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 RF and electron beam tests comparing two similar geometry linac structures, each tested with a 17 gigahertz dual resonant ring power amplifier under the same RF 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 low carbon stainless steel. These enhanced gradient results present a compelling argument for avoiding the use of copper surfaces in the beam apertures of high gradient linacs and encourages the investigation of strong candidate, high temperature brazed iris materials that retain low surface resistivity and low vapor pressure characteristics during operation at high pulse temperatures. During this Phase I effort, in anticipation of high power testing in Phase II, a full length linac structure having molybdenum inserts brazed into the copper disc irises shall be designed such that the comparatively low resistivity of this refractory metal, and the desirable high temperature characteristics, can be expected to provide Q values acceptable for a linear collider while further enhancing the high gradient performance by the removal of copper surfaces from the high electric field, dark current interception regions of the beam aperture irises. Commercial Applications and Other Benefits: The Phase I and Phase II efforts, if successful, would lead to a better understanding of radio-frequency breakdown in traveling wave linear accelerator mixed metal structures, and result in linac operation at higher gradients while avoiding structure lifetime foreshortening caused by surface erosion. This would have a positive impact on the design of future linear collider systems and compact commercial accelerators for medical, radiographic and homeland security applications.