Breakdown of the back-diode of the pulse modulator for FEL linac at Nihon University

The breakdown of the diodes and the resistors in the back-diode circuit, which is a protection against damages of the klystron and other elements, was found at the pulse modulator No.2 after a series of operations since 1997. The breakdown of the resistor can be explained by assuming a preceding breakdown of the diodes possibly suffered an excessive surge voltage. However, there has been no problem found in the back-diode circuit at the pulse modulator No.1. As one of efforts made at LEBRA of Nihon University to reduce the fluctuation of the energy and the current of the beam in the linac, an optimum reservoir voltage of the thyratron in each modulator has been investigated in order to reduce the fluctuation of the modulator output voltage between successive pulses. (author)