High-Power Operation of Accelerator Structures at Liquid Nitrogen Temperature

The microwave properties of two S-band resonant structures were measured under pulsed high-power operation at liquid nitrogen temperature. One was a simple cylindrical pillbox cavity operating in the TM010 mode, and the other was a high surface gradient side-coupled standing-wave structure. Both were machined from OFHC copper. At low power levels, the Q of both structures measured at liquid nitrogen temperature (-197°C) increased by the same factor over that at 20°C. When measured at high power levels, the enhancement factor was much lower. The low-temperature Q was found to be a function of the peak power of the RF pulse, rather than the average power level. The test method is described, and results of the tests and theoretical studies are presented.