| 1. |
- Andersson Sundén, Erik, et al.
(författare)
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An assessment of nitrogen concentrations from spectroscopic measurements in the JET and ASDEX upgrade divertor
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 18, s. 147-152
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Tidskriftsartikel (refereegranskat)abstract
- The impurity concentration in the tokamak divertor plasma is a necessary input for predictive scaling of divertor detachment, however direct measurements from existing tokamaks in different divertor plasma conditions are limited. To address this, we have applied a recently developed spectroscopic N II line ratio technique for measuring the N concentration in the divertor to a range of H-mode and L-mode plasma from the ASDEX Upgrade and JET tokamaks, respectively. The results from both devices show that as the power crossing the separatrix, P-sep, is increased under otherwise similar core conditions (e.g. density), a higher N concentration is required to achieve the same detachment state. For example, the N concentrations at the start of detachment increase from approximate to 2% to approximate to 9% as P-sep, is increased from approximate to 2.5 MW to approximate to 7 MW. These results tentatively agree with scaling law predictions (e.g. Goldston et al.) motivating a further study examining the parameters which affect the N concentration required to reach detachment. Finally, the N concentrations from spectroscopy and the ratio of D and N gas valve fluxes agree within experimental uncertainty only when the vessel surfaces are fully-loaded with N.
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| 2. |
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| 3. |
- Angioni, C., et al.
(författare)
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Dependence of the turbulent particle flux on hydrogen isotopes induced by collisionality
- 2018
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674 .- 1070-6631 .- 1089-7666. ; 25:8
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Tidskriftsartikel (refereegranskat)abstract
- The impact of the change of the mass of hydrogen isotopes on the turbulent particle flux is studied. The trapped electron component of the turbulent particle convection induced by collisionality, which is outward in ion temperature gradient turbulence, increases with decreasing thermal velocity of the isotope. Thereby, the lighter is the isotope, the stronger is the turbulent pinch, and the larger is the predicted density gradient at the null of the particle flux. The passing particle component of the flux increases with decreasing mass of the isotope and can also affect the predicted density gradient. This effect is however subdominant for usual core plasma parameters. The analytical results are confirmed by means of both quasi-linear and nonlinear gyrokinetic simulations, and an estimate of the difference in local density gradient produced by this effect as a function of collisionality has been obtained for typical plasma parameters at mid-radius. Analysis of currently available experimental data from the JET and the ASDEX Upgrade tokamaks does not show any clear and general evidence of inconsistency with this theoretically predicted effect outside the errorbars and also allows the identification of cases providing weak evidence of qualitative consistency.
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| 4. |
- Bobkov, V, et al.
(författare)
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Impact of ICRF on the scrape-off layer and on plasma wall interactions : From present experiments to fusion reactor
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 18, s. 131-140
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Tidskriftsartikel (refereegranskat)abstract
- Recent achievements in studies of the effects of ICRF (Ion Cyclotron Range of Frequencies) power on the SOL (Scrape-Off Layer) and PWI (Plasma Wall Interactions) in ASDEX Upgrade (AUG), Alcator C-Mod, and JET-ILW are reviewed. Capabilities to diagnose and model the effect of DC biasing and associated impurity production at active antennas and on magnetic field connections to antennas are described. The experiments show that ICRF near-fields can lead not only to E x B convection, but also to modifications of the SOL density, which for Alcator C-Mod are limited to a narrow region near antenna. On the other hand, the SOL density distribution along with impurity sources can be tailored using local gas injection in AUG and JET-ILW with a positive effect on reduction of impurity sources. The technique of RF image current cancellation at antenna limiters was successfully applied in AUG using the 3-strap AUG antenna and extended to the 4-strap Alcator C-Mod field-aligned antenna. Multiple observations confirmed the reduction of the impact of ICRF on the SOL and on total impurity production when the ratio of the power of the central straps to the total antenna power is in the range 0.6 < P-cen / P-total < 0.8. Near-field calculations indicate that this fairly robust technique can be applied to the ITER ICRF antenna, enabling the mode of operation with reduced PWI. On the contrary, for the A2 antenna in JET-ILW the technique is hindered by RF sheaths excited at the antenna septum. Thus, in order to reduce the effect of ICRF power on PWI in a future fusion reactor, the antenna design has to be optimized along with design of plasmafacing components.
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| 5. |
- Bombarda, F., et al.
(författare)
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Runaway electron beam control
- 2019
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 61:1
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Tidskriftsartikel (refereegranskat)
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| 6. |
- Bonanomi, N., et al.
(författare)
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Role of fast ion pressure in the isotope effect in JET L-mode plasmas
- 2019
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:9
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents results of JET ITER-like wall L-mode experiments in hydrogen and deuterium (D) plasmas, dedicated to the study of the isotope dependence of ion heat transport by determination of the ion critical gradient and stiffness by varying the ion cyclotron resonance heating power deposition. When no strong role of fast ions in the plasma core is expected, the main difference between the two isotope plasmas is determined by the plasma edge and the core behavior is consistent with a gyro-Bohm scaling. When the heating power (and the fast ion pressure) is increased, in addition to the difference in the edge region, also the plasma core shows substantial changes. The stabilization of ion heat transport by fast ions, clearly visible in D plasmas, appears to be weaker in H plasmas, resulting in a higher ion heat flux in H with apparent anti-gyro-Bohm mass scaling. The difference is found to be caused by the different fast ion pressure between H and D plasmas, related to the heating power settings and to the different fast ion slowing down time, and is completely accounted for in non-linear gyrokinetic simulations. The application of the TGLF quasi-linear model to this set of data is also discussed.
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| 7. |
- Bravenec, R., et al.
(författare)
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Benchmarking the GENE and GYRO codes through the relative roles of electromagnetic and E x B stabilization in JET high-performance discharges
- 2016
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 58:12
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinear gyrokinetic simulations using the GENE code have previously predicted a significant nonlinear enhanced electromagnetic stabilization in certain JET discharges with high neutral-beam power and low core magnetic shear (Citrin et al 2013 Phys. Rev. Lett. 111 155001, 2015 Plasma Phys. Control. Fusion 57 014032). This dominates over the impact of E x B flow shear in these discharges. Furthermore, fast ions were shown to be a major contributor to the electromagnetic stabilization. These conclusions were based on results from the GENE gyrokinetic turbulence code. In this work we verify these results using the GYRO code. Comparing results (linear frequencies, eigenfunctions, and nonlinear fluxes) from different gyrokinetic codes as a means of verification (benchmarking) is only convincing if the codes agree for more than one discharge. Otherwise, agreement may simply be fortuitous. Therefore, we analyze three discharges, all with a carbon wall: a simplified, two-species, circular geometry case based on an actual JET discharge; an L-mode discharge with a significant fast-ion pressure fraction; and a low-triangularity high-beta hybrid discharge. All discharges were analyzed at normalized toroidal flux coordinate rho = 0.33 where significant ion temperature peaking is observed. The GYRO simulations support the conclusion that electromagnetic stabilization is strong, and dominates E x B shear stabilization.
