Abstract
Plasma-surface interactions strongly influence the properties of plasmas produced in the ZT-40M reversed-field pinch, which generally operates without discrete limiters. Z eff data and spectroscopy, together with total radiated power, and measured impurity accumulations on collector probes demonstrate that metal impurities play a determining role in ZT-40M's performance. Plasma currents of 60–400 kA routinely generate total betas (assuming T i = T e ) of 5–10%. Pulse duration at the lower currents has reached ∼ 35 ms. Energy confinement time τ E has reached 0.7 ms for discharges with n n e = 8 × 10 19 m −3 and T e = 330 eV at a current of 330 kA. 400 kA flat-topped discharges yield T e ≥ 500 eV and n n e ∼ 4 × 10 19 m −3 . Magnetic field errors intrinsic to the winding and core configuration create localized, intense plasma-surface interactions that define the present operating boundary for ZT-40M. Discharge cleaning, either pulsed or glow, of the Inconel vacuum vessel increases the deuterium recycling rate and decreases the impurity fractions, leading to discharges with higher density and lower temperature, lower Z eff, lower resistive anomaly factor, and better overall confinement of particles and energy. Carbon films ∼ 40 monolayers thick are used in ZT-40M to simulate the recycling, fueling, and sputtering conditions that may be encountered in next generation devices with large areas of carbon.
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