Abstract

The experimental work presented in this thesis is done to develop an innovative procedure to create a protective nanostructured coating inside the X-band radio frequency cavity, a key component in future particle accelerator. The scope of the multilayer coating is to prevent the breakdown due to high electric and magnetic field. In fact the electrical discharges damage, in irreversible way, the internal surface of the cavity and compromise the final operation of the device. The keen interest on the topic is due to decrease the length and the cost of the next generation linear accelerator. To do this it is essential to enhance the performance of X-band Linacs up to 100MV/m accelerating gradient and to maintain, high as possible, the electrical breakdown reliability. Several studies are made on different materials in order to develop these cavities [1] [2], but the use of physical vapor deposition technique (PVD), to obtain nanostructured coating directly on internal wall of these small sized cavities is not reported in literature. The size of the cavities is of order of few millimeters and the iris aperture ranges from 2 to 6mm: for this reason the direct PVD coating is not possible. Hence a mandrel, that is the negative shape of the cavity, is first coated using PVD technique and finally chemically dissolved after copper electroforming[3]. The novel nanostructured coating is a multilayer composed by two high purity and immiscible metals. One is Copper to guarantee electrical conductivity of the cavity and the second is Molybdenum because it is a refractory metal. Moreover the choice of immiscible materials is important, because these materials do not form alloy during the deposition phase. Keeping a well-defined interface is important to guarantee a barrier effect to the motion of the defects inside the cavity’s material[4][5]. The experimental part of the thesis is divided in three different parts: design and setup of the PVD deposition system, plasma discharge analysis and, finally, the characterization of the coatings. This work is a collaboration between Industrial Engineering Department (University of Trento) and the National Laboratory of Legnaro (National Institute of Nuclear Physics LNL-INFN), but this research involves several institutes in different countries: SLAC (USA), KEK (Japan) and UCLA (Los Angeles USA).