The dependence of neutron generation on the discharge current as well as the voltage in an IEC device is studied experimentally. We consider that the fusion events mainly occur between the accelerated ions and the background neutral gas from the linear dependence of neutron yield on the discharge current. The result of neutron generation proportional to the 3/2 power of the applied voltage indicates that the current within the hollow cathode may be limited by space charge. The particle simulation shows that the perveance in the experiments satisfies the conditions for space charge limited current.