Experimental evidence of runaway electron tail generation via localized helical structure in pellet-triggered tokamak disruptions

Abstract

A novel detector, using stacked BGO crystals, is developed for runaway electron (REs) studies in the DIII-D tokamak. It is able to resolve fast dynamics of high-energy tail formation of REs with an ultra-high time resolution of ∼1 μs. As a cost, the detector estimates the ‘effective’ energy of a given shape of γ-ray spectra and sacrifices the energy resolution. In aid of the new measurement capability, a rapid, inhomogeneous growth of RE tail is observed in detail during a major disruption triggered by an argon pellet. It is found that both the population and energy of a well-confined RE tail significantly oscillate at the early period of the growth. The oscillation phase is locked to a slow rotating magnetohydrodynamic instability, which is briefly destabilized for only ∼1 ms at the early period of the current quench. The oscillation ceases promptly, when the mode disappears. The data suggests that the high-energy RE tail is well-confined and accelerated via a localized helical structure in the plasma core.

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…Once again, the key for the control is to detect the approach to vertical instability and to modify the plasma shape to maintain stability control. Finally, a novel technique for healing locked mode flux surfaces with 3D fields demonstrated promise for providing current quench (CQ) control by allowing the plasma to partially reheat and thereby extend the CQ to a long timescale [7]. In this technique the stochastic field line state at the start of the thermal quench is healed by the applied 3D fields into a 3D helical equilibrium with intact flux surfaces, allowing the plasma temperature to partially recover and substantially extending the quench time to final loss of current.…”