Surface Analysis of OFE-Copper X-Band Accelerating Structures and Possible Correlation to RF Breakdown Events

Abstract

X-band accelerator structures meeting the Next Linear Collider (NLC) design requirements have been found to suffer vacuum surface damage caused by radio frequency (RF) breakdown, when processed to high electric-field gradients. Improved understanding of these breakdown events is desirable for the development of structure designs, fabrication procedures, and processing techniques that minimize structure damage. RF reflected wave analysis and acoustic sensor pickup have provided breakdowns localization in RF structures [1] [2]. Particle contaminations found following clean autopsy of four RF-processed travelling wave structures, have been catalogued and analyzed. Their influence on RF breakdown, as well as that of several other material-based properties, will be discussed.

Electromigration occurences and its effects on metallic surfaces submitted to high electromagnetic field: A novel approach to breakdown in accelerators

“…The melted areas are usually not observed in the high current density area, but close to the high electrical field although not at its maximum, which suggests that current, field and temperature gradients all play a role. Breakdowns localized in a high magnetic field area, hence low electric field area, seem to be rather triggered by the presence of foreign material or perhaps by the presence of a gap between joint elements [ [10,141]].…”

“…Note that breakdown at grain boundaries was also observed at SLAC on similar Cu made RF structures. In this case segregation (diffusion) of sulfur compounds inside the grain boundaries was evidenced [10,11].…”

Localization of RF Breakdown in Copper RF Structure by the Use of Acoustic Sensors

“…Visual inspection and an autopsy were performed. On the predicted cell, we discover more craters due to a more intense activity as well as a piece of aluminium foil sitting on the bottom of the structure [20], labelled 180 • in Fig.21. Sensors were located in a similar way as shown in Fig.1, 0 • being the top of the structure.…”

Thoughts on Limitation in the Use of Acoustic Sensors in RF Breakdown Localization

“…We have observed during autopsy of some RF structures, cf. [1] and [33], slip bands. This matter was investigated separately.…”