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

Abstract

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.

Interaction between runaway electrons and internal kink in a post-disruption plasma

“…Understanding the RE beam behavior in a post-disruption plasma is therefore critical to ensure machine safety in future reactor-scale tokamak devices [2]. In particular, how to properly control and dissipate a well established RE beam is one of the key research areas addressed during recent years [3][4][5][6][7][8], alongside studies of RE avoidance [9][10][11][12][13][14][15]. This work offers a toroidal modeling effort to investigate the behavior of a well formed steady state RE beam.…”

“…There has been strong experimental evidence of sawtoothlike relaxation in a post thermal quench (TQ) RE beam [8], suggesting that the safety factor q in the plasma core region drops below 1. Toroidal modeling of plasma equilibrium evolution during a disruption for ITER also found cases where the on-axis safety factor q 0 drops below 1 [23].…”

Understanding how minority relativistic electron populations may dominate charge state balance and radiative cooling of a post-thermal quench tokamak plasma

“…From Lvovskiy et al, 72 a recent indirect measurement of an experimentally produced runaway EEDF is reported for a RE beam, following the purge of higher Z impurities. In that study, the EEDF was obtained via inverting a measured hard x-ray spectra.…”

“…IV A, a comparison was made between potential analytic EEDF forms and some experimental results of a reconstructed RE spectra. From the efforts of the community to understand RE distributions from experimental studies, 10,72 as well as theoretical and computational studies, 7,32,73,[78][79][80] we know there are many different forms and shapes that a RE tail distribution may take. The shape of a RE EEDF will be heavily influenced by electric fields experienced, collisionality (with both electrons and impurity ions), and/or radiation such as synchrotron or Bremsstrahlung.…”

Toroidal modeling of runaway avalanche in DIII-D discharges

“…The study mostly focuses on three aspects: the runaway seed generation [3][4][5], the RE avalanche multiplication [6][7][8][9] and the resulting runaway current evolution [10][11][12][13], and the runaway avoidance and mitigation [14][15][16][17][18][19][20][21]. Besides, the interaction between RE and magneto-hydrodynamic (MHD) instabilities has also generated a significant amount of interest during recent years [22][23][24][25].…”