24-MW, 24-/spl mu/s 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 /spl mu/s 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%/sub p-p/ at 20 MW, 24 /spl mu/s pulse operation. The 24 /spl mu/s 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 /spl mu/s at 5.5 /spl mu/m FEL oscillation and for 12 /spl mu/s at 1.88 /spl mu/m FEL oscillation, respectively.