parts near the injection port, while around 4% has been found in the divertor. The ASDEX-Upgrade results have also been modeled. Helium trapping has been measured in deposits containing tungsten and nitrogen.
  •  
48.
  • Ratynskaia, Svetlana V., et al. (author)
  • Capture by aerogel-characterization of mobile dust in tokamak scrape-off layer plasmas
  • 2009
  • In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 49:12
  • Journal article (peer-reviewed)abstract
    • The aim of this letter is to demonstrate the feasibility and potential of the novel in situ dust diagnostic method-capture by aerogel targets. Aerogel, a highly porous material with a density of a few tens of kg m(-3), allows capturing of dust particles present during the discharge without destroying them. The first exposures in the TEXTOR scrape-off layer plasma showed that such targets are able to capture both slow and fast particles with sizes in the range from submicrometre to similar to 100 mu m. The technique provides information on dust velocity and size distribution as well as dust flux estimates. The composition and texture of the captured dust can also be studied in detail to shed light on dust formation processes.
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49.
  •  
50.
  • Rubel, Marek J., et al. (author)
  • An overview of fuel retention and morphology in a castellated tungsten limiter
  • 2008
  • In: Fusion engineering and design. - : Elsevier BV. - 0920-3796 .- 1873-7196. ; 83:7-9, s. 1049-1053
  • Journal article (peer-reviewed)abstract
    • A castellated tungsten test limiter composed of detachable segments was exposed to plasma discharges in the TEXTOR to kamak operated with graphite main limiters. Dismantling of the limiter enabled the analysis Of Surfaces located inside the castellation, The emphasis was on the determination of: (i) deposition and fuel retention; (ii) material mixing and new Compound formation on plasma-facing Surfaces and in the grooves of castellation. The investigation performed by means of accelerator-based ion beam analysis methods, microscopy and X-ray diffraction has brought several essential results: (i) deuterium retention oil plasma-facing Surfaces and in the castellation of metal PFC is strongly related to the co-deposition with carbon; (ii) both carbon and deuterium are detected only in narrow belts, a few millimetre broad, clown the gap with the decay length of around 1.2-1.8 mm; (iii) the presence of copper droplets and tungsten oxide (WO(2)) has been identified in the gaps. Different pathways leading to the oxide formation are considered.
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