Abstract
Acceleration gradient is a critical parameter for the design of future TeV-scale linear colliders. The major obstacle to higher gradient in room-temperature accelerators is rf breakdown, which is still a very mysterious phenomenon that depends on the geometry and material of the accelerator as well as the input power and operating frequency. Pulsed heating has been associated with breakdown for many years; however, there have been no experiments that clearly separate field and heating effects on the breakdown rate. Recently, such experiments have been performed at SLAC with both standing-wave and traveling-wave structures. These experiments have demonstrated that pulsed heating is limiting the gradient. Nevertheless the $X$-band structures breakdown studies show damage to the iris surfaces in locations of high electric field rather than of high magnetic field after thousands of breakdowns. It is not yet clear how the relative roles of electric field, magnetic field, and heating factor into the damage caused by rf breakdown. Thus, a dual-moded cavity has been designed to better study the electric field, magnetic field, and pulsed heating effects on breakdown damage.
Highlights
The highest practical acceleration gradient achieved in a metal structure is about 108 V=m, far below the boundary material ionization gradient of about 1010 V=m. It is limited by material damage and rf breakdown in cavities as well as local surface field enhancement factor
From the high-power test results on the X-band single cell standing-wave (SW) structures and travelingwave (TW) structures for JLC/NLC and CLIC, it indicates that the rf breakdown rate has a very strong dependence on electric field of Ea 30, where Ea donates the gradient at fixed pulse length [3]
The above high-power test results on SW and TW cavities establish a clear relationship between pulsed heating and rf breakdown rate
Summary
The highest practical acceleration gradient achieved in a metal structure is about 108 V=m, far below the boundary material ionization gradient of about 1010 V=m It is limited by material damage and rf breakdown in cavities as well as local surface field enhancement factor. Studies using single cell cavities [6,7] and multicell cavities [8] show damage to the surfaces on the irises in locations of high electric field instead of high magnetic field after thousands of breakdowns. All the experimental results are still repeatable with these cavities, which show that the cavities are not damaged by the cumulative breakdowns yet This is the first clear demonstration that pulsed heating is limiting structure performance.
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