Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas

TL;DR

This paper demonstrates solenoid-free plasma current drive in the EXL-50 spherical tokamak using electron cyclotron resonance heating, achieving significant current and stable operation without a central solenoid.

Contribution

It introduces a novel solenoid-free current drive method in a spherical tokamak using ECRH, with detailed analysis of energetic electron dynamics and plasma equilibrium.

Findings

Achieved plasma currents over 80kA with 150kW ECRH power.

Maintained plasma current for more than 1 second.

Discovered energetic electrons create smaller flux surfaces aiding confinement.

Abstract

As a new spherical tokamak (ST) designed to simplify engineering requirements of a possible future fusion power source, the EXL-50 experiment features a low aspect ratio (A) vacuum vessel (VV), encircling a central post assembly containing the toroidal field coil conductors without a central solenoid. Multiple electron cyclotron resonance heating (ECRH) resonances are located within the VV to improve current drive effectiveness. Copious energetic electrons are produced and measured with hard X-ray detectors, carry the bulk of the plasma current ranging from 50kA to 150kA, which is maintained for more than 1s duration. It is observed that over one Ampere current can be maintained per Watt of ECRH power issued from the 28-GHz gyrotrons. The plasma current reaches Ip>80kA for high density (>5e18me-2) discharge with 150kW ECHR heating. An analysis was carried out combining reconstructed multi-fluid equilibrium, guiding-center orbits of energetic electrons, and resonant heating mechanisms. It is verified that in EXL-50 a broadly distributed current of energetic electrons creates smaller closed magnetic-flux surfaces of low aspect ratio that in turn confine the thermal plasma electrons and ions and participate in maintaining the equilibrium force-balance.