Characterization of a core ion mode in a linear magnetized helicon plasma device

Controlled Shear Decorellation eXperiment (CSDX) is a Radio-Frequency (RF) heated helicon plasma device used to study plasma turbulence and transport. Previous studies in CSDX show that the plasma equilibrium undergoes a self organized global transition as the magnetic field is raised above a certain threshold [1, 2]. This transition occurs simultaneously with axial plasma detachment at fixed power. At the transition, we also observe a prominent new “ion feature” at the core (r < 2 cm), which rotates in the ion diamagnetic drift direction, has very high azimuthal mode number (10-15) and occurs when an ion temperature gradient develops. Based on preliminary experimental signatures, we investigate if this ion mode is due to an Ion Temperature Gradient (ITG) instability. We use linear ITG theory and CSDX experimental profiles to compute the dispersion relations and compare them with those calculated from fast camera imaging of CSDX plasmas. We also experimentally measure the parallel wavenumber of this ion mode to compare with our linear theory. Finally, our recent upgrades to CSDX (20 kW of RF power) allow us to study the ion mode independent of axial detachment.

[1] S. C. Thakur et. al., Plasma Sources Science and Technology (2014)

[2] L. Cui. et. al., Physics of Plasmas (2016)