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| 8. |
- Cannas, Barbara, et al.
(författare)
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Recurrence Plots for Dynamic Analysis of Type-I ELMs at JET With a Carbon Wall
- 2019
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Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 47:4, s. 1871-1877
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the dynamic characteristics of type-I edge-localized modes (ELM) time series from the JET tokamak, the world's largest magnetic confinement plasma physics experiment, have been investigated through recurrence plots (RPs). The analysis has been focused on RPs of pedestal temperature, line averaged electron density, and outer divertor D-alpha time series during experiments with a carbon wall. The analysis of RPS shows the patterns similar to those characteristics of signals exhibiting type-2 intermittency, in particular, a characteristic kite-like shape; this gives useful hints to model the temperature signal as well as the D-alpha radiation time series, with simple nonlinear maps capturing the nearly periodic behavior of type-I ELMs.
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| 9. |
- Carvalho, D. D., et al.
(författare)
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Deep neural networks for plasma tomography with applications to JET and COMPASS
- 2019
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Ingår i: Journal of Instrumentation. - : Institute of Physics Publishing (IOPP). - 1748-0221. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- Convolutional neural networks (CNNs) have found applications in many image processing tasks, such as feature extraction, image classification, and object recognition. It has also been shown that the inverse of CNNs, so-called deconvolutional neural networks, can be used for inverse problems such as plasma tomography. In essence, plasma tomography consists in reconstructing the 2D plasma profile on a poloidal cross-section of a fusion device, based on line-integrated measurements from multiple radiation detectors. Since the reconstruction process is computationally intensive, a deconvolutional neural network trained to produce the same results will yield a significant computational speedup, at the expense of a small error which can be assessed using different metrics. In this work, we discuss the design principles behind such networks, including the use of multiple layers, how they can be stacked, and how their dimensions can be tuned according to the number of detectors and the desired tomographic resolution for a given fusion device. We describe the application of such networks at JET and COMPASS, where at JET we use the bolometer system, and at COMPASS we use the soft X-ray diagnostic based on photodiode arrays.
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| 10. |
- Chankin, A. , V, et al.
(författare)
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EDGE2D-EIRENE simulations of the influence of isotope effects and anomalous transport coefficients on near scrape-off layer radial electric field
- 2019
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 61:7
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Tidskriftsartikel (refereegranskat)abstract
- EDGE2D-EIRENE (the 'code') simulations show that radial electric field, Er, in the near scrape-off layer (SOL) of tokamaks can have large variations leading to a strong local E x B shear greatly exceeding that in the core region. This was pointed out in simulations of JET plasmas with varying divertor geometry, where the magnetic configuration with larger predicted near SOL E-r was found to have lower H-mode power threshold, suggesting that turbulence suppression in the SOL by local E. x. B shear can be a player in the L-H transition physics (Delabie et al 2015 42nd EPS Conf. on Plasma Physics (Lisbon, Portugal, 22-26 June 2015) paper O3.113 (http://ocs.ciemat.es/EPS2015PAP/pdf/O3.113.pdf), Chankin et al 2017 Nucl. Mater. Energy 12 273). Further code modeling of JET plasmas by changing hydrogen isotopes (H-D-T) showed that the magnitude of the near SOL E-r is lower in H cases in which the H-mode threshold power is higher (Chankin et al 2017 Plasma Phys. Control. Fusion 59 045012). From the experiment it is also known that hydrogen plasmas have poorer particle and energy confinement than deuterium plasmas, consistent with the code simulation results showing larger particle diffusion coefficients at the plasma edge, including SOL, in hydrogen plasmas (Maggi et al 2018 Plasma Phys. Control. Fusion 60 014045). All these experimental observations and code results support the hypothesis that the near SOL E x B shear can have an impact on the plasma confinement. The present work analyzes neutral ionization patterns of JET plasmas with different hydrogen isotopes in L-mode cases with fixed input power and gas puffing rate, and its impact on target electron temperature, T-e, and SOL E-r. The possibility of a self-feeding mechanism for the increase in the SOL E-r via the interplay between poloidal E x B drift and target T-e is discussed. It is also shown that reducing anomalous turbulent transport coefficients, particle diffusion and electron and ion heat conductivities, leads to higher peak target T-e and larger E-r, suggesting the possibility of a positive feedback loop, under an implicitly made assumption that the E x B shear in the SOL is capable of suppressing turbulence.
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| 11. |
- Craciunescu, Teddy, et al.
(författare)
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Evaluation of reconstruction errors and identification of artefacts for JET gamma and neutron tomography
- 2016
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 87:1
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Tidskriftsartikel (refereegranskat)abstract
- The Joint European Torus (JET) neutron profile monitor ensures 2D coverage of the gamma and neutron emissive region that enables tomographic reconstruction. Due to the availability of only two projection angles and to the coarse sampling, tomographic inversion is a limited data set problem. Several techniques have been developed for tomographic reconstruction of the 2-D gamma and neutron emissivity on JET, but the problem of evaluating the errors associated with the reconstructed emissivity profile is still open. The reconstruction technique based on the maximum likelihood principle, that proved already to be a powerful tool for JET tomography, has been used to develop a method for the numerical evaluation of the statistical properties of the uncertainties in gamma and neutron emissivity reconstructions. The image covariance calculation takes into account the additional techniques introduced in the reconstruction process for tackling with the limited data set (projection resampling, smoothness regularization depending on magnetic field). The method has been validated by numerically simulations and applied to JET data. Different sources of artefacts that may significantly influence the quality of reconstructions and the accuracy of variance calculation have been identified.
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| 12. |
- Denis, J., et al.
(författare)
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Dynamic modelling of local fuel inventory and desorption in the whole tokamak vacuum vessel for auto-consistent plasma-wall interaction simulations
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 19, s. 550-557
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Tidskriftsartikel (refereegranskat)abstract
- An extension of the SolEdge2D-EIRENE code package, named D-WEE, has been developed to add the dynamics of thermal desorption of hydrogen isotopes from the surface of plasma facing materials. To achieve this purpose, D-WEE models hydrogen isotopes implantation, transport and retention in those materials. Before launching autoconsistent simulation (with feedback of D-WEE on SolEdge2D-EIRENE), D-WEE has to be initialised to ensure a realistic wall behaviour in terms of dynamics (pumping or fuelling areas) and fuel content. A methodology based on modelling is introduced to perform such initialisation. A synthetic plasma pulse is built from consecutive SolEdge2D-EIRENE simulations. This synthetic pulse is used as a plasma background for the D-WEE module. A sequence of plasma pulses is simulated with D-WEE to model a tokamak operation. This simulation enables to extract at a desired time during a pulse the local fuel inventory and the local desorption flux density which could be used as initial condition for coupled plasma-wall simulations. To assess the relevance of the dynamic retention behaviour obtained in the simulation, a confrontation to post-pulse experimental pressure measurement is performed. Such confrontation reveals a qualitative agreement between the temporal pressure drop obtained in the simulation and the one observed experimentally. The simulated dynamic retention during the consecutive pulses is also studied.
