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RF Test Facility (RFTF) has been constructed to support present and future needs in testing, processing and conditioning of various high power RF components of normal conducting and superconducting systems at the SNS. The facility is expected to have additional subsystems that are needed for complete superconducting RF (SRF) testing and processing. A full capacity high voltage converter modulator (HVCM) with 11 MW peak power at 8% duty cycle has been installed for driving one or two klystron RF amplifiers. The waveguides are completed in WR-2100 and WR-1150 for the 402.5 MHz and 805 MHz klystrons being used in the SNS. The 805 MHz system has been used for RF processing the coaxial fundamental power couplers (FPCs) for the SNS superconducting linac (SCL) [1]. The high power RF system can be reconfigured or modified for various tests and conditioning processes along with the neighboring cryo-plant.

The APS [1,2] linac beam energy must be stable to within 21% to match the energy acceptance of the positron accumulator ring. The klystron pulse modulators [3,4] must therefore provide a pulse-to-pulse repeatability of 0.1% in order for the beam to have the required energy stability. The modulators have had difficulty achieving the necessary repeatability since the pulse forming network (PFN) charging scheme does not include a deQing circuit. Several of the major charging circuit components are also less reliable than desired. In order to increase operating reliability and to improve pulse-to-pulse stability, it is planned to replace the high voltage power supplies in all modulators with constantcurrent power supplies. A new modulator charging supply that contains two EM1 [SI series 303 constant-current power supplies was constructed. Each of these EM1 supplies delivers 1.5 A at up to 40 kV. One supply is sufficient for linac operation at up to 15 Hz, and two supplies in parallel enable linac operation at the nominal rf repetition rate of 60 Hz. This paper discusses test results from the new modulator, and ais0 describes the existing modulators and their performance limitations.