Understanding the electromagnetic topology during the ohmic breakdown in tokamaks considering self-generated electric fields

Abstract

The physical mechanisms of the ohmic breakdown in a tokamak have been understood based on the classical Townsend avalanche theory. However, a new systematic theory [Yoo, Nat. Commun. \textbf{9} 3523 (2018)] recently demonstrated that electron avalanches during the ohmic breakdown are completely different from the Townsend avalanche due to strong self-generated electric fields. In this study, we elucidate the multi-dimensional effects of the self-generated electric field on plasma dynamics during the ohmic breakdown. We also propose a novel electromagnetic topology analysis method that can easily predict the overall plasma behavior and where the main plasma is generated. The topology analysis method is validated by a state-of-art particle simulation for various magnetic configurations. New physical insights into the complex electromagnetic topology would facilitate designing more reliable and optimized ohmic breakdown scenarios in future tokamaks, such as ITER and beyond.

Modelling of ohmic startup and runaway electron formation in support of JT-60SA initial operation

“…The analyses of the breakdown time in NSTX [52] and KSTAR [87] have shown that the Townsend model of equation is insufficient for reproducing the timescale of experimental I p and n e rises, and a mechanism for shielding applied electric fields is required. Numerical studies using PIC simulations [87][88][89][90] have proposed that when ionisation proceeds, self-consistent electric fields formed by the charge separation of electrons and ions shield the applied electric field and retard the breakdown. A simple expression for estimating the self-consistent toroidal electric field that shields the external drive is [87]:…”