Measurement of parallel transport of B+ and Al2+ impurities in a linear He+ plasma

Laser ablation is used to inject trace quantities (∼1%) of boron or aluminum impurities into steady-state, cylindrical He+ plasmas. The He+ plasmas have an axial flow velocity of about 2 × 103 m/s, corresponding to Mach number M ∼ 0.2. The parallel flow of the resulting B+ or Al2+ impurity ions is measured directly using photomultiplier tubes with line filters. Perpendicular loss of the impurity ions is estimated indirectly from particle conservation. Rapid (<100 μs) entrainment of B+ in the background flow is observed; Al2+ ions appear to accelerate up to perhaps half the background flow velocity in the same time period. Preliminary comparisons with expected transport rates indicate that the observed parallel transport is ≈2-3× faster than expected from classical collisional transport, while perpendicular transport appears slightly slower than expected.