Evidence for non-negligible temperature fluctuations in a helicon plasma

We present preliminary evidence for the existence of finite electron temperature fluctuations in a helicon plasma, the Controlled Shear Decorrelation Experiment (CSDX). Results indicate the presence of fluctuations with amplitude $\widetilde{T}_e/\langle T_e\rangle \sim 0.1$ in spite of a relatively flat temperature profile. CSDX is a well-characterized linear machine that uses a helicon antenna to produce dense plasmas relevant to the tokamak edge ($T_e \sim 3$ eV, $n_e \sim 10^{13}$/cc). Visible light from ArI and ArII line emission is collected using a fast digital camera, while floating potential and ion-saturation current are measured by an array of electrostatic probe tips. Time-average profiles are obtained using a compensated Langmuir probe and laser-induced fluorescence. We identify several features that indicate contributions of electron temperature fluctuations to both probe and imaging measurements. Striking radial dependence of the observed probability distribution functions (PDFs) for imaging fluctuations is explained by a simple theoretical model based on normally distributed $T_e$ fluctuations and the temperature sensitivity of the observed argon ion emission. Our results imply that particular care is needed when neglecting electron temperature fluctuations.