Experimental study on edge energetic electrons in EXL-50 spherical torus

TL;DR

This study investigates energetic electrons outside the Last Closed Flux Surface in the EXL-50 spherical torus, revealing their confinement, distinct boundary, and implications for plasma modeling.

Contribution

First application of multiple diagnostics to characterize energetic electrons outside the LCFS in a spherical torus, highlighting the need for multi-fluid models.

Findings

Existence of high-temperature, low-density energetic electrons outside LCFS.

Clear separation between thermal plasma and energetic electron boundary.

Energetic electrons can be well confined outside the LCFS, consistent with simulations.

Abstract

A significant number of confined energetic electrons have been observed outside the Last Closed Flux Surface (LCFS) of the solenoid-free, ECRH sustained plasmas in the EXL-50 spherical torus. Several diagnostics have been applied, for the first time, to investigate the key characters of energetic electrons. Experiments reveal the existence of high-temperature low density electrons, which can carry relatively a large amount of the stored energy. The boundary between the thermal plasma and the energetic electron fluid appears to be clearly separated and the distance between the two boundaries can reach tens of centimeters (around the size of the minor radius of the thermal plasma). This implies that the Grad-Shafranov equilibrium is not suitable to describe the equilibrium of the EXL-50 plasma and a multi-fluid model is required. Particle dynamics simulations of full orbits show that energetic electrons can be well confined outside the LCFS. This is consistent with the experimental observations.