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| 13. |
- Eriksson, Frida, 1986, et al.
(författare)
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Interpretative and predictive modelling of Joint European Torus collisionality scans
- 2019
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 61:11
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Tidskriftsartikel (refereegranskat)abstract
- Transport modelling of Joint European Torus (JET) dimensionless collisionality scaling experiments in various operational scenarios is presented. Interpretative simulations at a fixed radial position are combined with predictive JETTO simulations of temperatures and densities, using the TGLF transport model. The model includes electromagnetic effects and collisions as well as (E)over-right-arrow x (b)over-right-arrow shear in Miller geometry. Focus is on particle transport and the role of the neutral beam injection (NBI) particle source for the density peaking. The experimental 3-point collisionality scans include L-mode, and H-mode (D and H and higher beta D plasma) plasmas in a total of 12 discharges. Experimental results presented in (Tala et al 2017 44th EPS Conf.) indicate that for the H-mode scans, the NBI particle source plays an important role for the density peaking, whereas for the L-mode scan, the influence of the particle source is small. In general, both the interpretative and predictive transport simulations support the experimental conclusions on the role of the NBI particle source for the 12 JET discharges.
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| 14. |
- Garcia, J., et al.
(författare)
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First principles and integrated modelling achievements towards trustful fusion power predictions for JET and ITER
- 2019
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:8
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Tidskriftsartikel (refereegranskat)abstract
- Predictability of burning plasmas is a key issue for designing and building credible future fusion devices. In this context, an important effort of physics understanding and guidance is being carried out in parallel to JET experimental campaigns in H and D by performing analyses and modelling towards an improvement of the understanding of DT physics for the optimization of the JET-DT neutron yield and fusion born alpha particle physics. Extrapolations to JET-DT from recent experiments using the maximum power available have been performed including some of the most sophisticated codes and a broad selection of models. There is a general agreement that 11-15 MW of fusion power can be expected in DT for the hybrid and baseline scenarios. On the other hand, in high beta, torque and fast ion fraction conditions, isotope effects could be favourable leading to higher fusion yield. It is shown that alpha particles related physics, such as TAE destabilization or fusion power electron heating, could be studied in ITER relevant JET-DT plasmas.
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| 15. |
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| 16. |
- Kirschner, A., et al.
(författare)
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Modelling of tungsten erosion and deposition in the divertor of JET-ILW in comparison to experimental findings
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 18, s. 239-244
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Tidskriftsartikel (refereegranskat)abstract
- The erosion, transport and deposition of tungsten in the outer divertor of JET-ILW has been studied for an H-Mode discharge with low frequency ELMs. For this specific case with an inter-ELM electron temperature at the strike point of about 20 eV, tungsten sputtering between ELMs is almost exclusively due to beryllium impurity and self-sputtering. However, during ELMs tungsten sputtering due to deuterium becomes important and even dominates. The amount of simulated local deposition of tungsten relative to the amount of sputtered tungsten in between ELMs is very high and reaches values of 99% for an electron density of 5E13 cm(-3) at the strike point and electron temperatures between 10 and 30 eV. Smaller deposition values are simulated with reduced electron density. The direction of the B-field significantly influences the local deposition and leads to a reduction if the E x B drift directs towards the scrape-off-layer. Also, the thermal force can reduce the tungsten deposition, however, an ion temperature gradient of about 0.1 eV/mm or larger is needed for a significant effect. The tungsten deposition simulated during ELMs reaches values of about 98% assuming ELM parameters according to free-streaming model. The measured WI emission profiles in between and within ELMs have been reproduced by the simulation. The contribution to the overall net tungsten erosion during ELMs is about 5 times larger than the one in between ELMs for the studied case. However, this is due to the rather low electron temperature in between ELMs, which leads to deuterium impact energies below the sputtering threshold for tungsten.
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| 17. |
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| 18. |
- Kotschenreuther, M., et al.
(författare)
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Gyrokinetic analysis and simulation of pedestals to identify the culprits for energy losses using 'fingerprints'
- 2019
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:9
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Tidskriftsartikel (refereegranskat)abstract
- Fusion performance in tokamaks hinges critically on the efficacy of the edge transport barrier (ETB) in suppressing energy losses. The new concept of 'fingerprints' is introduced to identify the instabilities that cause transport losses in the ETBs of many of today's experiments, from among widely posited candidates. Analysis of the gyrokinetic-Maxwell equations and gyrokinetic simulations of experiments reveals that each mode type produces characteristic ratios of transport in the various channels: density, heat, and impurities. This, together with experimental observations of transport in some channel or of the relative size of the driving sources of channels, can identify or determine the dominant modes causing energy transport. In multiple H-mode cases with edge-localized modes that are examined, these fingerprints indicate that magnetohydrodynamic (MHD)-like modes are apparently not the dominant agent of energy transport; rather, this role is played by micro-tearing modes (MTMs) and electron temperature gradient (ETG) modes, and in addition, possibly by ion temperature gradient/ trapped electron modes (ITG/TEM) on JET (Joint European 'Torus). MHD-like modes may dominate the electron particle losses. Fluctuation frequency can also be an important means of identification, and is often closely related to the transport fingerprint. The analytical arguments unify and explain previously disparate experimental observations on multiple devices, including DIII-D, JET, and ASDEX-U. Detailed simulations of two DIII-D ETBs also demonstrate and corroborate this.
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| 19. |
- Krivska, A., et al.
(författare)
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RF sheath modeling of experimentally observed plasma surface interactions with the JET ITER-Like Antenna
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 19, s. 324-329
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Tidskriftsartikel (refereegranskat)abstract
- Waves in the Ion Cyclotron Range of Frequencies (ICRF) enhance local Plasma-Surface Interactions (PSI) near the wave launchers and magnetically-connected objects via Radio-Frequency (RF) sheath rectification. ITER will use 20MW of ICRF power over long pulses, questioning the long-term impact of RF-enhanced localized erosion on the lifetime of its Beryllium (Be) wall. Recent dedicated ICRF-heated L-mode discharges documented this process on JET for different types of ICRF antennas. Using visible spectroscopy in JET ICRF-heated L-mode discharges, poloidally-localized regions of enhanced (by similar to 2-4x) Be I and Be II light emission were observed on two outboard limiters magnetically connected to the bottom of the active ITER-Like Antenna (ILA). The observed RF-PSI induced by the ILA was qualitatively comparable to that induced by the JET standard, type-A2 antennas, for similar strap toroidal phasing and connection geometries. The Be II line emission was found more intense when powering the bottom half of the ILA rather than its top half. Conversely, more pronounced SOL density modifications were observed with only top array operation, on field lines connected to the top half of the ILA. So far the near-field modeling of the ILA with antenna code TOPICA (Torino Polytechnic Ion Cyclotron Antenna), using curved antenna model, was partially able to reproduce qualitatively the observed phenomena. A quantitative discrepancy persisted between the observed Be source amplification and the calculated, corresponding increases in E-// field at the magnetically connected locations to the ILA when changing from only top to only bottom half antenna operation. This paper revisits these current drive phased and half-ILA powered cases using for the new simulations flat model of the ILA and more realistic antenna feeding to calculate the E-// field maps with TOPICA code. Further, the Self-consistent Sheaths and Waves for Ion Cyclotron Heating Slow Wave (SSWICH-SW) code, which couples slow wave evanescence with DC Scrape-Off Layer (SOL) biasing, is used to estimate the poloidal distribution of rectified RF-sheath Direct Current (DC) potential V-DC in the private SOL between the ILA poloidal limiters. The approach so far was limited to correlating the observed, enhanced emission regions at the remote limiters to the antenna near-electric fields, as calculated by TOPICA. The present approach includes also a model for the rectification of these near-fields in the private SOL of the ILA. With the improved approach, when comparing only top and only bottom half antenna feeding, we obtained good qualitative correlation between all experimental measurements and the calculated local variations in the E-// field and V-DC potential.
