| 1. |
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| 2. |
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| 3. |
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| 4. |
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| 5. |
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| 6. |
- Appel, L.C., et al.
(författare)
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Observation of CAEs on MAST
- 2004
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Ingår i: In Proc. of 31st EPS Conf. of Plasma Physics, London, 2004, ECA. ; 28B, s. 4.195-
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Konferensbidrag (refereegranskat)
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| 7. |
- Berger, Esmée, 1998, et al.
(författare)
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Runaway dynamics in reactor-scale spherical tokamak disruptions
- 2022
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 88:6
-
Tidskriftsartikel (refereegranskat)abstract
- Understanding generation and mitigation of runaway electrons in disruptions is important for the safe operation of future tokamaks. In this paper we investigate the runaway dynamics in reactor-scale spherical tokamaks, focusing on a compact nominal design with a plasma current of 21 megaamperes (MA), 1.8 T magnetic field on axis and major radius of approximately 3 m. We study both the severity of runaway generation during unmitigated disruptions, and the effect that typical mitigation schemes based on massive material injection have on runaway production. The study is conducted using the numerical framework DREAM (Disruption Runaway Electron Analysis Model). We find that, in many cases, mitigation strategies are necessary to prevent the runaway current from reaching multi-MA levels. Our results indicate that, with a suitably chosen deuterium–neon mixture for mitigation, it is possible to achieve a tolerable runaway current and ohmic current evolution. However, this does not account for the runaway source due to wall activation, which has been found to severely limit successful mitigation at conventional aspect ratios, but whose definition requires a more complete wall specification. Furthermore, the majority of the thermal energy loss is found to happen through radial transport rather than radiation, which poses a risk of unacceptable localised heat loads.
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| 8. |
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| 9. |
- Cantono, Giada, et al.
(författare)
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Laser-driven proton acceleration from ultrathin foils with nanoholes
- 2021
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- Structured solid targets are widely investigated to increase the energy absorption of high-power laser pulses so as to achieve efficient ion acceleration. Here we report the first experimental study of the maximum energy of proton beams accelerated from sub-micrometric foils perforated with holes of nanometric size. By showing the lack of energy enhancement in comparison to standard flat foils, our results suggest that the high contrast routinely achieved with a double plasma mirror does not prevent damaging of the nanostructures prior to the main interaction. Particle-in-cell simulations support that even a short scale length plasma, formed in the last hundreds of femtoseconds before the peak of an ultrashort laser pulse, fills the holes and hinders enhanced electron heating. Our findings reinforce the need for improved laser contrast, as well as for accurate control and diagnostics of on-target plasma formation.
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| 10. |
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| 11. |
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| 12. |
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| 13. |
- Creely, A. J., et al.
(författare)
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Overview of the SPARC tokamak
- 2020
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:5
-
Tidskriftsartikel (refereegranskat)abstract
- The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field (B-0 = 12.2 T), compact (R-0 = 1.85 m, a = 0.57 m), superconducting, D-T tokamak with the goal of producing fusion gain Q > 2 from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of Q > 2 is achievable with conservative physics assumptions (H-98,H- y2 = 0.7) and, with the nominal assumption of H-98,H- y2 = 1, SPARC is projected to attain Q approximate to 11 and P-fusion approximate to 140 MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density (< n(e)> approximate to 3 x 10(20) m(-3)), high temperature (< Te > approximate to 7 keV) and high power density (P-fusion/V-plasma approximate to 7 MWm(-3)) relevant to fusion power plants. SPARC's place in the path to commercial fusion energy, its parameters and the current status of SPARC design work are presented. This work also describes the basis for global performance projections and summarizes some of the physics analysis that is presented in greater detail in the companion articles of this collection.
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| 14. |
- Decker, Joan, 1977, et al.
