Current and heat flux to the wall and electron density control in reversed field pinches

Abstract Investigations of HBTX1B ( R / a = 0.8/0.26 m, I = 0.2 MA) edge phenomena are reported. Calculations show that the 160 small graphite limiters intercept half of P Ω = 10 MW and photography shows strong and intermittent heating of edges exposed to the poloidal electron current. Experiments and simulation are described of density maintenance by preloading the vessel with gas to achieve RFP with low I / N ≤ 5 × 10 −14 A m ( N = 2 π ∝ n e r d r ). A movable limiter probe collects large spikes of electron current ≲ 500 A and the average current is much larger than that required for RFP equilibrium. The conserving ion current to the wall will produce j r B θ forces on the plasma. From energy and potential measurements we infer that the SOL is collisionless and because of the low magnetic field is less than a few ion gyro radii thick.