Investigation of the dc vacuum breakdown mechanism

Abstract

Breakdowns occurring in rf accelerating structures will limit the ultimate performance of future linear colliders such as the Compact Linear Collider (CLIC). Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN to better understand the vacuum breakdown mechanism in a simple setup. Measurements of the field enhancement factor show that the local breakdown field is constant and depends only on the electrode material. With copper electrodes, the local breakdown field is around 10:8 GV=m, independent of the gap distance. The value characterizes the electrode surface state, and the next macroscopic breakdown field can be well predicted. In breakdown rate experiments, where a constant field is applied to the electrodes, clusters of consecutive breakdowns alternate with quiet periods. The occurrence and lengths of these clusters and quiet periods depend on the evolution of. The application of a high field can even modify the electrode surface in the absence of breakdown. Measurements of time delays to breakdown show two distinct populations, immediate and delayed breakdowns, indicating that two different mechanisms could exist. The ratio of these two populations depends on the conditioning state of the electrodes and on material. Gas release during breakdown is dominated by H 2 and CO. This degassing is mainly due to electron-stimulated desorption. During the quiet periods without breakdown, gases are also released but the quantities are much smaller. All the measurements presented here emphasize the crucial role of field emission in the breakdown triggering.

Advances in the Understanding of the Physical Processes of Vacuum Breakdown

“…This discussion of breakdown will continue with the common assumption [15,] that these enhance emission sites are the locations at which breakdown can occur. Indeed experimental evidence of a threshold local electric field, βE value, has been obtained in the test in the CERN dc spark system [13], which supports this assumption.…”

“…After high-gradient testing, structures are often cut open and inspected with optical and scanning electron microscopes. The craters seen in the rf test look very similar looking breakdown 'craters' are observed from sparks in the CERN dc spark system [12,13] as shown in figs. 9a and 9b.…”

X-band photonic band-gap accelerator structure breakdown experiment

“…16 for the data set corresponding to an accelerating gradient of 114 MV=m at 150 ns pulse length. A large number of breakdown events are followed immediately by a subsequent breakdown; a phenomenon that is usually observed in rf processing [32], and quantified in DC breakdown studies [33], but is readily quantifiable with the large collection of data resulting from this testing process. The percentage of immediate breakdown events ranges across all data sets from $25%-65%.…”

A passive measurement of dissociated atom densities in atmospheric pressure air discharge plasmas using vacuum ultraviolet self-absorption spectroscopy

“…This plasma (in the form of spark breakdown) is especially prevalent when the device is deployed to environments with a large background of UV or x-ray radiation which can lead to free electron buildup. Such applications include power modulators in high current sources for high power microwave (HPM) vacuum diodes, 3 mega-ampere current pulse generators in dense nuclear fusion experiments, 4 induction accelerator cavities used to generate intense electron beams for radiographic studies, 5 and as well as general problems with propagating high power electromagnetic waves for applications in communications. 6 Studies 7 have discussed the effectiveness of UV radiation in seeding the background electron density required for spark breakdown at atmospheric pressures, but most investigations are limited to studying optical wavelengths longer than 200 nm due to the experimental difficulty in measuring vacuum ultraviolet (VUV) radiation.…”