Feasibility Study of Resistivity Measurement of Metal Surfaces to Address Potential Dislocations Caused by Surface Conditioning
Abstract: High electrical fields are needed inside the accelerating cavities of particle accelerators in order to accelerate the particles to higher energies in shorter distances. But high electrical fields will lead to electrical breakdowns. The electrical breakdowns are events in which the insulating proprieties of a typically electrically insulating medium are weakened due to the presence of high electrical fields. One of the best insulating mediums is the ultra high vacuum because there are no molecules that will ionize and that will conduct the electricity. But even in vacuum, there will be electrical breakdowns. They are called vacuum arc breakdowns. The conducting medium in this case is given by the ions and the electrons emitted from the metal surface of the electrodes that create the high electrical fields. It has been observed that applying repeatedly high electrical fields on the surface of the electrodes reduces the number of breakdowns. This process is called conditioning. One explanation is that the large electric fields create dislocations near the surface of the metal that reduce the probability of having new vacuum arc breakdowns. These dislocations should also increase the electrical resistivity of the metal near its surface. To test if new dislocations are formed during conditioning, precise measurements of the surface resistivity are needed. These measurements will be made with radio pulses in the GHz range. In this range of frequencies, the electromagnetic fields penetrate only a few microns inside the surface of the metal and it will be possible to measure only the resistivity of the metal near its surface. The surface resistivity data is encoded in the quality factor (Q-factor) of a resonant cavity. This parameter describes how fast the energy is dissipated inside the cavity. A larger surface resistivity leads to a larger dissipation of energy in the walls of the cavity and to a lower Q-factor. It is advantageous to perform the measurements in cryogenic conditions because the increase in resistivity due to the formation of dislocations is much more pronounced at very low temperatures. The measurements are planned for the discharge system available in FREIA laboratory, that consists of two electrodes, separated by a small gap (60 µm), and placed inside a cryostat cooled with liquid helium. In this thesis, I describe the algorithms used to extract the Q-factor from experimental data and the results of some experiments done using the electrodes and test cavities. Small changes in resistivity (less than 0.6%), induced by temperature changes, were measured. The final chapter explores the results of the 3D EM simulations, where the electrode system in the cryogenic setup in FREIA laboratory is modified to act as a resonant cavity.
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