High-power test of the single-periodic magnetically coupled standing-wave accelerating structure for a proton therapy linac

A single-periodic magnetically coupled standing-wave accelerating structure prototype operating at 3 GHz for low phase velocity particles has been designed as the first tank of the 30–230 MeV accelerating section for the compact proton therapy linac developed by the Institute of Modern Physics, Chinese Academy of Sciences. Fabrication and rf measurement of the prototype have been completed. High-power test of the prototype was performed. The cavity could operate stably at the effective input power of around 5.7 MW, corresponding to the accelerating gradient of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>38</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>MV</a:mi> <a:mo>/</a:mo> <a:mi mathvariant="normal">m</a:mi> </a:mrow> </a:math> at the pulse length of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mn>5</d:mn> <d:mtext> </d:mtext> <d:mtext> </d:mtext> <d:mi mathvariant="normal">μ</d:mi> <d:mi mathvariant="normal">s</d:mi> </d:mrow> </d:math> and the repetition rate of 70 Hz. Further increasing the duty factor to 0.1%, the accelerating gradient decreased to <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mrow> <h:mn>24</h:mn> <h:mtext> </h:mtext> <h:mtext> </h:mtext> <h:mi>MV</h:mi> <h:mo>/</h:mo> <h:mi mathvariant="normal">m</h:mi> </h:mrow> </h:math> due to the uneven cooling of each single cell. The details of the high-power test setup and preparation, the process of rf conditioning, and the high-power test results are described in this paper. Limiting factors for higher accelerating gradients with high duty factors are discussed, and two novel cooling channel schemes are described.