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| 20. |
- Likonen, J., et al.
(författare)
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Investigation of deuterium trapping and release in the JET divertor during the third ILW campaign using TDS
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 19, s. 300-306
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Tidskriftsartikel (refereegranskat)abstract
- Selected set of samples from JET ITER-Like Wall (JET-ILW) divertor tiles exposed in 2015-2016 has been analysed using Thermal Desorption Spectrometry (TDS). The deuterium (D) amounts obtained with TDS were compared with Nuclear Reaction Analysis (NRA). The highest amount of D was found on the top part of inner divertor which has regions with the thickest deposited layers as for divertor tiles removed in 2014. This area resides deep in the scrape-off layer and plasma configurations for the second (ILW-2, 2013-2014) and the third (ILW-3, 2015-2016) JET-ILW campaigns were similar. Agreement between TDS and NRA is good on the apron of Tile 1 and on the upper vertical region whereas on the lower vertical region of Tile 1 the NRA results are clearly smaller than the TDS results. Inner divertor Tile 3 has somewhat less D than Tiles 0 and 1, and the D amount decreases towards the lower part of the tile. The D retention at the divertor inner and outer corner regions is not symmetric as there is more D retention poloidally at the inner than at the outer divertor corner. In most cases the TDS spectra for the ILW-3 samples are different from the corresponding ILW-2 spectra because HD and D-2 release occurs at higher temperatures than from the ILW-2 samples indicating that the low energy traps have been emptied during the plasma operations and that D is either in the energetically deep traps or located deeper in the sample.
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| 21. |
- Likonen, J., et al.
(författare)
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Investigation of deuterium trapping and release in the JET ITER-like wall divertor using TDS and TMAP
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 19, s. 166-178
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Tidskriftsartikel (refereegranskat)abstract
- Selected set of samples from JET ITER-Like Wall (JET-ILW) divertor tiles exposed both in 2013-2014 and 2011-2014 has been analysed using Thermal Desorption Spectrometry (TDS). The deuterium (D) amounts obtained with TDS were compared with Ion Beam Analysis (IBA) and Secondary Ion Mass Spectrometry (SIMS) data. The highest amount of D was found on the top part of inner divertor which has regions with the thickest deposited layers. This area resides deep in the scrape-off layer. Changes in plasma configurations between the first (2011-2012) and the second (2013-2014) JET-ILW campaign altered the material migration towards the inner and the outer divertor corner increasing the amount of deposition in the shadowed areas of the divertor base tiles. D retention on the outer divertor tiles is clearly smaller than on the inner divertor tiles. Experimental TDS spectra for samples from the top part of inner divertor and from the outer strike point region were modelled using TMAP program. Experimental deuterium profiles obtained with SIMS have been used and the detrapping and the activation energies have been adjusted. Analysis of the results of the TMAP simulations enabled to determine the nature of traps in different samples.
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| 22. |
- Maggi, C. F., et al.
(författare)
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Isotope identity experiments in JET-ILW with H and D L-mode plasmas
- 2019
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:7
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Tidskriftsartikel (refereegranskat)abstract
- NBI-heated L-mode plasmas have been obtained in JET with the Be/W ITER-like wall (JET-ILW) in H and D, with matched profiles of the dimensionless plasma parameters, rho*, nu*, beta and q in the plasma core confinement region and same T-i/T-e and Z(eff). The achieved isotope identity indicates that the confinement scale invariance principle is satisfied in the core confinement region of these plasmas, where the dominant instabilities are Ion Temperature Gradient (ITG) modes. The dimensionless thermal energy confinement time, Omega(i) tau(E,th), and the scaled core plasma heat diffusivity, A chi(eff)/B-T, are identical in H and D within error bars, indicating lack of isotope mass dependence of the dimensionless L-mode thermal energy confinement time in JET-ILW. Predictive flux driven simulations with JETTO-TGLF of the H and D identity pair is in very good agreement with experiment for both isotopes: the stiff core heat transport, typical of JET-ILW NBI heated L-modes, overcomes the local gyro-Bohm scaling of gradient-driven TGLF, explaining the lack of isotope mass dependence in the confinement region of these plasmas. The effect of E x B shearing on the predicted heat and particle transport channels is found to be negligible for these low beta and low momentum input plasmas.
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| 23. |
- Makepeace, C., et al.
(författare)
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The effect of beryllium oxide on retention in JET ITER-like wall tiles
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 19, s. 346-351
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Tidskriftsartikel (refereegranskat)abstract
- Preliminary results investigating the microstructure, bonding and effect of beryllium oxide formation on retention in the JET ITER-like wall beryllium tiles, are presented. The tiles have been investigated by several techniques: Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray (EDX), Transmission Electron microscopy (TEM) equipped with EDX and Electron Energy Loss Spectroscopy (EELS), Raman Spectroscopy and Thermal Desorption Spectroscopy (TDS). This paper focuses on results from melted materials of the dump plate tiles in JET. From our results and the literature, it is concluded, beryllium can form micron deep oxide islands contrary to the nanometric oxides predicted under vacuum conditions. The deepest oxides analyzed were up to 2-micron thicknesses. The beryllium Deuteroxide (BeOxDy) bond was found with Raman Spectroscopy. Application of EELS confirmed the oxide presence and stoichiometry. Literature suggests these oxides form at temperatures greater than 700 degrees C where self-diffusion of beryllium ions through the surface oxide layer can occur. Further oxidation is made possible between oxygen plasma impurities and the beryllium ions now present at the wall surface. Under Ultra High Vacuum (UHV) nanometric Beryllium oxide layers are formed and passivate at room temperature. After continual cyclic heating (to the point of melt formation) in the presence of oxygen impurities from the plasma, oxide growth to the levels seen experimentally (approximately two microns) is proposed. This retention mechanism is not considered to contribute dramatically to overall retention in JET, due to low levels of melt formation. However, this mechanism, thought the result of operation environment and melt formation, could be of wider concern to ITER, dependent on wall temperatures.
