Investigation of ring-like runaway electron beams in the EAST tokamak

Abstract

In the EAST tokamak, asymmetrical ring-like runaway electron beams with energy more than 30 MeV and pitch angle about 0.1 were investigated. Those runaway beams carried about 46% of the plasma current and located around the q = 2 rational surface when m/n = 1/1 and m/n = 2/1 MHD modes existed in the plasma. Those runaway beams changed from a hollow to a filled structure during the slow oscillations in the discharge about every 0.2 s, which correlated with a large step-like jump in electron cyclotron emission (ECE) signals, a big spike-like perturbation in Mirnov signals and a large decrease in runaway energy. Between those slow oscillations with large magnitude, fast oscillations with small magnitude also existed about every 0.02 s, which correlated with a small step-like jump in ECE signals, a small spike-like perturbation in Mirnov signals, but no clear decrease in runaway energy and changes in the runaway beam structure. Resonant interactions occurred between runaway electrons and magnetohydrodynamic instabilities, which gave rise to fast pitch angle scattering processes of those resonant runaway electrons, and hence increased the synchrotron radiation. Theoretical calculations of the resonant interaction were given based on a test particle description. Synchrotron radiation of those resonant runaway electrons was increased by about 60% until the end of the resonant interaction.

Long-Lived Banana Orbit Formation of Suprathermal Electrons During MHD Spikes in Runaway Tokamak Discharges

“…In the present paper, it is shown the additional possibility of long-lived banana orbit formation of suprathermal electrons during MHD spikes in runaway tokamak discharges (EAST experiments [8][9][10]). This is the objective of this investigation.…”

“…In shot #28957 three types of events were observed with the stepwise jump increases of the non-thermal ECE signal that are described and discussed in details in Refs. [8][9][10]. Here, we give only a short summary of these papers results:…”

“…During EAST experiments with runaway electrons [8][9][10], peculiar events have been investigated: rapid changes in the synchrotron spot shape and its intensity that coincided with step-like jumps in the electron cyclotron emission (ECE) signal (cyclotron radiation of suprathermal electrons) and the MHD (m/n=2/1) Mirnov signal spikes (m and n are the poloidal and toroidal mode numbers, respectively). These phenomena were initially observed in TEXTOR (Tokamak Experiment for Technology Oriented Research), where these events only occurred in the current decay phase or in discharges with thin stable runaway beams at the q = 1 drift surface [11] (q is the safety factor).…”

“…The runaway ohmic discharge #28957 in EAST was performed in the limiter configuration with the toroidal magnetic field B0 = 2 T, the plasma current I p = 250 kA, the inductive equilibrium electric field E∥ = V loop /2πR 0 = E 0 ≈ 0.1 V/m, the central line-averaged density <n e > = 2.2 10 19 m −3 , the plasma major radius R 0 = 1.86 m and the minor radius a = 0.45 m [8][9][10]. At the plasma center, the electron temperature T e ≈ 0.55 keV was obtained using a soft x-ray pulse height analysis (PHA) system during the plasma current flat-top phase (duration of impulse was 5 s).…”

“…Secondary runaway region in the phase space (p∥, p⊥) is filled by these ellipses. In case of EAST shot #28957 the ring-like runaway beam contained high energy electrons with energy more than 20 MeV and p⊥/p∥ ~ 0.15 (p∥ >> p⊥) [8,9]. The bulk electrons knocked out in runaway region by these very high energy electrons are born near the parabola [ (6) This curve (6) is shown in Fig.…”