24-MW, 24-μs pulse RF power supply for linac-based FELs

Two infrared free electron lasers (FELs) of the FELI, operating using an S-band 80 MeV LINAC with a thermionic gun and a 165 MeV LINAC and UV-FEL facility, are under construction. Since an RF system for LINAC-based FELs is required of long pulse duration and high quality, improved pulse klystrons (Toshiba E3729) have been operated in 24 MW, 24 μs pulse mode for the FELI LINAC. The klystron modulator developed by the FELI and NISSIN Electric Corp. has a pulse forming network consisting of 4 parallel networks of 24 capacitors and 24 variable reactors. The line switch consists of 30 light triggered thyristors (Toshiba SL1500GX22). A saturable reactor is used in series to protect the 30 thyristors from overvoltage caused by a delay in thyristor turn-on time. The flatness of modulator pulses is 0.08%_p-p at 20 MW, 24 μs pulse operation. The 24 μs stable RF pulses can increase conversion efficiency from electron beam power to FEL power at short wavelength FELs. Saturated FEL outputs have been observed for 18 μs at 5.5 μm FEL oscillation and for 12 μs at 1.88 μm FEL oscillation, respectively