| 1. |
- Dubois, Timothy, 1982, et al.
(författare)
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Origins of plateau formation in ion energy spectra under target normal sheath acceleration
- 2017
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 24:12, s. 123114-
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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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| 2. |
- Embréus, Ola, 1991, et al.
(författare)
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Numerical calculation of ion runaway distributions
- 2015
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:5, s. 052122-
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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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| 3. |
- Ferri, Julien, 1990, et al.
(författare)
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Proton acceleration by a pair of successive ultraintense femtosecond laser pulses
- 2018
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 25
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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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| 4. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Alfvénic instabilities driven by runaways in fusion plasmas
- 2014
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 21:8, s. 080702-
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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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| 5. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas
- 2011
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Ingår i: Physics of Plasmas. - 1089-7674 .- 1070-664X. ; 18:3, s. 030703-
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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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| 6. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Impurity transport driven by ion temperature gradient turbulence in tokamak plasmas
- 2010
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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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| 7. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Magnetic field threshold for runaway generation in tokamak disruptions
- 2009
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 16:2, s. 022502-
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Tidskriftsartikel (refereegranskat)abstract
- Experimental observations show that there is a magnetic field threshold for runaway electrongeneration in tokamak disruptions. In this work, two possible reasons for this threshold are studied.The first possible explanation for these observations is that the runaway beam excites whistler wavesthat scatter the electrons in velocity space prevents the beam from growing. The growth rates of themost unstable whistler waves are inversely proportional to the magnetic field strength. Taking intoaccount the collisional and convective damping of the waves it is possible to derive a magnetic fieldthreshold below which no runaways are expected. The second possible explanation is the magneticfield dependence of the criterion for substantial runaway production obtained by calculating howmany runaway electrons can be produced before the induced toroidal electric field diffuses out of theplasma. It is shown, that even in rapidly cooling plasmas, where hot-tail generation is expected togive rise to substantial runaway population, the whistler waves can stop the runaway formationbelow a certain magnetic field unless the postdisruption temperature is very low.
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| 8. |
- Fülöp, Tünde, 1970, et al.
(författare)
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Turbulent and neoclassical impurity transport in tokamak plasmas
- 2009
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Ingår i: Physics of Plasmas. - 1089-7674 .- 1070-664X. ; 16:3, s. 032306-
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Tidskriftsartikel (refereegranskat)abstract
- Impurity particle transport in tokamaks is studied using an electrostatic fluid model for main ion andimpurity temperature gradient (ITG)͒ mode and trapped electron (TE͒) mode turbulence in thecollisionless limit and neoclassical theory. The impurity flux and impurity density peaking factorobtained from a self-consistent treatment of impurity transport are compared and contrasted with theresults of the often used trace impurity approximation. Comparisons between trace andself-consistent turbulent impurity transport are performed for ITER-like profiles. It is shown that forsmall impurity concentrations the trace impurity limit is adequate if the plasma is dominated by ITGturbulence. However, in case of TE mode dominated plasmas the contribution from impurity modesmay be significant, and therefore a self-consistent treatment may be needed.
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| 9. |
- Hollmann, E M, et al.
(författare)
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Status of research toward the ITER disruption mitigation system
- 2015
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:2, s. 021802-
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Tidskriftsartikel (refereegranskat)abstract
- An overview of the present status of research toward the final design of the ITER disruption mitigation system (DMS) is given. The ITER DMS is based on massive injection of impurities, in order to radiate the plasma stored energy and mitigate the potentially damaging effects of disruptions. The design of this system will be extremely challenging due to many physics and engineering constraints such as limitations on port access and the amount and species of injected impurities. Additionally, many physics questions relevant to the design of the ITER disruption mitigation system remain unsolved such as the mechanisms for mixing and assimilation of injected impurities during the rapid shutdown and the mechanisms for the subsequent formation and dissipation of runaway electron current.
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| 10. |
- Kómár, A, et al.
(författare)
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Electromagnetic waves destabilized by runaway electrons in near-critical electric fields
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 20:1, s. 012117-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway electron distributions are strongly anisotropic in velocity space. This anisotropy is a source of free energy that may destabilize electromagnetic waves through a resonant interaction between the waves and the energetic electrons. In this work, we investigate the high-frequency electromagnetic waves that are destabilized by runaway electron beams when the electric field is close to the critical field for runaway acceleration. Using a runaway electron distribution appropriate for the near-critical case, we calculate the linear instability growth rate of these waves and conclude that the obliquely propagating whistler waves are most unstable. We show that the frequencies, wave numbers, and propagation angles of the most unstable waves depend strongly on the magnetic field. Taking into account collisional and convective damping of the waves, we determine the number density of runaways that is required to destabilize the waves and show its parametric dependences.
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