Impedance of an intense plasma-cathode electron source for tokamak startup

An impedance model is formulated and tested for the similar to 1 kV, 1 kA/cm(2), arc-plasma cathode electron source used for local helicity injection tokamak startup. A double layer sheath is established between the high-density arc plasma (n(arc) approximate to 10(21) m(-3)) within the electron source, and the less dense external tokamak edge plasma (n(edge) approximate to 10(18) m(-3)) into which current is injected at the applied injector voltage, V-inj. Experiments on the Pegasus spherical tokamak show that the injected current, I-inj, increases with V-inj according to the standard double layer scaling I-inj similar to V-inj(3/2) at low current and transitions to I-inj similar to V-inj(1/2) at high currents. In this high current regime, sheath expansion and/ or space charge neutralization impose limits on the beam density n(b) similar to I-inj/V-inj(1/2). For low tokamak edge density n(edge) and high I-inj, the inferred beam density nb is consistent with the requirement n(b) <= n(edge) imposed by space-charge neutralization of the beam in the tokamak edge plasma. At sufficient edge density, n(b) similar to n(arc) is observed, consistent with a limit to n(b) imposed by expansion of the double layer sheath. These results suggest that n(arc) is a viable control actuator for the source impedance. Published by AIP Publishing.