AbstractA vacuum insulated beam source (VIBS), where all the components including the negative ion source and the acceleration grids are immersed in the vacuum, has been designed at Japan Atomic Energy Research Institute. The beam source is characterized by high gas flow conductance that reduces stripping losses of negative ions in the accelerator and improves the acceleration efficiency. The gas flow in VIBS was calculated using a 3-D Monte-Carlo code. At a typical operating pressure of 0.3 Pa in the negative ion source, the stripping loss of D− ions in VIBS is about 23%, which is about half of stripping loss in the gas insulated beam source (GIBS) that was designed in the ITER-FDR design. The acceleration efficiency is improved from 84 to 94%. To enhance the acceleration efficiency further, the possibility for reducing the operating pressure was experimentally studied. It was found the negative ion production in the KAMABOKO source adopted for ITER is dominated by surface production, therefore, the operating pressure can be reduced by improving plasma confinement. It is predicted that the operating pressure in KAMABOKO source can be reduced to 0.15 Pa, where the stripping loss and the acceleration efficiency in VIBS are improved to be 13 and 97%, respectively.