室温CH腔体的单元腔近似优化

Optimization of room temperature CH-cavity with cell-cavity approximation

CH（cross-bar H-typestructure）结构是近几年提出的一种适用于低β的新型DTL（drift tube linac）加速结构，同IH（interdigital H-type structure）结构相比，CH结构可以工作在更高的频率（150～700MHz）下，从而可以得到更高的输出能量（150MeV）。由于DTL腔体为准周期结构，通过对单元腔的MWS（microwave studio）模拟及优化，得到了工作频率为350MHz，单核能从6MeV到66MeV时的腔体并联阻抗及其它腔体参数，并对腔体单元数对腔体特性参数及谐振频率的影响做了定性分析。分析表明：对于CH结构，其有效并联阻抗远大于传统的DTL结构，对于350MHz的工作频率，在6MeV时将近100MΩ／m，即使在能量高达66MeV时，其有效并联阻抗也大于40MΩ／m；单元腔近似是一种非常有效的分析DTL加速结构的方法，单元腔计算结果和整腔计算结果相比，谐振频率的相对偏差小于1％；对于有效并联阻抗的计算，误差也在10％之内。

Compared with the IH structure （110-mode）, the CH structure （210-mode） can operate at higher frequencies （150-700 MHz） and can accelerate ions to higher energies （up to 150 MeV）. Detailed microwave studio（MWS） simulations were performed for this structure. Since a multi-gap cavity can be approximated as a quasi-periodic structure, it is possible to analyze one βλ/2-cell at an energy corresponding to the cavity centre. A reduced copper conductivity of 85 % was adopted in effective shunt impedance calculations. Geometry variations with respect to RF frequency and shunt impedance can be performed rapidly by that method in the first stage of optimization. Using the transit time factor calculated by the beam dynamics simulation code LORASR, effective shunt impedances from 100 MΩ/m down to 45 MΩ/m were obtained for the energy range from 6 MeV to 66 MeV by this method. The RF frequency was 352 MHz. A systematic analysis of the influence of the cell number in long CH-cavities on the effective shunt impedance is presented and the following conclusions can be drawn from the analysis： Cell cavity approximation is an effective method in CH structure simulation. Compared with the whole cavity simulation results, the errors of the cell approximation for frequency and effective shunt impedance are within 1% and 10% respectively.