Effect of boronization in VEST: Achieving 0.1 MA discharge

Boronization procedure for Versatile Experiment Spherical Torus (VEST) is developed in order to reduce the oxygen impurity during the Ohmic plasma discharge. Carborane (C2B10H12) powder has been chosen for a safe operation. During the helium glow discharge, the sublimated carborane is diffused into the vacuum vessel, which results in thin boron-containing coatings on the vacuum vessel wall. After the boronization, qualitative changes in visible light intensities of oxygen and boron lines are observed indicating that the water concentration is reduced globally. The quality of discharges is greatly improved: a discharge with less radiative loss is achieved, which leads to a record shot of 0.1 MA plasma current and a nearly doubled pulse length.