Design, Operation, and Test Results of 350 Mhz Leda Rf System

The Low Energy Demonstration Accelerator (LEDA) being constructed at Los Alamos National Laboratory will serve as the prototype for the low energy section of Acceleration Production of Tritium (APT) accelerator. The APT accelerator requires over 244 RF systems each with a continuous wave output power of 1 MW. The reliability and availability of these RF systems is critical to the successful operation of APT plant and prototypes of these systems are being developed and demonstrated on LEDA. The first completed LEDA RF systems are three, 1.2 MW, 350 MHz, continuous wave, klystrons driving a radio frequency quadrapole (RFQ). This paper present the design and test results for these RF systems including the klystrons, cathode power supply, circulators, RF vacuum windows, and RF components. The three RF systems driving the RFQ use the accelerating structure as a power combiner and this places some unique requirements on the RF systems. These requirements and corresponding operational implications will be discussed. 1 LEDA RF SYSTEM DESIGN The LEDA RF system provides RF power to an RFQ which accelerates a proton beam to a final energy of 6.7 MeV. To accomplish this acceleration the RFQ requires structure and beam power of 1900 kW. The power to the RFQ is provided by three 1.2 MW klystron amplifiers with the structure serving as the power combiner. The LEDA RF systems utilize modulating-anode klystrons with individual power supplies to maximize operating flexibility and efficiency. Each klystron has an individual power supply, and the modulating anode voltage is derived from the cathode voltage using a regulator tube. The high-efficiency klystrons are protected from reflected power by circulators. The power from a klystron is divided into four equal parts using 3 dB hybrids to reduce the power passed through the accelerating structure vacuum windows. For the RFQ waveguide runs, the power from the three klystrons is carried in three full height WR2300 through a waveguide switch to hybrid splitters. The hybrid splitters divide the power into twelve waveguide feeds that transition to half-height WR2300 and deliver the power to the RFQ through coaxial vacuum windows. Because the LEDA systems are, in part, meant to serve as prototypes for the APT plant, the approach to achieving high availability is also being prototyped on LEDA. Only two of the three RF systems connected to the RFQ are required for operation of the RFQ. The RFQ serves as the power combiner. Should any component of any one of the RFQ RF systems fail, a waveguide switch is used to isolate the failed system from the RFQ. The waveguide switch also reflects a short circuit at the appropriate phase to the RFQ irises associated with the failed system allowing the RFQ to continue to operate with the two remaining systems. This approach results in some additional requirements on the RF system. With the RFQ serving as the power combiner, it is necessary to balance the output power and phase of the two or three RF systems connected to the RFQ or unwanted reflected power will result. To accomplish this, local phase control loops are implemented around each klystron and the group of klystrons is treated as if it were a single RF source by the low level control system, which modulates the amplitude and phase of the klystron drive to control the RFQ field amplitude and phase. Also, an RF arc in a window or circulator requires that all RFQ RF systems be disabled, not just the system associated with the arc, to prevent the other RFQ systems from continuing to drive the arc through the RFQ. This logic also holds true for a crowbar or interlock trip in any one of the RF systems. The other RFQ RF systems must be disabled to prevent the disabled system from being driven through the cavity by the remaining RF systems. The low-level RF control (LLRF) system performs various functions. Foremost is feedback control of the accelerating fields within the cavity in order to maintain field stability within ±1% amplitude and 1° phase. Other functions of the LLRF control system are implementation of the local phase control loops of each klystron and RFQ resonance condition monitoring. The resonance of the RFQ is controlled by varying cooling water temperature. Because the RFQ will rapidly cool when RF is shut down, drive frequency agility in the main feedback control subsystem is incorporated to quickly restore the cavity to resonance with RF heating. 2 RF COMPONENT DESCRIPTIONS