Materials Considerations for a High Power-Density Accelerator

A 100-mA 50-MeV H- accelerator is being designed at Los Alamos. The accelerating structures will operate at 425-MHz and will consist of a radio-frequency quadrupole (RFQ) to 2-MeV and a drift-tube linac (DTL) from 2 to 50-MeV. Design parameters have been specified to match the maximum operating capabilities of the rf system: 2-ms pulse length and 6% duty factor. The accelerating gradient in the DTL will be 4-MV/m; the maximum electric field will be approximately 1.2 times the Kilpatrick limit. These design parameters are substantially more ambitious than those of the accelerator test stand (ATS). The larger accelerating gradient and increased duty factor will increase the average power density in the DTL to approximately 25 times the design values for the ATS DTL. The increased duty factor will raise the average power density on the proposed RFQ to approximately 6 times the operating values of the ATS RFQ. The instantaneous power density on the drift tubes is predicted to be 60 W/sq cm, and the average power density is predicted to be 4 W/sq cm. These power densities are not excessive, but do represent design challenges in specific areas. Some components where thermal difficulties may be encountered include RFQ vanes, drift tubes, post couplers, slug tuners, rf seals, and tuning bars. That copper will be used on the rf-structure surfaces is not in question. Concern with residual activation and thermal management forces the investigation of materials other than the traditional carbon- and stainless-steel base materials used at Los Alamos.