Direct measurements and modeling of gradient-aligned cross-field ion flows near an absorbing boundary

Direct measurements of cross-field ion transport near boundaries are sought for validating transport models in magnetically confined plasmas. Using laser-induced fluorescence, we measured ion flows normal to an absorbing boundary that was aligned to be parallel to a uniform axial magnetic field in a helicon plasma. We used Langmuir and emissive probes to measure local density, temperature and plasma potential profiles in the same region. We then scanned ion-neutral collisionality by varying the ratio of the ion gyro-radius, $\rho_{i}$, and ion-neutral collision length, $\lambda $, over the range 0.34 $\le \rho_{i}$/$\lambda \le $1.60. Classical diffusion along density and potential gradients is sufficient to describe flow profiles for most cases but did not describe measurements well for 0.44 $\le \rho_{i}$/$\lambda \le $ 0.65. In these cases, cross-sections $\approx $3 times the classical prediction produced acceptable fits, and flow to the boundary was enhanced significantly. These enhanced flow cases exhibit spectra with low-frequency electrostatic fluctuations (f \textless 10 kHz) that are not observed in data described well by a classical diffusion model.