Spatial and temporal evolutions of blue-core helicon discharge driven by planar antenna with concentric rings

TL;DR

This study investigates the spatial and temporal behavior of blue-core helicon plasma driven by a novel planar antenna with concentric rings, revealing new mode characteristics, rotation directions, and instability behaviors.

Contribution

It provides the first detailed characterization of blue-core helicon plasma driven by a planar antenna, including mode transitions, rotation directions, and centrifugal instability analysis.

Findings

Blue-core plasma can be driven to higher modes with off-axis peaking.

Blue-core mode exhibits counterclockwise rotation, opposite to non-blue-core mode.

Centrifugal instability is stronger at small axial wave numbers and peaks at specific radial locations.

Abstract

The spatial and temporal evolutions of blue-core helicon discharge driven by a planar antenna with four concentric rings are explored on the Linear Experimental Advanced Device (LEAD). The discharge experiences distinct density jumps from E mode to H mode, W mode, and blue-core mode, when RF input power increases. This is similar to previous observations using other typical helicon antennas; however, this special antenna could drive modes of even higher levels for which the blue-core plasma column is actually hollow in radius, i.e. peaking off-axis, which was not presented before. The column shows counterclockwise rotation for blue-core mode and clockwise rotation for non-blue-core mode. The reason could be attributed to the radial electric field differenceses for both modes which reverses the rotation direction via ExB drive. Moreover, the centrifugal instability of blue-core helicon plasma is computed using a two-fluid flowing plasma model. It shows that the instability is strong for small axial wave number but becomes weak for large axial wave number. Perturbed density peaks at radius of 0.045 m, while the equilibrium density gradient peaks at radius of 0.055 m. The coincidence of their radial locations suggests that it is a resistive drift mode driven by density gradient. The blue-core mode weakens once the magnetic field or flow rate exceeds the threshold value. Increasing power further leads to a smoother plasma density gradient. The electron temperature profiles decrease with increased power, and the radial gradient of the electron temperature inside the core is smaller as the magnetic field changes. To our best knowledge, it is the first detailed characterization of blue-core helicon plasma driven by planar antenna, especially in terms of azimuthal rotation and centrifugal instability.