UC San Diego

Understanding material migration in tokamaks is fundamental to understanding the problem of the lifetime of the plasma facing components in the tokamak. However, diagnostic access and plasma parameter

control in tokamaks is difficult. In order to address the migration of wall material in the scrape off layer of tokamaks, parallel transport of heavy impurities in a lighter flowing plasma is studied in an Argon plasma.

The device used is CSDX, a 3m long, 20 cm diameter helicon plasma device.

Bismuth is injected using a technique called laser blow off (LBO). Impurity transport is found to be classically collisional, which agrees with basic plasma physics theories. A careful study of the plasma flows with multiple diagnostics, Laser Induced Fluorescence (LIF) and Mach probes, reveals a systematic error in the interpretation of Mach probe data. It is shown that the probe creates a low density wake on the downstream side. When the

ion-neutral mean free path is comparable to the length of the shadow downstream of the probe, the Mach probe reports an artificially high Mach number when the shadow is not taken into account.

A theory describing the low density wake is presented, which results in successful correction of the Mach probe results. It is demonstrated that the ion-neutral collision effect can be important and should be taken into account when constructing Mach probes for velocity measurement in plasma devices.