Generation of energetic electrons by an electron cyclotron wave through stochastic heating in a spherical tokamak

Abstract

This study presents novel findings on stochastic electron heating via a random electron cyclotron wave (ECW) in a spherical tokamak. Hard x ray measurements demonstrate the time evolution of hard x ray counts at different energy bands, consistent with predictions from the stochastic heating model. The ECW heating rate shows a positive correlation with applied power, confirming the effectiveness of stochastic heating. Remarkably, the ECW-driven plasma current remains insensitive to ECW incidence angle, consistent with model predictions. The observed stochastic heating of electrons offers potential for exploring innovative non-inductive current drive modes in spherical tokamaks. This research contributes to the understanding of plasma behaviour and motivates the development of new models for non-inductive current drive in fusion devices.

Experimental confirmation of the linear relation between plasma current and external vertical magnetic field in EXL-50 spherical torus energetic electron plasmas

“…The ECW injected power was approximately 140 kW and was switched off after 3 s. Throughout the discharge, a consistently low loop voltage was maintained, and the plasma current remained stable at around 118 kA during the 2-3 s time period, indicating that the ECW primarily drove the plasma current. Notably, an increase in plasma current corresponds to a significant rise in hard X-ray intensity, suggesting the presence of energetic electrons in the EXL-50 system and a correlation between the plasma current and hard X-ray emission [16,31].…”

“…At present, EXL-50 uses three ECW systems (28 GHz) to heat the plasma and drive the plasma current. System A (gyrotron source power of ~ 50 kW) is mainly used to produce the initial plasma and form closed flux surfaces [31], systems B and C (gyrotron source power of ~ 400 kW) are used to increase the plasma current and maintain the current flattop for multiple seconds. Discharges with plasma currents substantially above 100 kA with a 100 kW ECW are routinely obtained in EXL-50 [29], and the main current is identified as the contribution of energetic electrons [27], which are driven by stochastic ECW [36].…”

“…These equations have been subsequently applied in the EXL-50 spherical torus for magnetic flux reconstruction purposes. This paper presents the results of the equilibrium experiments of the EXL-50 spherical torus, which is a medium-sized spherical torus with a large number of energetic electrons carrying the main plasma current [29][30][31]. According to previous experiment results [30,[32][33][34][35], the energetic electrons exist and play an important role in the EXL-50 plasma, making it no longer suitable to be treated as a single-fluid plasma.…”