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- Moradi, Sara, 1981, et al.
(författare)
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Charged particle dynamics in the presence of non-Gaussian Lévy electrostatic fluctuations
- 2016
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 23:9
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Tidskriftsartikel (refereegranskat)abstract
- Full orbit dynamics of charged particles in a 3-dimensional helical magnetic field in the presenceof a-stable Levy electrostatic fluctuations and linear friction modeling collisional Coulomb drag isstudied via Monte Carlo numerical simulations. The Levy fluctuations are introduced to model theeffect of non-local transport due to fractional diffusion in velocity space resulting from intermittentelectrostatic turbulence. The probability distribution functions of energy, particle displacements,and Larmor radii are computed and showed to exhibit a transition from exponential decay, in thecase of Gaussian fluctuations, to power law decay in the case of Levy fluctuations. The absolutevalue of the power law decay exponents is linearly proportional to the Levy index a. The observedanomalous non-Gaussian statistics of the particles’ Larmor radii (resulting from outlier transportevents) indicate that, when electrostatic turbulent fluctuations exhibit non-Gaussian Levy statistics,gyro-averaging and guiding centre approximations might face limitations and full particle orbiteffects should be taken into account.
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- Paz-Soldan, C., et al.
(författare)
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Recent DIII-D advances in runaway electron measurement and model validation
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:6
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Tidskriftsartikel (refereegranskat)abstract
- Novel measurements and modeling of runaway electron (RE) dynamics in DIII-D have resolved experimental discrepancies and validated predictions for ITER, improving confidence that RE avoidance and mitigation can be predictably achieved. Considering RE formation, first experimental assessments of the RE seed current demonstrates that present hot-tail theories are not yet accurate and require improved treatment of the pellet dynamics. Novel measurements of kinetic instabilities in the MHz-range have been made in the RE formation phase, with the intensity of these modes correlated with previously unexplained empirical thresholds for RE generation. Controlled RE dissipation experiments in quiescent regimes have validated RE distribution function dependencies on collisional and synchrotron damping, both in terms of distribution function shape and dissipation rates. Measurements of RE bremsstrahlung and synchrotron emission are now used in tandem to resolve energy and pitch-angle effects. A resolution to long-standing dissipation anomalies in the quiescent regime is offered by taking into account kinetic instability effects on RE phase-space dynamics. Kinetic instabilities in the 100-200 MHz range are directly observed, though modeling finds the largest dissipation arises from GHz range instabilities that are beyond the reach of existing diagnostics. Kinetic instabilities are also observed in the mature post-disruption RE plateau phase, so long as the collisional damping rate is reduced with low-Z injection. Experiments with high-Z injection find that the dissipation rate saturates with injection quantity, likely due to neutral diffusion rates being slower than vertical instability rates in DIII-D. Considering the final loss, a 0D model for first-wall Joule heating is found to be in agreement with experiment, and controlled access to RE equilibria with edge safety factor of two identifies novel dynamics brought about by large-scale kink instabilities. These dynamics are typified by fast (tens of microseconds) RE loss rates without RE beam regeneration. The above measurements and comparison with theory represent significant advances in the understanding of RE dynamics and indicate possible new opportunities for RE avoidance or mitigation via kinetic instabilities.
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