(författare)
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Numerical characterization of bump formation in the runaway electron tail
- 2016
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 58:2, s. 025016-
-
Tidskriftsartikel (refereegranskat)abstract
- Runaway electrons are generated in a magnetized plasma when the parallel electric field exceeds a critical value. For such electrons with energies typically reaching tens of MeV, the Abraham–Lorentz–Dirac (ALD) radiation force, in reaction to the synchrotron emission, is significant and can be the dominant process limiting electron acceleration. The effect of the ALD force on runaway electron dynamics in a homogeneous plasma is investigated using the relativistic finite-difference Fokker–Planck codes LUKE (Decker and Peysson 2004 Report EUR-CEA-FC-1736, Euratom-CEA), and CODE (Landreman et al 2014 Comput. Phys. Commun. 185 847). The time evolution of the distribution function is analyzed as a function of the relevant parameters: parallel electric field, background magnetic field, and effective charge. Under the action of the ALD force, we find that runaway electrons are subject to an energy limit, and that the electron distribution evolves towards a steady-state. In addition, a bump is formed in the tail of the electron distribution function if the electric field is sufficiently strong. The mechanisms leading to the bump formation and energy limit involve both the parallel and perpendicular momentum dynamics; they are described in detail. An estimate for the bump location in momentum space is derived. We observe that the energy of runaway electrons in the bump increases with the electric field amplitude, while the population increases with the bulk electron temperature. The presence of the bump divides the electron distribution into a runaway beam and a bulk population. This mechanism may give rise to beam-plasma types of instabilities that could, in turn, pump energy from runaway electrons and alter their confinement.
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| 15. |
- Dubois, Timothy, 1982, et al.
(författare)
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Origins of plateau formation in ion energy spectra under target normal sheath acceleration
- 2017
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 24:12, s. 123114-
-
Tidskriftsartikel (refereegranskat)abstract
- Target normal sheath acceleration (TNSA) is a method employed in laser–matter interaction experiments to accelerate light ions (usually protons). Laser setups with durations of a few 10 fs and relatively low intensity contrasts observe plateau regions in their ion energy spectra when shooting on thin foil targets with thicknesses of the order of 10 μm. In this paper, we identify a mechanism which explains this phenomenon using one-dimensional particle-in-cell simulations. Fast electrons generated from the laser interaction recirculate back and forth through the target, giving rise to time-oscillating charge and current densities at the target backside. Periodic decreases in the electron density lead to transient disruptions of the TNSA sheath field: peaks in the ion spectra form as a result, which are then spread in energy from a modified potential driven by further electron recirculation. The ratio between the laser pulse duration and the recirculation period (dependent on the target thickness, including the portion of the pre-plasma which is denser than the critical density) determines if a plateau forms in the energy spectra.
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| 16. |
- Ekmark, Ida, 1998, et al.
(författare)
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Bayesian optimization of disruption scenarios with fluid-kinetic models
- 2023
-
Ingår i: 49th EPS Conference on Plasma Physics, EPS 2023. - : European Physical Society (EPS).
-
Konferensbidrag (refereegranskat)abstract
- Tokamak disruptions can damage the machine due to localized heat loads, mechanical stresses and impact of energetic runaway electron beams. We use a Bayesian optimization framework to optimize massive material injection of deuterium and neon in an ITER-like tokamak set up. The optimization is performed using both fluid and kinetic plasma models. The fluid model allows the exploration of a large parameter space. Once promising parameter regions are located, these are studied in higher physics fidelity using kinetic simulations. The kinetic model predicts more optimistic results regarding the success of the disruption mitigation.
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| 17. |
- Embréus, Ola, 1991, et al.
(författare)
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Conservative large-angle collision operator for runaway avalanches
- 2015
-
Ingår i: 57th Annual Meeting of the APS Division of Plasma Physics. ; 60:19, s. PP12.00107-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Avalanche runaway generation is the phenomenon whereby runaway electrons (REs) are generated due to large-angle collisions of thermal electrons with existing REs, leading to an exponential growth of the runaway current. These large-angle collisions are not described by the Fokker-Planck operator commonly employed to model collisions in plasmas, and have previously been accounted for by the addition of a particle source term in the kinetic equation [M. Rosenbluth et al., 1997, Nucl. Fusion 37, 1355; S. C. Chiu et al. 1998, Nucl. Fusion 38, 1711]. In this contribution we describe a new large-angle collision operator, derived as the high-energy limit of the linearized relativistic Boltzmann collision integral. This operator generalizes previous models of large-angle collisions to account for the full momentum dependence of the primary distribution and conserves particle number, momentum and energy, while also avoiding double counting of small- and large-angle collisions. The new operator is implemented in the 2D Fokker-Planck solver CODE [M. Landreman et al. 2014, Comp. Phys. Comm. 185, 847], with which we investigate its effect on the evolution of the runaway distribution.
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| 18. |
- Embréus, Ola, 1991, et al.
