NSRRC Activity Report 2023

102 NSRRC ACTIVITY REPORT 2023 noise levels from high-voltage switching in traditional klystron-type RF transmitters, further minimizing disturbances to the electron beam. Figure 2 illustrates the architecture of the 300-kW SSPA. This RF transmitter comprises a total of four 80-kW SSPA towers, achieving the required output of 300 kW through a two-stage power combination. Each 80-kW tower consists of 100 SSPA modules, each capable of delivering a maximum output power of 1 kW. These modules, designed by the NSRRC, include planar balun push-pull power amplification circuits, the RF power chip BLF578XR, a circulator, a strip-line load, and analog circuits for monitoring purposes. In addition to the SSPA modules, each tower includes two 2-way isolated power dividers/ combiners, ten 10-way 300-W nonisolated power dividers, ten 10-way 10-kW nonisolated power combiners, one 10- way 80-kW nonisolated final-stage coaxial power combiner, two 96-kW DC power supply racks, and a programmable logic controller with a man machine interface. This 300-kW SSPA RF transmitter has been in continuous operation at the TPS since August 2023, where it is undergoing regular module checks during shutdown maintenance periods. After approximately five months of operation with an RF power output of approximately 250 kW, a total of nine SSPA modules were damaged. However, there were no RF trip events caused by the SSPA during the user beam time, indicating that the occurrence of a few module failures would not impact the system operation. This finding highlights an enhancement in the reliability of the RF system, as the transmitter operation remains unaffected despite a few module failures. Each of the four 80-kW SSPA towers undergoes a two-stage power combination at a 1:1 ratio. After ensuring identical phase and power amplitudes between the towers, the overall combining efficiency exceeds 93%. With this 300-kW SSPA RF transmitter, the DC drain voltage of the chips can be adjusted between 42 and 56 V based on the operational conditions, impacting both the maximum output power and efficiency. If the DC voltage is adjusted to 48 V, the 300-kW SSPA RF transmitter can output a maximum of 300 kW to the load. At this output level, the overall AC-to-RF efficiency, including the power combination, is approximately 50%. However, as the output power decreases, the efficiency decreases accordingly. Considering the current operation at an RF output of approximately 250 kW, selecting a drain voltage of 45 V for operation in a single 80-kW SSPA tower yields a DC-to-RF efficiency of approximately 58%. Considering the power combination, the overall AC-to-RF efficiency of the entire 300-kW SSPA RF transmitter reaches 50%, which is slightly better than the power efficiency of the original Klystron-based system at the same output power (approximately 50% for DC to RF). Further enhancements in efficiency can be pursued in the future to achieve energy-saving benefits. In terms of system stability, the original klystron-based RF transmitter generates high voltage by sequentially switching 86 sets of 800 V DC voltage modules, introducing approximately 7 kHz of high-frequency noise into the RF output power. This noise can affect the stability of the orbit of the electron beam. However, the SSPA RF transmitter, which does not rely on high voltage, avoids this high- frequency noise and thus prevents any impact on the stability of the orbit of the electron beam. Figure 3 shows the spectra of the electron beam orbit when using the klystron-based RF transmitter and the SSPA RF transmitter. It is evident that when operating with the SSPA RF transmitter, the orbit of the electron beam in the horizontal direction shows no disturbance at a high frequency of Fig. 2 : Architecture of the 300-kW SSPA RF transmitter.