Overview of impurity control and wall conditioning in NSTX

The national spherical torus experiment (NSTX) started plasma operations in February 1999. In the first extended period of experiments, NSTX achieved high current, inner wall limited, double null, and single null plasma discharges, initial Coaxial Helicity Injection, and high harmonic fast wave results. As expected, discharge reproducibility and performance were strongly affected by wall conditions. In this paper, we describe the internal geometry, and initial plasma discharge, impurity control, wall conditioning, erosion, and deposition results.