Investigation of the energy dependence of breakdown properties with a DC spark setup

The Compact LInear Collider (CLIC) study is a site independent feasibility study aiming at the development of a realistic technology at an affordable cost for a future linear electron-positron collider. The European Organization for Nuclear Research (CERN) is one of the collaborators for the CLIC study.The CLIC Test Facility (CTF3) positioned at CERN provides testing of this technology, including the testing of the proposed radio-frequency (RF) structures in a two-beam concept to produce the necessary accelerating electric field as high as 100 MV/m to reach the goal of a nominal total energy of 3 TeV. One problem at such high accelerating fields is electrical discharges, i.e. sparks, damaging the inside of the RF structures as well as deflecting the trajectories of accelerated particles. A Direct Current (DC) spark test setup is in use at CERN to aid the understanding of electrical discharges under vacuum conditions, also called vacuum arcs. In contrast to the more complex CTF3 setup, the DC spark setup is simple, fast and is solely devoted to study the discharge phenomenon, in-between two electrodes in ultra high vacuum. For this thesis, several parameters governing a vacuum breakdown have been studied at different discharge energies, i.e. the energy that is available for a spark. The energy dependence of several parameters was examined for two RF structure candidate metals, copper and molybdenum, with discharges in the energy range 1 to 1000 mJ. The different parameters examined in this work are the size of the surface damage on the electrodes, the initial number of sparks needed to reach a stable, saturated field, and the saturated field. The size of the surface damage was found to increase for increasing energy for both materials. At any energy, Cu reaches its saturated field within 2-8 sparks, while Mo need 30-50 sparks to achieve a saturated field. For Cu the saturated field decreases with increasing energy, contrary to Mo where the saturated field increases with increasing energy. Furthermore, the field enhancement factor and the corresponding local field have also been studied. The field enhancement factor is seen to increase with increasing energy and the local field is found to be constant for Cu. For Mo, the field enhancement factor is constant, while the local field is increasing with increasing energy. The lower limit of the discharge energy feasible with the present DC setup has been identified, as well as limitations of measuring the field enhancement factor on the cathode surface. In addition, another approach to explain the process of DC conditioning has been presented as well as a possible criterion to predict and thus avoid breakdowns.