(författare)
-
Dynamics of positrons during relativistic electron runaway
- 2018
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:5, s. 905840506-
-
Tidskriftsartikel (refereegranskat)abstract
- Sufficiently strong electric fields in plasmas can accelerate charged particles to relativistic energies. In this paper we describe the dynamics of positrons accelerated in such electric fields, and calculate the fraction of created positrons that become runaway accelerated, along with the amount of radiation that they emit. We derive an analytical formula that shows the relative importance of the different positron production processes, and show that, above a certain threshold electric field, the pair production by photons is lower than that by collisions. We furthermore present analytical and numerical solutions to the positron kinetic equation; these are applied to calculate the fraction of positrons that become accelerated or thermalized, which enters into rate equations that describe the evolution of the density of the slow and fast positron populations. Finally, to indicate operational parameters required for positron detection during runaway in tokamak discharges, we give expressions for the parameter dependencies of detected annihilation radiation compared to bremsstrahlung detected at an angle perpendicular to the direction of runaway acceleration. Using the full leading-order pair-production cross-section, we demonstrate that previous related work has overestimated the collisional pair production by at least a factor of four.
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| 19. |
- Embréus, Ola, 1991, et al.
(författare)
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Effect of bremsstrahlung radiation emission on fast electrons in plasmas
- 2016
-
Ingår i: 43rd European Physical Society Conference on Plasma Physics, EPS 2016.
-
Konferensbidrag (refereegranskat)abstract
- Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this contribution we investigate the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, using a Boltzmanntransport model fully accounting for the emission of finite-energy photons. We implement the model in a 2D continuum kinetic-equation solver, and study the solutions to determine the effect of bremsstrahlung on the electron distribution function. We find that electrons acceleratedby electric fields can reach significantly higher energies than predicted in previous work,which considered only the average energy loss of a test particle. We demonstrate that significantfractions of electrons reach twice the expected energy or more, due to the difference betweenthe average and Boltzmann model of bremsstrahlung radiation losses. Furthermore, we show that the emission of low-energy photons, which have previously been neglected because theydo not contribute to net energy loss, can contribute significantly to the dynamics of electrons with an anisotropic distribution by enhancing the angular-deflection rate.
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| 20. |
- Embréus, Ola, 1991, et al.
(författare)
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Effect of bremsstrahlung radiation emission on fast electrons in plasmas
- 2016
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:9, s. 093023-
-
Tidskriftsartikel (refereegranskat)abstract
- Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this paper we investigate the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, fully accounting for the emission of finite-energy photons. We find that electrons accelerated by electric fields can reach significantly higher energies than what is expected from energy-loss considerations. Furthermore, we show that the emission of soft photons can contribute significantly to the dynamics of electrons with an anisotropic distribution.
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| 21. |
- Embréus, Ola, 1991, et al.
(författare)
-
Numerical calculation of ion runaway distributions
- 2015
-
Ingår i: 42nd European Physical Society Conference on Plasma Physics, EPS 2015.
-
Konferensbidrag (refereegranskat)abstract
- Numerical solver of the 2D ion Fokker-Planck equation has been presented, and its usefulness demonstrated in investigating the conditions required for ion runaway in cold and hot tokamak plasmas. It is shown that Alfvénic activity observed in disruption experiments are unlikely to be explained by the runaway mechanism alone.
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| 22. |
- Embréus, Ola, 1991, et al.
(författare)
-
Numerical calculation of ion runaway distributions
- 2015
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:5, s. 052122-
-
Tidskriftsartikel (refereegranskat)abstract
- Ions accelerated by electric fields (so-called runaway ions) in plasmas may explain observations in solar flares and fusion experiments; however, limitations of previous analytic work have prevented definite conclusions. In this work, we describe a numerical solver of the 2D non-relativistic linearized Fokker-Planck equation for ions. It solves the initial value problem in velocity space with a spectral-Eulerian discretization scheme, allowing arbitrary plasma composition and time-varying electric fields and background plasma parameters. The numerical ion distribution function is then used to consider the conditions for runaway ion acceleration in solar flares and tokamak plasmas. Typical time scales and electric fields required for ion acceleration are determined for various plasma compositions, ion species, and temperatures, and the potential for excitation of toroidal Alfvén eigenmodes during tokamak disruptions is considered.
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| 23. |
- Embréus, Ola, 1991, et al.
