Abstract
Choke-mode accelerating structures are higher-order-mode (HOM) damped structures that are relatively simple to fabricate and have low surface magnetic fields. $C$-band choke-mode accelerating structures have been successfully applied in free electron lasers. However, studies on $X$-band choke-mode structures for the main linac of the Compact Linear Collider (CLIC) currently remain at the theoretical design stage, and the high-gradient performance of a choke remains unknown. In this study, we designed and fabricated five different single-cell choke-mode accelerating structures and subjected them to high-gradient tests to study related radio frequency (rf) breakdown characteristics. High electrical field and small choke gap caused serious breakdowns in the choke, which mainly limited high-gradient performance. The choke-mode accelerating structure reached $121\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown rate of $8.30\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ per pulse per meter and a flat top pulse width of 250 ns. A new quantity proposed to give the high-gradient performance limitation of choke-mode accelerating structures due to rf breakdown. The new quantity was obtained from the summary of the high-gradient experiments and can be used as reference for high-gradient choke-mode accelerating structure design.
Highlights
The choke-mode accelerating structure was first proposed by Tsumoru Shintake [1]
The X-band choke-mode accelerating structure is currently being studied as an alternative design for the accelerating structure of the compact linear collider (CLIC) main linac in collaboration between CERN and Tsinghua University [2,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]
One of the choke-mode structure was based on the current prototype choke-mode accelerating structure design for CLIC [2,13,31], whereas the reference structure was based on the single-cell structure with the iris aperture of 3.75 mm and the iris thickness of 2.6 mm from SLAC [50,51]
Summary
The choke-mode accelerating structure was first proposed by Tsumoru Shintake [1]. It was designed to use choke to contain the fundamental accelerating mode inside the cavity while allowing for parasitic higher-order-modes (HOMs) to be propagated to rf loads where their energy is absorbed. Single-cell experimental study is an important way to study the performance of accelerating structures It was first conducted by V.A. Dolgashev from SLAC National Accelerator Laboratory (SLAC) and Y. Given that single-cell accelerating structures are easy to fabricate and have short high-gradient test time, experimental studies are suitable for comparing the high-gradient performance of different accelerating structures [53,54]. Five different choke-mode single-cell accelerating structures and one reference structure were designed, fabricated, assembled, and tuned in Tsinghua University [55]. A new quantity was proposed to give the high-gradient performance limitation of choke-mode accelerating structures due to rf breakdown
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