Commissioning and initial heating results of the neutral beam injection system in Versatile Experiment Spherical Torus

The initial neutral beam heating experiment in Versatile Experiment Spherical Torus (VEST) has been conducted via neutral beam characterizations such as neutralization efficiency, beam fraction, and shine-through loss. For the neutralization efficiency and shine-through loss estimation, a secondary electron emission (SEE) detector, which can be used in rapidly changing plasma, has been developed. The measurement results of the monatomic fraction by Doppler shift spectroscopy and the neutralization efficiency by SEE detector are 65%, and 70%, respectively. The shine-through loss measurement shows that the injection of a 12 keV neutral beam into a typical ohmic discharge of VEST results in a large amount of shine-through loss. To reduce the shine-through loss, a new target plasma discharge (0.17 T, 120 kA, 13 ms) with increased plasma density of -2 x 1019m-3 is developed by providing additional gas injection. The electron-heating effect has been successfully observed for this target plasma with a 200 kW neutral beam injection at the beam energy of 12 keV. The electron temperature increases from 20 to 68 eV without significant density change when the neutral beam is injected.