Electromagnetic instabilities driven by ion thermal anisotropy in helicon plasmas*

Temperature anisotropies provide a source of free energy for instability growth in space and laboratory plasmas. The growth of electromagnetic ion temperature anisotropy driven instabilities is dependent on the anisotropy parameter T_⟘/T_|| and the local plasma β. These instabilities reduce the ion temperature anisotropy by inducing velocity-space diffusion, resulting in a limit on the ion temperature anisotropy that scales with plasma β. Here we report measurements of the spectra and parameter dependence of magnetic fluctuations in a helicon plasma source where the RF antenna responsible for plasma creation is amplitude modulated at the ion cyclotron frequency to increase the perpendicular ion temperature and thereby increase the ion temperature anisotropy. Magnetic fluctuation spectra up to 500 kHz, parallel and perpendicular to the magnetic field, are reported as a function of ion temperature anisotropy. Ion velocity distribution functions in the same region, measured by laser induced fluorescence, provide direct measurements of the ion temperatures parallel and perpendicular to the magnetic field.