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| 24. |
- Nabais, F., et al.
(författare)
-
Energetic ion losses 'channeling' mechanism and strategy for mitigation
- 2019
-
Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 61:8
-
Tidskriftsartikel (refereegranskat)abstract
- Results from two different sets of JET experiments are presented. In experiments in which toroidicity-induced Alfven eigenmodes (TAEs) localized at different radial locations had the same frequencies and toroidal mode numbers, the occurrence of enhanced losses after the excitation of TAEs in the core of the plasma was observed. On the contrary, enhanced losses were not observed if the TAEs localized at different radial locations had different frequencies and toroidal mode numbers. Numerical modeling indicates that, in the first set of experiments, the enhanced losses were caused by a combined effect of the TAEs localized at different radial locations. The TAEs localized in the plasma core transported energetic ions from the core to outer regions of the plasma. Then, the TAEs localized in outer regions of the plasma interacted with these ions just transported by the core-localized TAEs causing a further radial displacement of the ions to the plasma edge. This process eventually ends up causing the loss of the resonant ions. In the second set of experiments, it was found that TAEs localized in the plasma core and in outer regions did not interact with the same ions and so no enhanced losses were measured. Sheared profiles of the safety factor combined with flat mass density profiles lead to larger differences on the frequencies of the TAEs localized at different radial locations, eventually avoiding loss of energetic ions through the described mechanism.
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| 25. |
- Neverov, V. S., et al.
(författare)
-
Determination of isotope ratio in the divertor of JET-ILW by high-resolution H alpha spectroscopy : H-D experiment and implications for D-T experiment
- 2019
-
Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:4
-
Tidskriftsartikel (refereegranskat)abstract
- The data of the H alpha high-resolution spectroscopy, collected on the multiple lines of sight, which cover the entire divertor space in poloidal cross-section, during the recent hydrogen-deuterium experiments in JET-ILW (ITER-like wall), are processed. A strong spatial inhomogeneity of the hydrogen concentration, H/(H + D), in divertor is found in many pulses. Namely, the H/(H + D) ratio may be lower in the inner divertor than that in the outer divertor by the values of 0.15-0.35, depending on the conditions of gas puffing and plasma heating. This effect suggests the necessity of spatially-resolved measurements of isotope ratio in the divertor in the upcoming deuterium-tritium experiments. Also, separation of the overlapped T alpha and D alpha spectral lines is shown to be a challenging task especially when the local Doppler-broadened (Gaussian) line shapes are noticeably distorted by the net inward flux of fast non-Maxwellian neutral atoms. We use the respective, formerly developed model of an asymmetric spectral line shape, while analysing the data of the first deuterium-tritium experiment in JET-C (carbon wall), and test the model via comparing the isotope ratio results with another diagnostic's measurements. This model is shown to increase the accuracy of tritium concentration measurements in the divertor.
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| 26. |
- Pamela, S., et al.
(författare)
-
A wall-aligned grid generator for non-linear simulations of MHD instabilities in tokamak plasmas
- 2019
-
Ingår i: Computer Physics Communications. - : Elsevier. - 0010-4655 .- 1879-2944. ; 243, s. 41-50
-
Tidskriftsartikel (refereegranskat)abstract
- Block-structured mesh generation techniques have been well addressed in the CFD community for automobile and aerospace studies, and their applicability to magnetic fusion is highly relevant, due to the complexity of the plasma-facing wall structures inside a tokamak device. Typically applied to non-linear simulations of MHD instabilities relevant to magnetically confined fusion, the JOREK code was originally developed with a 2D grid composed of isoparametric bi-cubic Bezier finite elements, that are aligned to the magnetic equilibrium of tokamak plasmas (the third dimension being represented by Fourier harmonics). To improve the applicability of these simulations, the grid-generator has been generalised to provide a robust extension method, using a block-structured mesh approach, which allows the simulations of arbitrary domains of tokamak vacuum vessels. Such boundary-aligned grids require the adaptation of boundary conditions along the edge of the new domain. Demonstrative non-linear simulations of plasma edge instabilities are presented to validate the robustness of the new grid, and future potential physics applications for tokamak plasmas are discussed. The methods presented here may be of interest to the wider community, beyond tokamak physics, wherever imposing arbitrary boundaries to quadrilateral finite elements is required.
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| 27. |
- Silva, C., et al.
(författare)
-
Geodesic acoustic mode evolution in L-mode approaching the L-H transition on JET
- 2019
-
Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 61:7
-
Tidskriftsartikel (refereegranskat)abstract
- Geodesic acoustic modes (GAMs) may generate strong oscillations in the radial electric field and therefore are considered as a possible trigger mechanism for the L-H transition. This contribution focuses on the characterization of GAMs in JET plasmas when approaching the L-H transition aiming at understanding their possible role in triggering the transition. GAM and turbulence characteristics are measured at the plasma edge using Doppler backscattering for different plasma current and line-averaged densities. The radial location of the GAM often moves further inside when neutral beam injection is applied possibly as a response to changes in the turbulence drive. GAMs are found to have modest amplitude at the transition except for high density discharges where GAMs are stronger, suggesting that the GAM is not responsible for facilitating the transition as the L-H power threshold also increases with density in the high density branch of the L-H transition. Our results suggest that the GAM alone does not play a leading role for causing the L-H transition at JET.
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| 28. |
- Tardocchi, M., et al.
(författare)
-
Modeling of neutron emission spectroscopy in JET discharges with fast tritons from (T)D ion cyclotron heating
- 2006
-
Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 77:12
-
Tidskriftsartikel (refereegranskat)abstract
- The measurement of fast ion populations is one of the diagnostic capabilities provided by neutron emission spectroscopy (NES). NES measurements were carried out during JET trace tritium campaign with the magnetic proton recoil neutron spectrometer. A favorable plasma scenario is (T)D where the resulting 14 MeV neutron yield is dominated by suprathermal emission from energetic tritons accelerated by radio frequency at their fundamental cyclotron frequency. Information on the triton distribution function has been derived from NES data with a simple model based on two components referred to as bulk (B) and high energy (HE). The HE component is based on strongly anisotropic tritium distribution that can be used for routine best-fit analysis to provide tail temperature values (T-HE). This article addresses to what extent the T-HE values are model dependent by comparing the model above with a two-temperature (bi-) Maxwellian model featuring parallel and perpendicular temperatures. The bi-Maxwellian model is strongly anisotropic and frequently used for radio frequency theory.
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| 29. |
- Telesca, G., et al.
