Thermal design of drift tubes for high-gradient linacs

We designed high-intensity drift tube linacs at Los Alamos using copper drift tubes capable of very high duty-factor operation, limited only by beam-loss heating in the bore. The drift-tube bodies are cooled by a peripheral, full-length cooling jacket that effectively clamps the drift tube at the average coolant temperature and shields the samarium-cobalt quadrupole from excessive heating caused by rf dissipation. The bore tube is thick-walled copper that is split to provide a gap for thermal-expansion, beam-diagnostic instrumentation, and vacuum relief of the drift-tube body. The magnet pole pieces are isolated from the bore tube by a 0.2-mm thermal gap that is very effective in preventing beam-loss heating from degrading the magnet properties of the quadrupoles. We discuss thermal studies that explore the capabilities of this design concept. The design features of the quadrupoles include a technique for precision shimming using captive screws and guide slots. We also present results of tests on prototype quadrupole assemblies