(författare)
-
On the relativistic large-angle electron collision operator for runaway avalanches in plasmas
- 2018
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 84:1, s. 905840102-
-
Tidskriftsartikel (refereegranskat)abstract
- Large-angle Coulomb collisions lead to an avalanching generation of runaway electrons in a plasma. We present the first fully conservative large-angle collision operator, derived from the relativistic Boltzmann operator. The relation to previous models for large-angle collisions is investigated, and their validity assessed. We present a form of the generalized collision operator which is suitable for implementation in a numerical kinetic-equation solver, and demonstrate the effect on the runaway-electron growth rate. Finally we consider the reverse avalanche effect, where runaways are slowed down by large-angle collisions, and show that the choice of operator is important if the electric field is close to the avalanche threshold.
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| 24. |
- Embréus, Ola, 1991, et al.
(författare)
-
Relativistic Boltzmann collision operator for runaway-avalanche studies
- 2017
-
Ingår i: International Sherwood Fusion Theory Conference, Annapolis, USA.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Avalanche runaway generation is a critical threat to large tokamaks such as ITER: according to the Rosenbluth-Putvinski theory[1], a small seed population of fast electrons will multiply through knock-on collisions by a factor of order R~exp(3 I[MA]), which for plasma currents of order I=10-15 MA means that a single fast electron can convert the entire ohmic current to runaway current.The exponential sensitivity of the avalanche multiplication factor R to the details of the runaway-generation dynamics shows a need for more accurate models of large-angle collisions.Existing models of large-angle collisions that have been used in magnetic-confinement fusion studies[1,2] have two main flaws: (i) they do not conserve electron momentum or energy, and (ii) they double count collisions with small-angle collisions, which are typically accounted for with a Fokker-Planck collision operator.We have developed a new large-angle collision operator based on the full relativistic Boltzmann equation which resolves these issues.We use kinetic simulations to show how the new improved collision operator modify the avalanche growth rates obtained with previous models. In particular, we compare results with the steady-state theory of Rosenbluth and Putvinski, as well as with theoretical predictions of a recent study[3] for avalanche generation in near-threshold electric fields E~E_c in the presence of synchrotron-radiation losses.References:[1] M. N. Rosenbluth and S. V. Putvinski, Nucl. Fusion 37, 1355 (1997).[2] S. C. Chiu, M. N. Rosenbluth, R. W. Harvey and V. S. Chan, Nucl. Fusion 38, 1711 (1998).[3] P. Aleynikov and B. N. Breizman, Phys. Rev. Lett. 114, 155001 (2015).
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| 25. |
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| 26. |
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| 27. |
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| 28. |
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| 29. |
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| 30. |
- Fehér, Tamás, 1984, et al.
(författare)
-
Simulation of runaway electron generation during plasma shutdown by impurity injection in ITER
- 2011
-
Ingår i: Plasma Physics and Controlled Fusion. - 1361-6587 .- 0741-3335. ; 53:3, s. 035014-
-
Tidskriftsartikel (refereegranskat)abstract
- Disruptions in a large tokamak can cause serious damage to the device and should be avoided or mitigated. Massive gas or killer pellet injection are possible ways to obtain a controlled fast plasma shutdown before a natural disruption occurs. In this work, plasma shutdown scenarios with different types of impurities are studied for an ITER-like plasma. Plasma cooling, runaway generation and the associated electric field diffusion are calculated with a 1D-code taking the Dreicer, hot-tail and avalanche runaway generation processes into account. Thin, radially localized sheets with high temperature can be created after the thermal quench, and the Dreicer and avalanche processes produce a high runaway current inside these sheets. At high impurity concentration the Dreicer process is suppressed but hot-tail runaways are created. Favorable thermal and current quench times can be achieved with a mixture of deuterium and neon or argon. However, to prevent the avalanche process from creating a significant runaway current fraction, it is found to be necessary to include runaway losses in the model.
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| 31. |
- Ferri, Julien, 1990, et al.
