The role of kinetic instabilities in formation of the runaway electron current after argon injection in DIII-D

Lvovskiy, A.; Paz-Soldan, C.; Eidietis, N. W.; Molin, A. Dal; Du, Xiaodi; Giacomelli, L.; Herfindal, J. L.; Hollmann, E. M.; Martinelli, L.; Moyer, R. A.; Nocente, M.; Rigamonti, D.; Shiraki, D.; Tardocchi, M.; Thome, K. E.

doi:10.1088/1361-6587/aae95a

Cited by 42 publications

(59 citation statements)

References 33 publications

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“…Excitation of whistler and Alfvénic waves by REs, recently observed in the series of experiments on DIII-D [21,[80][81][82], is usually diagnosed by clear magnetic fluctuations in the MHz range. Despite the high-frequency (digitized at 200 MHz) magnetic diagnostic is enabled in the present experiment, such magnetic fluctuations are not found.…”

Section: Candidate Instabilitymentioning

confidence: 96%

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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Section: Candidate Instabilitymentioning

confidence: 96%

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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“…Previous experimental and theoretical work proposed a role for MHD activity in preventing the RE seed amplification [24,9] and aimed to explain RE formation dependencies on plasma shape and magnetic field. Recent studies propose a novel candidate mechanism whereby kinetic instabilities excited by the RE beam can lead to enhanced RE spatial transport that can inhibit RE seed survival into the plateau [22]. This work employs a unique combination of high- frequency magnetic probes [25] to observe the kinetic instability and hard X-ray (HXR) spectrometers [26] to diagnose the RE properties that excite instabilities.…”

Section: Kinetic Instability and Re Seed Survivalmentioning

confidence: 99%

“…(b)The discharge that does not form the plateau exhibits intense and long-lasting instability activity in the MHz range, while (c) the discharge that does form a RE plateau has shorter and less intense mode activity. Modes appear above a critical RE energy (E RE ) (green) of 2.5 MeV, and are correlated with RE loss (white) as measured by a distant HXR detector[22].…”

mentioning

confidence: 94%

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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“…Observations at DIII-D indicate that when the power in the instabilities exceeds a threshold, runaway plateau formation is absent (Lvovskiy et al. 2018). Perturbations imposed by external magnetic coils have also been shown to suppress the formation of runaway beams in several tokamaks (Yoshino & Tokuda 2000; Lehnen et al.…”

Section: Introductionmentioning

confidence: 99%

Effects of magnetic perturbations and radiation on the runaway avalanche

et al. 2021

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The electron runaway phenomenon in plasmas depends sensitively on the momentum- space dynamics. However, efficient simulation of the global evolution of systems involving runaway electrons typically requires a reduced fluid description. This is needed, for example, in the design of essential runaway mitigation methods for tokamaks. In this paper, we present a method to include the effect of momentum-dependent spatial transport in the runaway avalanche growth rate. We quantify the reduction of the growth rate in the presence of electron diffusion in stochastic magnetic fields and show that the spatial transport can raise the effective critical electric field. Using a perturbative approach, we derive a set of equations that allows treatment of the effect of spatial transport on runaway dynamics in the presence of radial variation in plasma parameters. This is then used to demonstrate the effect of spatial transport in current quench simulations for ITER-like plasmas with massive material injection. We find that in scenarios with sufficiently slow current quench, owing to moderate impurity and deuterium injection, the presence of magnetic perturbations reduces the final runaway current considerably. Perturbations localised at the edge are not effective in suppressing the runaways, unless the runaway generation is off-axis, in which case they may lead to formation of strong current sheets at the interface of the confined and perturbed regions.

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The role of kinetic instabilities in formation of the runaway electron current after argon injection in DIII-D

Cited by 42 publications

References 33 publications

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

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

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

Effects of magnetic perturbations and radiation on the runaway avalanche

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