AbstractIn neutral beam injection (NBI) systems for next generation tokamaks such as the International Thermonuclear Experimental Reactor (ITER), insulation gas around beam sources will be irradiated with neutrons and gamma rays generated by nuclear fusion reactions. To evaluate the influence of radiation, irradiation experiments were performed using the 60Co gamma rays. Ionization current and voltage-holding characteristics of various gases, such as air, SF6, C2F6, CO2, and mixing gas of air and SF6, were investigated up to an absorbed dose rate of 0.45 Gy/s. Saturation current flowing through the gases proved to increase linearly with gas volume, gas pressure, an absorbed dose rate. Saturation current also increased with molecular weight of the gases. During irradiation, voltage-holding capability was degraded by about 10%; the degree of the degradation did not depend on the absorbed dose rate. Mixture of a small quantity of SF6 gas and air proved to be effective from the viewpoint of suppressing ionization current, since air containing a small amount of SF6 gas has lower ionization current and higher voltage-holding capability. An experimental formula for estimating saturation current was obtained on the basis of experimental results. Using the experimental formula, ionization current at the ITER-NBI system was estimated to be higher than 2 A; such a high ionization current is not acceptable. Instead of gas insulation, other insulating method like vacuum insulation will be necessary.