(författare)
-
Effects of oblique incidence and colliding pulses on laser-driven proton acceleration from relativistically transparent ultrathin targets
- 2020
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:5
-
Tidskriftsartikel (refereegranskat)abstract
- The use of ultrathin solid foils offers optimal conditions for accelerating protons to high energies from laser-matter interactions. When the target is thin enough that relativistic self-induced transparency sets in, all of the target electrons get heated to high energies by the laser, which maximizes the accelerating electric field and therefore the final ion energy. In this work, we first investigate how ion acceleration by ultraintense femtosecond laser pulses in transparent CH2 solid foils is modified when turning from normal to oblique (45 degrees) incidence. Due to stronger electron heating, we find that higher proton energies can be obtained at oblique incidence but in thinner optimum targets. We then show that proton acceleration can be further improved by splitting the laser pulse into two half-pulses focused at opposite incidence angles. An increase by similar to 30% in the maximum proton energy and by a factor of similar to 4 in the high-energy proton charge is reported compared to the reference case of a single normally incident pulse.
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| 32. |
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| 33. |
- Ferri, Julien, 1990, et al.
(författare)
-
Enhanced target normal sheath acceleration using colliding laser pulses
- 2019
-
Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 2
-
Tidskriftsartikel (refereegranskat)abstract
- Laser-solid interaction can lead to the acceleration of protons to tens of MeV. Here, we show that a strong enhancement of this acceleration can be achieved by splitting the laser pulse to two parts of equal energy and opposite incidence angles. Through the use of two- and three-dimensional Particle-In-Cell simulations, we find that the multi-pulse interaction leads to a standing wave pattern at the front side of the target, with an enhanced electric field and a substantial modification of the hot electron generation process. This in turn leads to significant improvement of the proton spectra, with an almost doubling of the accelerated proton energy and five-fold enhancement of the number of protons. The proposed scheme is robust with respect to incidence angles for the laser pulses, providing flexibility to the scheme, which should facilitate its experimental implementation.
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| 34. |
- Ferri, Julien, 1990, et al.
(författare)
-
Enhancement of laser-driven ion acceleration in non-periodic nanostructured targets
- 2020
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:1
-
Tidskriftsartikel (refereegranskat)abstract
- Using particle-in-cell simulations, we demonstrate an improvement of the target-normal-sheath acceleration (TNSA) of protons in non-periodically nanostructured targets with micron-scale thickness. Compared to standard flat foils, an increase in the proton cutoff energy by up to a factor of two is observed in foils coated with nanocones or perforated with nanoholes. The latter nano-perforated foils yield the highest enhancement, which we show to be robust over a broad range of foil thicknesses and hole diameters. The improvement of TNSA performance results from more efficient hot-electron generation, caused by a more complex laser-electron interaction geometry and increased effective interaction area and duration. We show that TNSA is optimized for a nanohole distribution of relatively low areal density and that is not required to be periodic, thus relaxing the manufacturing constraints.
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| 35. |
- Ferri, Julien, et al.
(författare)
-
Generation of attosecond electron bunches and x-ray pulses from few-cycle femtosecond laser pulses
- 2021
-
Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 63:4
-
Tidskriftsartikel (refereegranskat)abstract
- Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons accelerated in wakefields emit radiation due to betatron oscillations. Such x-ray sources inherit the features of the electron beam; sub-femtosecond electron bunches produce betatron sources of the same duration, which in turn allow probing matter on ultrashort time scales. In this paper we show, via Particle-in-Cell simulations, that attosecond electron bunches can be obtained using low-energy, ultra-short laser beams both in the self-injection and the controlled injection regimes at low plasma densities. However, only in the controlled regime does the electron injection lead to a stable, isolated attosecond electron bunch. Such ultrashort electron bunches are shown to emit attosecond x-ray bursts with high brilliance.
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| 36. |
- Ferri, Julien, 1990, et al.
(författare)
-
Proton acceleration by a pair of successive ultraintense femtosecond laser pulses
- 2018
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 25
-
Tidskriftsartikel (refereegranskat)abstract
- © 2018 Author(s). We investigate the target normal sheath acceleration of protons in thin aluminum targets irradiated at a relativistic intensity by two time-separated ultrashort (35 fs) laser pulses. When the full-energy laser pulse is temporally split into two identical half-energy pulses, and using target thicknesses of 3 and 6 μm, we observe experimentally that the second half-pulse boosts the maximum energy and charge of the proton beam produced by the first half-pulse for time delays below ∼0.6-1 ps. Using two-dimensional particle-in-cell simulations, we examine the variation of the proton energy spectra with respect to the time-delay between the two pulses. We demonstrate that the expansion of the target front surface caused by the first pulse significantly enhances the hot-electron generation by the second pulse arriving after a few hundreds of fs time delay. This enhancement, however, does not suffice to further accelerate the fastest protons driven by the first pulse once three-dimensional quenching effects have set in. This implies a limit to the maximum time delay that leads to proton energy enhancement, which we theoretically determine.