(författare)
-
COREDIV numerical simulation of high neutron rate JET-ILW DD pulses in view of extension to JET-ILW DT experiments
- 2019
-
Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:5
-
Tidskriftsartikel (refereegranskat)abstract
- Two high performance JET-ILW pulses, pertaining to the 2016 experimental campaign, have been numerically simulated with the self-consistent code COREDIV with the aim of predicting the ELM-averaged power load to the target when extrapolated to DT plasmas. The input power of about 33 MW as well as the total radiated power and the average density are similar in the two pulses, but for one of them the density is provided by combined low gas puff and pellet injection, characterized by low SOL density, for the other one by gas fuelling only, at higher SOT. density. Considering the magnetic configuration of theses pulses and the presence of a significant amount of Ni (not included in the version of the code used for these simulations), a number of assumptions are made in order to reproduce numerically the main core and SOL experimental data. The extrapolation to DT plasmas at the original input power of 33 MW, and taking into account only the thermal component of the alpha-power, does not show any significant difference regarding the power to the target with respect to the DD case. In contrast, the simulations at auxiliary power 40 MW, both at the original I-p = 3 MA and at I-p = 4 MA, show that the power to the target for both pulses is possibly too high to be sustained for about 5 s by strike-point sweeping alone without any control by Ne seeding. Even though the target power load may decrease to about 13-15 MW with substantial Ne seeding for both pulses, as from numerical predictions, there are indications suggesting that the control of the power load may be more critical for the pulse with pellet injection, due to the reduced SOL radiation.
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| 30. |
- Varje, J., et al.
(författare)
-
Synthetic diagnostic for the JET scintillator probe lost alpha measurements
- 2019
-
Ingår i: Journal of Instrumentation. - : Institute of Physics Publishing (IOPP). - 1748-0221. ; 14
-
Tidskriftsartikel (refereegranskat)abstract
- A synthetic diagnostic has been developed for the JET lost alpha scintillator probe, based on the ASCOT fast ion orbit following code and the AFSI fusion source code. The synthetic diagnostic models the velocity space distribution of lost fusion products in the scintillator probe. Validation with experimental measurements is presented, where the synthetic diagnostic is shown to predict the gyroradius and pitch angle of lost DD protons and tritons. Additionally, the synthetic diagnostic reproduces relative differences in total loss rates in multiple phases of the discharge, which can be used as a basis for total loss rate predictions.
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| 31. |
- Weiland, M., et al.
(författare)
-
Simulation of neutron emission in neutral beam injection heated plasmas with the real-time code RABBIT
- 2019
-
Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:8
-
Tidskriftsartikel (refereegranskat)abstract
- In plasmas heated with deuterium beams a deficit of the expected fusion neutron rate is an indicator of the deterioration of the fast-ion confinement, caused, for instance, by magnetohydrodynamic instabilities. The capability of predicting this deficit during the discharge relies on the availability of real-time estimates of the neutron rate from NBI codes which must be fast and accurate at the same time. Therefore, the recently developed real-time RABBIT code for neutral beam injection (NBI) simulations has been extended to output the distribution function and calculate the neutron emission. After the description of this newly installed diagnostics in RABBIT, benchmarks with NUBEAM, a massively used and validated Monte Carlo NBI solver, are discussed on ASDEX-Upgrade and JET cases. A first application for control-room intershot analysis on DIII-D is presented, and the results are compared on a large database with a slower NUBEAM analysis. Further application possibilities, e.g. for real-time control of Alfven eigenmodes, are outlined.
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| 32. |
- Widdowson, A., et al.
(författare)
-
Deposition of impurity metals during campaigns with the JET ITER-like Wall
- 2019
-
Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 19, s. 218-224
-
Tidskriftsartikel (refereegranskat)abstract
- Post mortem analysis shows that mid and high atomic number metallic impurities are present in deposits on JET plasma facing components with the highest amount of Ni and W, and therefore the largest sink, being found at the top of the inner divertor. Sources are defined as "continuous" or "specific", in that "continuous" sources arise from ongoing erosion from plasma facing surfaces and "specific" are linked with specific events which decrease over time until they no longer act as a source. This contribution evaluates the sinks and estimates sources, and the balance gives an indication of the dominating processes. Charge exchange neutral erosion is found to be the main source of nickel, whereas erosion of divertor plasma facing components is the main source of tungsten. Specific sources are shown to have little influence over the global mid- and high-Z impurity concentrations in deposits.
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| 33. |
- Zanca, P., et al.
(författare)
-
A power-balance model of the density limit in fusion plasmas : application to the L-mode tokamak
- 2019
-
Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:12
-
Tidskriftsartikel (refereegranskat)abstract
- A power-balance model, with radiation losses from impurities and neutrals, gives a unified description of the density limit (DL) of the stellarator, the L-mode tokamak, and the reversed field pinch (RFP). The model predicts a Sudo-like scaling for the stellarator, a Greenwald- like scaling, alpha I-p(8/9), for the RFP and the ohmic tokamak, a mixed scaling, alpha (PIp4/9)-I-4/9, for the additionally heated L-mode tokamak. In a previous paper (Zanca et al 2017 Nucl. Fusion 57 056010) the model was compared with ohmic tokamak, RFP and stellarator experiments. Here, we address the issue of the DL dependence on heating power in the L-mode tokamak. Experimental data from high-density disrupted L-mode discharges performed at JET, as well as in other machines, arc taken as a term of comparison. The model fits the observed maximum densities better than the pure Greenwald limit.
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| 34. |
-
- 2018
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:9
-
Tidskriftsartikel (refereegranskat)
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| 35. |
-
- 2018
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:1
-
Forskningsöversikt (refereegranskat)
|
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| 36. |
- Ahlford, Marianne, et al.
(författare)
-
Uppsala Underdogs - A Robot Soccer Project
- 2006
-
Rapport (populärvet., debatt m.m.)abstract
- In this paper, we describe the four-legged soccer team Uppsala Underdogs developed by a group of 4th year computer science students at Uppsala University during the fall of 2004. The project is based on the experience from two similar previous projects. This year the emphasis of the project has been on distribution of data and on support for evaluation and reconfiguration of strategies. To support data distribution, a middleware has been developed, which implements a replication algorithm and provides a clean interface for the other software modules (or behaviors). To enable easy reconfiguration of strategies, an automata-based graphical description language has been developed, which can be compiled into code that uses the database and the lower level modules, such as tactics and positioning, to make decisions and control the robot. In addition, a graphical simulator has been developed in which the strategies can be evaluated.
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| 37. |
- Aho-Mantila, L., et al.
(författare)
-
Assessment of SOLPS5.0 divertor solutions with drifts and currents against L-mode experiments in ASDEX Upgrade and JET
- 2017
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP PUBLISHING LTD. - 0741-3335 .- 1361-6587. ; 59:3
-
Tidskriftsartikel (refereegranskat)abstract
- The divertor solutions obtained with the plasma edge modelling tool SOLPS5.0 are discussed. The code results are benchmarked against carefully analysed L-mode discharges at various density levels with and without impurity seeding in the full-metal tokamaks ASDEX Upgrade and JET. The role of the cross-field drifts and currents in the solutions is analysed in detail, and the improvements achieved by fully activating the drift and current terms in view of matching the experimental signals are addressed. The persisting discrepancies are also discussed.
