Observation of a strong correlation between the positive floating potential near the edge and plasma current on EXL-50 ECW plasma

TL;DR

This study investigates the relationship between floating potential and plasma current in ECW plasma on EXL-50, revealing a strong correlation influenced by magnetic field adjustments and electron confinement near the boundary.

Contribution

It demonstrates the correlation between floating potential and plasma current, highlighting the role of high-energy electrons and magnetic field control in non-inductive plasma start-up.

Findings

Floating potential is about 200V and correlates with plasma current.

Adjusting vertical magnetic field inversely affects potential and current.

Boundary potential measurements are crucial for electron confinement and current drive studies.

Abstract

Fully non-inductive plasma current start-up without the central solenoid in ECW plasma was used on EXL-50 Spherical Torus with a weak external vertical field (Bv). Generally, the number of electrons leaving to the vessel wall by the gradient Bt is larger than ions, and the positive potential was built up in plasma. The relationship between floating potential and the plasma current was studied using the Langmuir probes near the boundary. The results show that the floating potential is positive (about 200V) and has a strong correlation with plasma current. In open magnetic field, the plasma current is driven by the high energy electrons in preferential confinement, the plasma current and potential approximately positively correlated with total electron density. After forming the closed flux surface, the plasma current consists mainly of the ECW driven current, and potential is negatively correlated with plasma current. By actively adjusting the Bv, it demonstrated that the positive voltage is approximately inversely correlated with the Bv and plasma current (Ip). Considering that the plasma temperature near the boundary is quite low (~eV), the positive voltage near the boundary caused by the high-energy electron loss. Therefore, the measurements of the boundary potential are important for the study of high-energy electron confinement performance, noninductive plasma current start-up and current driven.