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| 37. |
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| 38. |
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| 39. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Alfvénic instabilities driven by runaways in fusion plasmas
- 2014
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:8, s. 080702-
-
Tidskriftsartikel (refereegranskat)abstract
- Runaway particles can be produced in plasmas with large electric fields. Here, we address the possibility that such runaway ions and electrons excite Alfvénic instabilities. The magnetic perturbation induced by these modes can enhance the loss of runaways. This may have important implications for the runaway electron beam formation in tokamak disruptions.
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| 40. |
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| 41. |
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| 42. |
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| 43. |
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| 44. |
- Fülöp, Tünde, 1970, et al.
(författare)
-
Effect of plasma elongation on current dynamics during tokamak disruptions
- 2020
-
Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:1
-
Tidskriftsartikel (refereegranskat)abstract
- Plasma terminating disruptions in tokamaks may result in relativistic runaway electron beams with potentially serious consequences for future devices with large plasma currents. In this paper, we investigate the effect of plasma elongation on the coupled dynamics of runaway generation and resistive diffusion of the electric field. We find that elongated plasmas are less likely to produce large runaway currents, partly due to the lower induced electric fields associated with larger plasmas, and partly due to direct shaping effects, which mainly lead to a reduction in the runaway avalanche gain. © Cambridge University Press 2020.
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| 45. |
- Fülöp, Tünde, 1970, et al.
(författare)
-
Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas
- 2011
-
Ingår i: Physics of Plasmas. - 1089-7674 .- 1070-664X. ; 18:3, s. 030703-
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that if the density of the impurity ions is poloidally asymmetric then the zero-flux impurity density gradient is significantly reduced and even a sign change in the impurity flux may occur if the asymmetry is sufficiently large. This effect is most effective in low shear plasmas with the impurity density peaking on the inboard side and may be a contributing factor to the observed outward convection of impurities in the presence of radio frequency heating.
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| 46. |
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| 47. |
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| 48. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Impurity transport driven by electrostatic turbulence in tokamak plasmas
- 2010
-
Ingår i: Proceedings of 23rd IAEA Fusion Energy Conference, Korea 2010.
-
Konferensbidrag (refereegranskat)abstract
- Impurity transport driven by electrostatic turbulence is analyzed in weakly-collisional tokamak plasmas using a semi-analytical model based on a boundary-layer solution of the gyrokinetic equation. Analytical expressions for the perturbed density responses are derived and used to calculate the stability boundaries, mode frequencies, growth rates and the quasilinear particle fluxes. Parametric dependencies of the above quantities with respect to impurity charge, effective charge, impurity density scale length, and collisionality, and the effect of the impurities on the stability boundaries, have been determined and compared with quasilinear gyrokinetic simulations with GYRO resulting in very good agreement. Ananalytical approximate expression of the zero-flux impurity density gradient is derived and used to discuss its parametric dependencies.
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| 49. |
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| 50. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Impurity transport driven by ion temperature gradient turbulence in tokamak plasmas
- 2010
-
Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 17:6, s. 062501-
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Tidskriftsartikel (refereegranskat)abstract
- Impurity transport driven by electrostatic turbulence is analyzed in weakly collisional tokamak plasmas using a semianalytical model based on a boundary layer solution of the gyrokinetic equation. Analytical expressions for the perturbed density responses are derived and used todetermine the stability boundaries and the quasilinear particle fluxes. For moderate impurity charge number Z, the stability boundaries are very weakly affected by the increasing impurity charge for constant effective charge, while for lower impurity charge the influence of impurities is larger, if the amount of impurities is not too small. Scalings of the mode frequencies and quasilinear fluxes withcharge number, effective charge, impurity density scale length, and collisionality are determined and compared to quasilinear gyrokinetic simulations with GYRO resulting in very good agreement. Collisions do not affect the mode frequencies, growth rates, and impurity fluxes significantly. The eigenfrequencies and growth ratesdepend only weakly on Z and Zeff but they are sensitive to the impurity density gradient scale length. An analytical approximate expression of the zero-flux impurity density gradient is derived and used to discuss its parametric dependencies.
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