Transport studies using laser blow-off injection of low-Z trace impurities injected into the TJ-II stellarator

Trace (<5%) impurity seeding of boron and carbon by laser blow-off is used to study low-Z parallel and radial impurity transport in the TJ-II stellarator. Toroidal transport is observed to be quite rapid τϕ ∼ 0.1 ms, roughly consistent with expectations of collisionless (kinetic) parallel ion transport. Radial impurity de-confinement of injected impurities to the plasma wall is measured to be much slower, τr ∼ 10 ms. The data suggest a slightly higher τr for carbon than for boron, but no clear charge state dependence is observed in contrast with expectations for neoclassical transport in the low collisionality regime.

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