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| 38. |
- Aiba, N., et al.
(författare)
-
Numerical analysis of ELM stability with rotation and ion diamagnetic drift effects in JET
- 2017
-
Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 57:12
-
Tidskriftsartikel (refereegranskat)abstract
- Stability to the type-I edge localized mode (ELM) in JET plasmas was investigated numerically by analyzing the stability to a peeling-ballooning mode with the effects of plasma rotation and ion diamagnetic drift. The numerical analysis was performed by solving the extended Frieman-Rotenberg equation with the MINERVA-DI code. To take into account these effects in the stability analysis self-consistently, the procedure of JET equilibrium reconstruction was updated to include the profiles of ion temperature and toroidal rotation, which are determined based on the measurement data in experiments. With the new procedure and MINERVA-DI, it was identified that the stability analysis including the rotation effect can explain the ELM trigger condition in JET with ITER like wall (JET-ILW), though the stability in JET with carbon wall (JET-C) is hardly affected by rotation. The key difference is that the rotation shear in JET-ILW plasmas analyzed in this study is larger than that in JET-C ones, the shear which enhances the dynamic pressure destabilizing a peeling-ballooning mode. In addition, the increase of the toroidal mode number of the unstable MHD mode determining the ELM trigger condition is also important when the plasma density is high in JET-ILW. Though such modes with high toroidal mode number are strongly stabilized by the ion diamagnetic drift effect, it was found that plasma rotation can sometimes overcome this stabilizing effect and destabilizes the peeling-ballooning modes in JET-ILW.
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| 39. |
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| 40. |
- Andersson Sundén, Erik, et al.
(författare)
-
Evaluation of neutron spectrometer techniques for ITER using synthetic data
- 2013
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 701, s. 62-71
-
Tidskriftsartikel (refereegranskat)abstract
- A neutron spectrometer at ITER is expected to provide estimates of plasma parameters such as ion temperature, Ti, fuel ion ratio, nt/nd, and Qthermal/Qtot, with 10-20% precision at a time resolution, Δt, of at least 100 ms. The present paper describes a method for evaluating different neutron spectroscopy techniques based on their instrumental response functions and synthetic measurement data. We include five different neutron spectrometric techniques with realistic response functions, based on simulations and measurements where available. The techniques are magnetic proton recoil, thin-foil proton recoil, gamma discriminating organic scintillator, diamond and time-of-flight. The reference position and line of sight of a high resolution neutron spectrometer on ITER are used in the study. ITER plasma conditions are simulated for realistic operating scenarios. The ITER conditions evaluated are beam and radio frequency heated and thermal deuterium-tritium plasmas. Results are given for each technique in terms of the estimated time resolution at which the parameter determination can be made within the required precision (here 10% for Ti and the relative intensities of NB and RF emission components). It is shown that under the assumptions made, the thin-foil techniques out-perform the other spectroscopy techniques in practically all measurement situations. For thermal conditions, the range of achieved Δt in the determination of Ti varies in time scales from ms (for the magnetic and thin-foil proton recoil) to s (for gamma discriminating organic scintillator).
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| 41. |
- Andersson Sundén, Erik, et al.
(författare)
-
Instrumentation for neutron emission spectrometry in use at JET
- 2010
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 623:2, s. 681-685
-
Tidskriftsartikel (refereegranskat)abstract
- The present contribution discusses two neutron spectrometers: the time-of-flight spectrometer (TOFOR) and the magnetic proton recoil spectrometer (MPRu). TOFOR uses fast plastic scintillators equipped with digital time-stamping electronics to register the time of each eligible scintillation event. The time trace for each detector is acquired practically dead-time free. The detectors of the MPRu are of phoswich type and each detector is connected to a digital transient recorder card that stores the full waveform for an event. By using phoswich detectors, pulse-shape discrimination techniques can be applied offline to distinguish signal events from background. A future upgrade of TOFOR could be digital “hybrid” cards, which store correlated time and waveform information. This information can be used to decrease the background level in the ttof spectrum, thereby increasing the operating range.
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| 42. |
- Andersson Sundén, Erik, et al.
(författare)
-
The thin-foil magnetic proton recoil neutron spectrometer MPRu at JET
- 2009
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 610:3, s. 682-699
-
Tidskriftsartikel (refereegranskat)abstract
- Neutrons are produced in fusion energy experiments with both deuterium (D) and deuterium–tritium (DT) plasmas. Neutron spectroscopy is a valuable tool in the study of the underlying fuel ion populations. The magnetic proton recoil neutron spectrometer, originally installed at JET in 1996 for 14-MeV neutron measurements, has been upgraded, with the main aim of improving its signal-to-background ratio (S/B), making measurements of the 2.5-MeV neutron emission in D plasmas possible. The upgrade includes a new focal-plane detector, based on the phoswich technique and consequently less sensitive to background, and a new custom-designed digital data acquisition system based on transient recorder cards. Results from JET show that the upgraded MPRu can measure 2.5-MeV neutrons with S/B=5, an improvement by a factor of 50 compared with the original MPR. S/B of 2.8×104 in future DT experiments is estimated. The performance of the MPRu is exemplified with results from recent D plasma operations at JET, concerning both measurements with Ohmic, ion cyclotron resonance (ICRH) and neutral beam injection (NBI) plasma heating, as well as measurements of tritium burn-up neutrons. The upgraded instrument allows for 2.5-MeV neutron emission and deuterium ion temperature measurements in plasmas with low levels of tritium, a feature necessary for the ITER experiment.
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| 43. |
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| 44. |
|
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| 45. |
- Angioni, C., et al.
(författare)
-
Gyrokinetic study of turbulent convection of heavy impurities in tokamak plasmas at comparable ion and electron heat fluxes
- 2017
-
Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 57:2
-
Tidskriftsartikel (refereegranskat)abstract
- In tokamaks, the role of turbulent transport of heavy impurities, relative to that of neoclassical transport, increases with increasing size of the plasma, as clarified by means of general scalings, which use the ITER standard scenario parameters as reference, and by actual results from a selection of discharges from ASDEX Upgrade and JET. This motivates the theoretical investigation of the properties of the turbulent convection of heavy impurities by nonlinear gyrokinetic simulations in the experimentally relevant conditions of comparable ion and electron heat fluxes. These conditions also correspond to an intermediate regime between dominant ion temperature gradient turbulence and trapped electron mode turbulence. At moderate plasma toroidal rotation, the turbulent convection of heavy impurities, computed with nonlinear gyrokinetic simulations, is found to be directed outward, in contrast to that obtained by quasi-linear calculations based on the most unstable linear mode, which is directed inward. In this mixed turbulence regime, with comparable electron and ion heat fluxes, the nonlinear results of the impurity transport can be explained by the coexistence of both ion temperature gradient and trapped electron modes in the turbulent state, both contributing to the turbulent convection and diffusion of the impurity. The impact of toroidal rotation on the turbulent convection is also clarified.
