PURPOSE: To improve the availability factor of a device by providing heat conduction members having the heat conductivity higher than or nearly equal to that of magnesium and having the surface area larger than that of magnesium grains in a vacuum container.

CONSTITUTION: Heat conduction members 4 having the heat conductivity larger than or nearly equal to that of magnesium and having the surface area larger than that of magnesium grains 2 are provided in addition to the magnesium grains 2 in a vacuum container 1. When the members 4 having the heat conductivity larger than that of magnesium and having the surface area larger than that of the magnesium grains 2 are inserted, the heat from the members 4 having high heat conductivity can be efficiently transferred to the magnesium grains 2. The heat conduction effect of the grains 2 themselves can be improved more than when heat is transferred by only the grains 2 so far, and the temperature rise time of the grains 2 can be shortened. The time required for beam extraction is shortened, and the availability factor of a device can be improved.