Macropulse electron beam production with a high-repetition-rate grid-pulser

The study of an infrared free-electron laser (FEL) is being conducting by using the macropulse electron beam with the duration of 8 μs generated by the L-band electron linac at the Institute of Scientific and Industrial Research, Osaka University. At present, the electron beam is generated from the thermionic electron gun with the duration of 8 μs in dc, and bunched into the micropulses with the bunch interval of 9.2 ns by using subharmonic-buncher cavity with the RF frequency of 108 MHz. On the other hand, the resonator length of the FEL is equal to the round-trip time for the light of 37 ns, thus, there are 4 electron and light bunches in this time. In order to increase the FEL gain, we have to increase the bunch charges of the electron beam, but the beam loading of the RF system has been already reached to the serious level. By reducing the electron bunch repetition to 27 MHz, it is possible to increase the bunch charges in 4 times keeping the present average beam loading to the RF system. Therefore, we have been developing the grid pulser system with the repetition rate of 27 MHz. Recently we have installed this system into the L-band electron linac and started the study of the 27 MHz beam operation. In this meeting, we show the results of the beam study of this grid pulser system. (author)