Installation and Operation of New Klystron Power Supply with Fast Solid-state Switch for Klystron Protection at the Photon Factory Storage Ring

In the 2.5-GeV Photon Factory (PF) storage ring at KEK, one of the klystron power supplies was upgraded during a summer shutdown in 2003. The new power supply can provide a DC high voltage of up to 45 kV with the maximum current of 9 A. To protect the klystron under sparks, a noble solid-state switch was incorporated. This paper describes the design and the performance of the new power supply. INTRODUCTION In the rf system [1] for the PF storage ring, four 500MHz damped accelerating cavities are driven by four 200-kW klystrons (Toshiba, E3774). Each of the four power supplies for the klystrons can provide a typical voltage of 40 kV with a current of 8 A. The original power supplies were fabricated during 1979-1987; each consists of an Automatic Voltage Regulator (AVR) using magnetic amplifiers, a step-up transformer and a rectifier. Klystron protection is provided by a conventional crowbar circuit [2] using three ignitrons in series. Although these power supplies have been operated well, we anticipated future difficulty in maintaining them and proposed to update them gradually. As the first step of this renewal, we upgraded one of the power supplies in the summer of 2003. High stability, low ripples, and good reliability are essential for the klystron power supply. To improve the performance mentioned above, we surveyed several modern designs in a similar manner to that presented in [3]. Initially, an existing power supply [4] for the NewSUBARU light source, which is based on an excellent high-frequency inverter technology, seemed to be very promising. However, we noticed that its modulation frequency (21.3 kHz) is close to our synchrotron frequency (23 kHz). We anticipated a potential problem that even small ripples at the modulation frequency can excite synchrotron oscillations. Thus, we adopted other design where both a conventional low-frequency converter and a noble solid-state switch for the klystron protection were used. DESIGN A block diagram of the new klystron power supply is shown in Fig. 1. A 3-phase input voltage of 6.6 kV is transformed to 440 V, and is regulated using thyristors. The voltage is then stepped up with a transformer, and is 12-phase rectified. The high voltage DC is smoothed using an LC filter, and is provided to the klystron through a fast solid-state switch. This switch is made up of 40 packages (two in a package) of Insulated Gate Bipolar Transistors (IGBT) which are connected in series. When the klystron experiences a spark, a short circuit current is initially limited by an inductance L of 10 mH. The above switch then turns the high voltage off quickly. We expect that this switch is more reliable than the conventional crowbar circuit, and is easier to maintain. In addition, the voltage drop in the input lines under sparks can be made small since this system does not shunt the currents. In a high-voltage cabinet, heater/anode power supplies are also incorporated. All control interfaces are so designed that they are compatible to those of the original one. The above power supply was designed and fabricated in Nichicon Corporation. It was then installed in the Photon Factory. The principal components of the new power supply are shown in Fig. 2, except for the high-voltage CONTROL PANEL STEP-UP TRANS. & RECTIFIER RECEIVING TERMINAL VOLTAGE REGULATOR HV CABINET