Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D

Shiraki, D.; Commaux, N.; Baylor, L. R.; Cooper, C. M.; Eidietis, N. W.; Hollmann, E. M.; Paz-Soldan, C.; Combs, S. K.; Meitner, S. J.

doi:10.1088/1741-4326/aab0d6

Cited by 55 publications

(68 citation statements)

References 26 publications

(36 reference statements)

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“…As first described in Ref. [64], D 2 injection is found to rapidly reduce the bulk n e by at 1-2 orders of magnitude, often to measurement zero. The mechanism for this process is not understood.…”

Section: Kinetic Instabilities In the Re Plateausupporting

confidence: 51%

Recent DIII-D advances in runaway electron measurement and model validation

Paz-Soldan¹,

Eidietis²,

Hollmann³

et al. 2019

Nucl. Fusion

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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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Section: Kinetic Instabilities In the Re Plateausupporting

confidence: 51%

Recent DIII-D advances in runaway electron measurement and model validation

Paz-Soldan¹,

Eidietis²,

Hollmann³

et al. 2019

Nucl. Fusion

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“…[289]) that is likely caused by long transport times to bring the impurities into the background plasma of the runaway beam. First attempts to inject impurities with SPI into a runaway beam confirm the expectation that also the ice fragments are not penetrating the beam since similar dissipation rates were achieved as for MGI [237].…”

Section: Runaway Electron Energy Dissipationmentioning

confidence: 57%

“…To-date there has been no clear evidence that the impurity injection method is important, i.e. either MGI or SPI (shattered pellet injection) of similar neon quantities have shown similar RE plateau current decay times, probably because cryogenic pellets are rapidly vaporized at the RE plateau edge and then behave like gas [237]. Successful assimilation of injected material and the resulting enhancement of the RE current dissipation has been observed in DIII-D [172] and Tore Supra [238].…”

Section: Lowmentioning

confidence: 99%

Physics of runaway electrons in tokamaks

et al. 2019

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Of all electrons, runaway electrons have long been recognized in the fusion community as a distinctive population. They now attract special attention as a part of ITER mission considerations. This review covers basic physics ingredients of the runaway phenomenon and the ongoing efforts (experimental and theoretical) aimed at runaway electron taming in the next generation tokamaks. We emphasize the prevailing physics themes of the last 20 years: the hot-tail mechanism of runaway production, runaway electron interaction with impurity ions, the role of synchrotron radiation in runaway kinetics, runaway electron transport in presence of magnetic fluctuations, micro-instabilities driven by runaway electrons in magnetized plasmas, and vertical stability of the plasma with runaway electrons. The review also discusses implications of the runaway phenomenon for ITER and the current strategy of runaway electron mitigation.

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“…This effect was firstly observed in Ref. [15], and though it is not yet understood, it is hypothesized to be due to complete recombination of the background plasma.…”

Section: Methodsmentioning

confidence: 78%

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

et al. 2020

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Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard X-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5-6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal MHD instability is accessed by ramping up the current. The instability leads to a sawtoothlike relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposed as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at ≈ 100 GHz, suggesting resonant absorption of the ECE at low frequencies.

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Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D

Cited by 55 publications

References 26 publications

Recent DIII-D advances in runaway electron measurement and model validation

Recent DIII-D advances in runaway electron measurement and model validation

Physics of runaway electrons in tokamaks

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

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