Particle control in the sustained spheromak physics experiment

In this paper we report on density and impurity measurements in the sustained spheromak physics experiment (SSPX) which has recently started operation. The SSPX plasma is sustained by coaxial helicity injection for a duration of 2 ms with peak toroidal currents of up to 0.5 MA. Plasma-facing components consist of tungsten-coated copper to minimize sputtering. The surfaces are conditioned by a combination of baking at 150°C, glow discharge cleaning, titanium gettering, and pulse-discharge cleaning with helium plasmas. In this way we achieve density control with ne∼1–4×1020 m−3. However, gas input has only a weak effect on plasma density; injector current is the dominant factor. Conditioning reduces the impurity radiation to the point where it is no longer important to the energy balance, so that the lifetime of the spheromak discharge is ultimately governed by MHD activity, which grows rapidly about 1.5–2.0 ms after helicity injection ends.