Hollow density formation in magnetically expanding helicon plasma

TL;DR

This study investigates how magnetic field strength influences the formation of hollow density profiles in helicon plasma devices, revealing the role of electron and ion magnetization and azimuthal electron rotation.

Contribution

It demonstrates the conditions under which hollow density profiles form, emphasizing the importance of electron and ion magnetization and electron azimuthal rotation in plasma density distribution.

Findings

Hollow density forms above a critical magnetic field where electrons and ions are magnetized.

Off-axis temperature peaks and tail electrons are observed in both chambers.

Azimuthal electron rotation due to gradient-B drift causes off-axis ionization leading to hollowness.

Abstract

Measurement of radial density profile in both the source and expansion chambers of a helicon plasma device have revealed that it is always centrally peaked in the source chamber, whereas in the expansion chamber near the diverging magnetic field it becomes hollow above a critical value of the magnetic field. This value corresponds to that above which both electrons and ions become magnetized. The temperature profile is always peaked off- axis and tail electrons are found at the peak location in both the source and expansion chambers. Rotation of the tail electrons in the azimuthal direction in the expansion chamber due to gradient-B drift produces more ionization off-axis and creates a hollow density profile; however, if the ions are not magnetized, the additional ionization does not cause hollowness.