Abstract
In an increasing number of high-gradient linear accelerator applications, accelerating structures must operate with both high surface electric fields and low breakdown rates. Understanding the statistical properties of breakdown occurrence in such a regime is of practical importance for optimizing accelerator conditioning and operation algorithms, as well as of interest for efforts to understand the physical processes which underlie the breakdown phenomenon. Experimental data of breakdown has been collected in two distinct high-gradient experimental set-ups: A prototype linear accelerating structure operated in the Compact Linear Collider Xbox 12 GHz test stands, and a parallel plate electrode system operated with pulsed DC in the kV range. Collected data is presented, analyzed and compared. The two systems show similar, distinctive, two-part distributions of number of pulses between breakdowns, with each part corresponding to a specific, constant event rate. The correlation between distance and number of pulses between breakdown indicates that the two parts of the distribution, and their corresponding event rates, represent independent primary and induced follow-up breakdowns. The similarity of results from pulsed DC to 12 GHz rf indicates a similar vacuum arc triggering mechanism over the range of conditions covered by the experiments.
Highlights
The target accelerating gradient for the 3 TeV center of mass energy version of Compact Linear Collider (CLIC) is 100 MV=m [1], which results in surface electric fields of around 250 MV=m in the accelerating structure [2]
The choice of where to place the upper bound of the fit is somewhat arbitrary, so out of consistency we set it in each case so that the top 10% of the probability density function (PDF) was excluded
This is not inconsistent since α is a fit parameter while overall breakdown rate (BDR) was obtained by counting breakdowns and pulses, whereby the breakdowns on the tail end of the PDF excluded from the fit contribute disproportionately to the total number of pulses
Summary
The target accelerating gradient for the 3 TeV center of mass energy version of Compact Linear Collider (CLIC) is 100 MV=m [1], which results in surface electric fields of around 250 MV=m in the accelerating structure [2]. Specification limits overall luminosity loss due to breakdown to 1 percent in the 3 TeV machine Other linac applications, such as XFELs and medical accelerators, require comparable combinations of high surface fields and low breakdown rate following similar rationales [5]. The objective of the analysis presented in this paper is to look more closely at the statistical properties of the occurrence of breakdown, of how breakdowns occur in relation to each other in both time and space This was initially motivated by observation in the experimental facilities at CERN that breakdown rate is not constant in time, but rather that there are more active and quiescent periods. Our main interest in breakdown localization is to investigate spatial correlation between subsequent breakdowns
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