Improved confinement in HBTX with removal of tile limiters

Removing the limiters from the HBTX experiment has had the beneficial effect of reducing the loop voltage by about a third. Since the poloidal beta, beta theta , remains largely unaltered there is a corresponding 50% improvement in the energy confinement time. Soft X-ray measurements show a suprathermal tail in the electron energy distribution for the first time. The 156 graphite tiles cover about 8% of the vacuum vessel and protrude 1 cm into the plasma. The consequent drop in loop voltage with their removal is in agreement with models for the loop voltage based on global magnetic helicity balance. The observed changes in the ion temperature are consistent with these models when the energy balance of electrons and ions is also included.