A 17 GHz High Gradient Linac having Molybdenum Surfaces in the Peak Electric Field, Dark Current Interception Regions of the Structure

While design improvements have been achieved, considerable difficulty continues to be encountered in the development of accelerator structures for future linear colliders required to operate with long term reliability at loaded average accelerating gradients of 100 megavolts per meter and radio-frequency pulse lengths in the range of 150 to 250 nanoseconds. A recently completed series of high power radio-frequency and electron beam tests comparing two similar geometry linac structures, each tested with a 17 gigahertz dual resonant ring power amplifier under the same radio-frequency conditions and breakdown rates, has demonstrated an electron beam verified, 25 percent increase in the accelerating gradient of a 22-cavity linac structure by replacing the copper surface around the periphery of each contoured disc iris with high temperature brazed stainless steel. This higher gradient performance presented a compelling argument for avoiding the use of copper surfaces in the beam apertures of high gradient linac structures, and encouraged investigation of alternate metals to further improve the gradient and to avoid the undesirable high losses associated with stainless steel. Ongoing studies and radio-frequency measurements have supported molybdenum as a strong candidate for replacing the copper irises because of the low surface resistivity and superior strength and thermal properties. The fabrication and evaluation of brazed copper/molybdenum short test structures during the Phase I effort confirmed the low loss radio-frequency characteristics of molybdenum, and verified that the structure resonant frequencies remained essentially unchanged after successive braze cycles. Also, during Phase I, microwave and physical design parameters, compatable with an existing dual resonant ring high power test amplifier, were established for a 2/3 mode, 22-cavity, copper/molybdenum linac structure, including the dual feed racetrack shaped input and output couplers. During Phase II, detailed designs and fabrication of the copper/molybdenum accelerator structure shall be completed, and the final tuned test linac and resonant ring system shall be installed in an existing 17 gigahertz linac test facility. A further Phase II objective is to complete an extended sequence of high power tests so that the high gradient performance of the new copper/molybdenum structure can be compared directly with that of the recently tested all-copper and copper/stainless steel structures. Commercial Applications and Other Benefits: Success of the Phase II effort would lead to a better understanding of radio-frequency breakdown in traveling wave linear accelerators and would provide design guidance for further miniaturization of commercial accelerators for medical, radiographic and homeland security applications.