Limiter Experiments and Edge Magnetic Field Structure in HSX

When operating at 1 Tesla, density control in HSX has been challenging, and although methane carbonization has mitigated the control issue, shot-to-shot density reproducibility needs improvement. A moveable graphite limiter for use both inside, and outside of the last closed flux surface has been implemented in HSX to improve plasma performance by reducing interactions between the plasma and the steel walls of the device. The magnetic islands that define the plasma edge in QHS allow a relatively small plate with primarily poloidal extent to intercept a large fraction of field lines that start near the last closed flux surface before they reach the vessel. These islands in the edge region can be moved with respect to the vessel by a small variation in the auxiliary field, allowing the relationship between island position and limiter effectiveness to be studied. The effect of edge electric fields set up by differential biasing between the limiter and an insertable electrode will be studied using langmuir probes.