Short Pulse High Power Test Study Of High-gradient Accelerating Structure

Increasing the accelerating gradient of radio-frequency（RF）linear accelerator（linac）can reduce the size and cost of accelerator devices.One of the important limitations of achieving higher gradient is RF breakdown,which will debase the beam quality or even cause the beam loss.Critical RF breakdown even might damage the cooper surface permanently.The statistical studies of RF breakdown propose that accelerating gradient and input RF pulse width have a negative correlation:E³⁰?⁵/=（89）.This conclusion indicates that the accelerating gradient can be increased by reducing the RF pulse width.No works about X-band breakdown experiment with input RF pulse length less than 10ns was reported to date.To explore the performance of X-band high gradient accelerating structures in short pulse case,a method to calculate accelerating gradient in time domain was developed in this thesis.An 11.7 GHz travelling-wave（TW）single cell accelerating structure was optimized based on this method.This accelerating structure was shipped to Argonne Wakefield Accelerator（AWA）after the fabrication and brazing conducted in Tsinghua University.RF measurements were conducted in AWA.The experiment scheme of X-band metallic power extraction and transfer structure（PETS）in AWA was improved by replacing the RF pickup with the 11.7 GHz directional coupler.The output waveform of PETS had 3-ns rising time,3-ns flat top,and 3-ns falling time.The maximum power achieved 400 MW in the experiment.The accelerating gradient of the single cell structure achieved 313 MV/m in the high power test.Electron multipacting and RF breakdown was observed in the experiment.The secondary pulse of transmission waveform was eliminated when a breakdown happened,but the main pulse was not deformed.This means that the RF breakdown in short pulse case will not affect the beam accelerating.For the purpose of studying the performance of this single cell structure in a range of pulse width,we scaled its frequency to 11.424 GHz and conducted high power test in Tsinghua X-band high power test facility.The transmission waveforms of RF breakdown cases showed that the breakdown affecting on transmission waveform pulse width decrease with input pulse width,agreeing well with the 11.7 GHz experiment.Distributed-coupling accelerating structures made by two-halves have a promising potential of short pulse high gradient accelerating.This thesis describes the design,fabrication,and high-power test of a 12-cells X-band travelling wave accelerating structure made by two-halves.This structure was brazed with silver-cooper alloy and can operated at～94 MV/m accelerating gradient with a pulse width of 90 ns.This is an attempting of fabrication and brazing of a two-half structure for the distributed two-half structure in the future.