Abstract
The rf accelerating structures of the Compact Linear Collider (CLIC) require a material capable of sustaining high electric field with a low breakdown rate and low induced damage. Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN in order to test candidate materials and surface preparations, and have a better understanding of the breakdown mechanism under ultrahigh vacuum in a simple setup. Conditioning speeds and breakdown fields of several metals and alloys have been measured. The average breakdown field after conditioning ranges from $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ for Al to $850\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ for stainless steel, and is around $170\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ for Cu which is the present base-line material for CLIC structures. The results indicate clearly that the breakdown field is limited by the cathode. The presence of a thin cuprous oxide film at the surface of copper electrodes significantly increases the breakdown field. On the other hand, the conditioning speed of Mo is improved by removing oxides at the surface with a vacuum heat treatment, typically at $875\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for 2 hours. Surface finishing treatments of Cu samples only affect the very first breakdowns. More generally, surface treatments have an effect on the conditioning process itself, but not on the average breakdown field reached after the conditioning phase. In analogy to rf, the breakdown probability has been measured in dc with Cu and Mo electrodes. The dc data show similar behavior as rf as a function of the applied electric field.
Highlights
The feasibility of the future 12 GHz multi-TeV eþeÀ Compact Linear Collider (CLIC) is under investigation at CERN [1,2,3,4]
The dc breakdown experiments can provide measurements of quantities such as breakdown fields and breakdown rates which are made with rf cavities
Even if these quantities depend on the specific features of the dc setup, these experiments could help selecting the best material for the production of rf accelerating structures
Summary
The feasibility of the future 12 GHz multi-TeV eþeÀ Compact Linear Collider (CLIC) is under investigation at CERN [1,2,3,4]. In order to limit this linear collider to an acceptable length, extremely high accelerating gradients of the order of 100 MV=m are required With such fields, rf breakdowns are likely to occur and produce damage on the accelerating cavities. Each structure must have a breakdown probability (or breakdown rate) as low as possible, typically in the order of 10À7 breakdowns per rf pulse In this context, a dc breakdown study is underway at CERN in order to test candidate materials and surface preparations, and to have a better comprehension of the breakdown mechanism under ultrahigh vacuum [5,6,7,8]. The dc breakdown rate measurements have been performed on Cu and Mo, and the results are compared to the rf data
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