Performance assessment of helicon wave heating and current drive in EXL-50 spherical torus plasmas

TL;DR

This paper evaluates helicon wave heating and current drive in EXL-50 spherical torus plasmas, revealing how parameters like frequency, plasma density, and $n_{||}$ influence driven current and efficiency.

Contribution

It provides new insights into the dependence of helicon wave current drive on frequency, plasma conditions, and the scrape-off layer in spherical torus plasmas.

Findings

Driven current peaks at $n_{||} = 4.0$

Higher frequency increases driven current

Driven current decreases with higher plasma density

Abstract

Analysis of helicon wave heating and current drive capability in EXL-50 spherical torus plasmas has been conducted. It is found that the driven current increases with the launched parallel refractive index $n_{||}$ and peaks around $n_{||} = 4.0$ when the frequency of the helicon wave is between 300~MHz and 380~MHz. The helicon wave current drive efficiency shows a relatively stable upward trend with increasing plasma temperature. Moreover, the driven current decreases as the plasma density increases. We also analyzed the current drive with helicon waves of 150~MHz and 170~MHz and found that the driven current at a lower frequency was lower than that at a higher frequency. A positive proportional relationship exists between the driven current and $n_{||}$. Besides, as $n_{||}$ increases, the profile of the driven current becomes wider. Finally, the effect of the scrape-off layer (SOL) region on the helicon wave current drive was also investigated.