dc voltage holding characteristics were investigated to obtain a data base for designing high‐energy and high‐power ion sources of neutral beam injectors. We confirmed that the voltage holding characteristics almost obey the clump theory in the experimental gap length of up to 50 mm. The magnetic field in the gap lowered the breakdown voltage in a gas discharge region higher than a pressure of 10−3Torr. The breakdown voltage of 30% was reduced by seeding cesium on the electrode with one order higher density than that of actual ion source at the pressure region of lower than several mTorr.