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| 46. |
- Angioni, C., et al.
(författare)
-
The impact of poloidal asymmetries on tungsten transport in the core of JET H-mode plasmas
- 2015
-
Ingår i: Physics of Plasmas. - : AMER INST PHYSICS. - 1070-664X .- 1089-7674. ; 22:5
-
Tidskriftsartikel (refereegranskat)abstract
- Recent progress in the understanding and prediction of the tungsten behaviour in the core of JET H-mode plasmas with ITER-like wall is presented. Particular emphasis is given to the impact of poloidal asymmetries of the impurity density. In particular, it is shown that the predicted reduction of temperature screening induced by the presence of low field side localization of the tungsten density produced by the centrifugal force is consistent with the observed tungsten behaviour in a JET discharge in H-mode baseline scenario. This provides first evidence of the role of poloidal asymmetries in reducing the strength of temperature screening. The main differences between plasma parameters in JET baseline and hybrid scenario discharges which affect the impact of poloidally asymmetric density on the tungsten radial transport are identified. This allows the conditions by which tungsten accumulation can be avoided to be more precisely defined.
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| 47. |
- Appel, L. C., et al.
(författare)
-
Equilibrium reconstruction in an iron core tokamak using a deterministic magnetisation model
- 2018
-
Ingår i: Computer Physics Communications. - : ELSEVIER. - 0010-4655 .- 1879-2944. ; 223, s. 1-17
-
Tidskriftsartikel (refereegranskat)abstract
- In many tokamaks ferromagnetic material, usually referred to as an iron-core, is present in order to improve the magnetic coupling between the solenoid and the plasma. The presence of the iron core in proximity to the plasma changes the magnetic topology with consequent effects on the magnetic field structure and the plasma boundary. This paper considers the problem of obtaining the free-boundary plasma equilibrium solution in the presence of ferromagnetic material based on measured constraints. The current approach employs, a model described by O'Brien et al. (1992) in which the magnetisation currents at the iron-air boundary are represented by a set of free parameters and appropriate boundary conditions are enforced via a set of quasi-measurements on the material boundary. This can lead to the possibility of overfitting the data and hiding underlying issues with the measured signals. Although the model typically achieves good fits to measured magnetic signals there are significant discrepancies in the inferred magnetic topology compared with other plasma diagnostic measurements that are independent of the magnetic field. An alternative approach for equilibrium reconstruction in iron-core tokamaks, termed the deterministic magnetisation model is developed and implemented in EFIT++. The iron is represented by a boundary current with the gradients in the magnetisation dipole state generating macroscopic internal magnetisation currents. A model for the boundary magnetisation currents at the iron-air interface is developed using B-Splines enabling continuity to arbitrary order; internal magnetisation currents are allocated to triangulated regions within the iron, and a method to enable adaptive refinement is implemented. The deterministic model has been validated by comparing it with a synthetic 2-D electromagnetic model of JET. It is established that the maximum field discrepancy is less than 1.5 mT throughout the vacuum region enclosing the plasma. The discrepancies of simulated magnetic probe signals are accurate to within 1% for signals with absolute magnitude greater than 100 mT; in all other cases agreement is to within 1 mT. The effect of neglecting the internal magnetisation currents increases the maximum discrepancy in the vacuum region to >20 mT, resulting in errors of 5%-10% in the simulated probe signals. The fact that the previous model neglects the internal magnetisation currents (and also has additional free parameters when fitting the measured data) makes it unsuitable for analysing data in the absence of plasma current. The discrepancy of the poloidal magnetic flux within the vacuum vessel is to within 0.1 Wb. Finally the deterministic model is applied to an equilibrium force-balance solution of a JET discharge using experimental data. It is shown that the discrepancies of the outboard separatrix position, and the outer strike-point position inferred from Thomson Scattering and Infrared camera data are much improved beyond the routine equilibrium reconstruction, whereas the discrepancy of the inner strike-point position is similar.
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| 48. |
- Arnichand, H., et al.
(författare)
-
Discriminating the trapped electron modes contribution in density fluctuation spectra
- 2015
-
Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 55:9
-
Tidskriftsartikel (refereegranskat)abstract
- Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently established between QC modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion 54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement of QC modes amplitude is observed in a region where TEM cause impurity transport and turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC) regimes. Evidencing TEM activity then becomes experimentally possible via analysis of fluctuation spectra.
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| 49. |
- Aslanyan, V, et al.
(författare)
-
Gyrokinetic simulations of toroidal Alfven eigenmodes excited by energetic ions and external antennas on the Joint European Torus
- 2019
-
Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 59:2
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Tidskriftsartikel (refereegranskat)abstract
- The gyrokinetic toroidal code (GTC) has been used to study toroidal Alfven eigenmodes (TAEs) in high-performance plasmas. Experiments performed at the Joint European Torus (JET), where TAEs were driven by energetic particles arising from neutral beams, ion cyclotron resonant heating, and resonantly excited by dedicated external antennas, have been simulated. Modes driven by populations of energetic particles are observed, matching the TAE frequency seen with magnetic probes in JET experiments. A synthetic antenna, composed of one toroidal and two neighboring poloidal harmonics has been used to probe the modes' damping rates and quantify mechanisms for this damping in GTC simulations. This method was also applied to frequency and damping rate measurements of stable TAEs made by the Alfven eigenmode active diagnostic in these discharges.
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- Augustinsson, Sören, et al.
(författare)
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Making sense of assignment : on the complexity of being a school leader
- 2018
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Ingår i: Nordic Journal of Comparative and International Education. - Oslo : OsloMet University Library. - 2535-4051. ; 2:2-3, s. 149-164
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Tidskriftsartikel (refereegranskat)abstract
- The primary aim of this paper is to narrow down the description of how school leaders interpret the assignment (the task) and identify the markers for how they look upon the conditions of doing a good job in Sweden. The aim is in the context of practice-based and process-oriented research. We use complexity and complexity theories to frame the emerging practice of leading and organizing. This is in contrast to technocratic homogenization—that is, law texts, steering documents, documentation, standardized methods, planning, and ceremonies. A questionnaire was conducted with three open questions (n=363 out of a possible 548 participants) and four focus groups (n=21). Complexity, dilemmas, and inconsistency emerge in the respondents’ answers the closer they are to everyday action. The results show that complexity theories put focus on a conflict between the image of schools as complicated and complex. Complicated is accompanied by generalizing and weak contextualizing of control systems, standardized methods, planning, law texts, and evidence-based education—that is, the concept of technocratic homogenization. Complexity theories emphasize the life in organizations, everyday practice as leaders, and a conflict between weak and robust contextualizing from the perspective as practice-based and process-oriented research.
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