Performance of the Klystron Modulators at the S-Band Test Facility at DESY

The klystrons at the S-band test facility at DESY require high voltage pulses of 550kV at a flat top pulse duration of 3$\mu$s, at a current of 700A and a repetition frequency of 50Hz. Two HV pulse modulators with a nominal power of 375MW were built. The design and results of the commissioning of the first system were described in [1]. The second system came into operation and was operated at an output power up to 475MW (610kV, 780A). In addition it was modified to investigate capacitor charging power supplies, which might be used in a linear collider tunnel. This type of power supply promises a better efficiency than the conventional resonant charging method and also saves space in the linear collider tunnel. This paper describes the operation experience with the HV modulators at the test facility especially with the second system. It reports the modifications, which were required to operate the modulator with the new type of HV power supply and presents results of the operation