ArI laser induced fluorescence system for measurement of neutral dynamics in a large scale helicon plasma

摘要

Neutral particles can play a significant role in the dynamics of plasma instabilities and flows through momentum transfer via ion-neutral collisions. When neutral and ion densities are spatially nonuniform, neutral-ion collisions can also exert a zero-order torque on a magnetized plasma column via the FxB force, where F is the force exerted on ions by neutrals (a neutral wind force). In order to investigate the role of neutral dynamics in helicon discharges in the HelCat (Helicon-Cathode) basic plasma science device at U. New Mexico, an ArI Laser Induced Fluorescence (LIF) system is being developed. Previous passive spectroscopic measurements of ArI and ArII lines indicate that the neutral density profile is hollow (higher n at larger radius). Additionally, we have not been able to reconcile azimuthal flows measured by Mach probes with those expected from ExB and diamagnetic torques. It is hypothesized that neutrals play an important role in the plasma flow. The LIF system is based on a > 250 mW, tunable diode-pumped solid state laser. The laser will pump the metastable (2P)4s level to the (2P)4p level using 696.543 nm light, and observe fluorescence radiation from decay to the (2P)4s level at 772.42 nm. The system design and initial results will be presented.