In this paper we report on density and impurity measurements in the Sustained Spheromak Physics Experiment (SSPX) which has recently started operation. The SSPX spheromak plasma is sustained by coaxial helicity injection for a duration of 2msec with peak toroidal currents of up to 0.5MA. The plasma-facing components consist of tungsten-coated copper to minimize sputtering. The surfaces are conditioned by a combination of baking at 150 o C, glow discharge cleaning, Titanium gettering, and pulse-discharge cleaning with helium plasmas. In this way we can achieve density control so that the plasma density (~ 1-4×10 20 m -3 ) matches the gas input. Low-density operation is presently limited by breakdown requirements, but we hope that new gas valves with supersonic nozzles will allow for a further reduction in density. We find that the conditioning reduces the impurity radiation to the point where it is no longer important to the energy balance, and long-lived spheromak plasmas are obtained (decay times of 1.5msec).
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IntroductionIn this paper we discuss power and particle control for the SSPX (Sustained Spheromak Physics Experiment) spheromak device. SSPX began operation in 1999 after it was constructed as part of a renewed US program in alternate confinement concepts.In spheromaks, a very low aspect ratio (A~1.1) toroidal confinement geometry is produced by currents in the plasma itself (the plasma dynamo), rather than by external coils which necessarily thread the vacuum vessel. Elimination of the linked coils could lead to smaller, cheaper power plants. Furthermore, DC or AC potentials applied to external electrodes can sustain the spheromak plasma. At present, it is unknown if the spheromak configuration can provide sufficient energy confinement to allow the plasma to be heated to thermonuclear temperatures (10keV). Recent analysis of previous experimental data [1,2] suggested that adequate core energy confinement could be obtained in these devices and that performance might scale favorably to power reactors. The SSPX device was built to explore this question.The spheromak plasma in SSPX is confined within an R=1.0m, h=0.5m, 1.2cm thick copper flux conserver which serves to maintain the plasma shape via image currents flowing in it. A cross section of the device appears in Fig. 1; magnetic flux surfaces for an ideal MHD equilibrium computed with the CORSICA code are included. The confined plasma (R=0.31m, a>0.25m is isolated from the flux conserver by a thin (less than 1cm wide at the midplane) scrape-off layer (SOL) plasma whose field lines encircle the plasma and connect to the electrode region at the top of the device. In the spheromak the magnetic field lines vary from almost completely poloidal (in the plane of the paper here) near the walls to completely toroidal at the magnetic axis at R=0.31m.The cross section of the shell was designed to minimize the volume of corner regions 3 having open field lines, so it is everywhere conformal to the magnetic flux surfaces except for a 5cm high toroidally unifo...
