Runaway electron seed formation at reactor-relevant temperature

Abstract

Systematic variation of the pre-disruption core electron temperature (T e ) from 1 to 12 keV using an internal transport barrier scenario reveals a dramatic increase in the production of 'seed' runaway electrons (REs), ultimately accessing near-complete conversion of the pre-disruption current into sub-MeV RE current. Injected Ar pellets are observed to ablate more intensely and promptly as T e rises. At high T e , the observed ablation exceeds predictions from published thermal ablation models. Simultaneously, the thermal quench (TQ) is observed to significantly shorten with increasing T e -a surprising result. While the reason for the shorter TQ is not yet understood, candidate mechanisms include: insufficiently accurate thermal ablation models, enhanced ablation driven by the seed RE population, or significant parallel heat transport along stochastic fields. Kinetic modeling that self-consistently treats the plasma cooling via radiation, the induced electric field, and the formation of the seed RE is performed. Including the combined effect of the inherent dependence of hot-tail RE seeding on T e together with the shortened TQ, modeling recovers the progression towards near-complete conversion of the pre-disruption current to RE current as T e rises. Measurement of the HXR spectrum during the early current quench (CQ) reveals a trend of decreasing energy with pre-disruption T e . At the very highest T e (≈ 12 keV), ≈ 100% conversion of the thermal current to runaway current is found. The energy of this peculiar RE beam is inferred to be sub-MeV as it emits vanishingly few MeV hard x-rays (HXRs). These measurements demonstrate novel TQ dynamics as T e is varied and illustrate the limitations of treating the RE seed formation problem without considering the inter-related dependencies of the pellet ablation, radiative energy loss, and resultant variations of the TQ duration. If the observed shortening of the TQ with increasing T e extends to fusion-grade plasmas, than their propensity to form large quantities of RE seeds at high T e may be far worse than previously thought. Positively, the high T e scenario in DIII-D produces REs so prodigiously that it can serve as a meaningful new platform for demonstrating RE avoidance techniques.

Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D

“…The increased avalanche rate due to partial screening is not included here; 20 this correction is not thought to be large in these shots due to the low plasma current and low resulting loop voltage. 12 As discussed later, it is possible that Dreicer hot electron formation is not negligible during the CQ of these shots; this results in significant uncertainty in the post-TQ hot electron current, as indicated by the estimated error bars in Figs. 2(d) and 2(e).…”

“…Additional details of these discharges were provided in a previous study. 12 The main diagnostics used in this work are a fast-framing visible camera, electron cyclotron emission (ECE), and a soft x-ray (SXR) array. The visible camera is bandpass filtered to isolate Ar-I 695 nm emission with a 5 nm bandpass and is aimed to view the injected Ar pellet injection region to measure neutral argon ablation and estimate local argon deposition into the plasma.…”

“…The standard method for estimating post-TQ hot electron current in DIII-D disruptions is to ignore radial loss and new hot electron formation, in which case hot electron current evolution is, typically, dominated by avalanche gain. 11,12 In this case, the post-TQ hot electron current can be estimated from RE plateau current at the end of the CQ and from the avalanche growth rate; 14 in these two shots, this method gives post-TQ hot electron currents of about 300 kA (early shutdown) and 100 kA (late shutdown), as shown in Figs. 2(d) and 2(e).…”

“…10,11 Post-TQ hot electron current is clearly seen to increase with increasing central plasma temperature, supporting the picture that the TQ is a net creator of superthermal electrons, i.e., more hot electrons are created by the hot tail mechanism than lost by radial transport during the TQ in mid-sized tokamaks. 12 After the TQ, the resulting cold (T e % 5 eV) current quench (CQ) plasma is thought to heal stochastic regions rapidly (on a 0.1 ms timescale), resulting in greatly reduced radial loss of hot electrons during the CQ, although this belief is based entirely on simulations. 8,9 There is experimental evidence that MHD modes or turbulence can cause loss of REs during the CQ, 13 but this loss term has not been quantified and is typically ignored.…”

Electron runaway in ASDEX Upgrade experiments of varying core temperature

“…Throughout the simulations performed, the preinjection on-axis electron temperature is varied between 4 and 20 keV, as SPI experiments in DIII-D suggest a growing seed runaway population as the electron temperature increases (Paz-Soldan et al. 2020). The simulation results obtained are compared against measurements of AUG disruption experiments.…”