Power modulators for FERMI's linac klystrons

The conventional line type modulators now in use for ELETTRA will have to be replaced for FERMI due to the increase in the pulse repetition frequency (PRF) from 10 to 50 Hz [1]. The requirements for the FERMI modulator are as follows. The klystron which will be used is a Thales TH2132 with a microperviance of 1.9-2.1 uA/V^3/2. The peak voltage from the modulator is 320 kV, and the current is 350 A. The pulse width is 4.5 us, with a PRF of 50 Hz. Flat top should be better than plusmn0.5% of the peak voltage and pulse to pulse ripple better than plusmn0.1% of the peak voltage. Prototypes for an upgraded line type modulator and a solid state induction type modulator [2] are in fabrication. The solid state design uses eight induction cells, each cell driven by two parallel insulated gate bipolar transistors (IGBT). Each IGBT will power an amorphous core with up to 4.16 kV and 1.925 kA for up to 5 us. A single turn is passed through the aperture of each of the cells, inductively adding the pulse voltages. The output from the modulator is then fed to a conventional pulse transformer to reach the 320 kV requirement. This paper presents the system design of both modulator types. Details of the IGBT drivers, control electronics, IGBT and klystron protection and the charging supply for the solid state modulator are also presented.