腔体双边二次电子倍增一阶与三阶模式瞬态特性对比

为评估高阶模式二次电子倍增对加速器射频腔动态建场过程的影响,采用蒙特卡罗与粒子模拟相结合的方法对金属无氧铜腔体双边二次电子倍增一阶与三阶模式的瞬态演化及饱和特性进行了数值模拟对比研究.研究结果表明:二次电子倍增过程中,二次电子数目、放电电流、放电功率、沉积功率随时间呈现以指数形式快速增长后趋于饱和振荡的物理规律;相对一阶模式,三阶模式的饱和值更低且上升时间更长.放电电流存在延时现象,由此引发了二次电子倍增过程中部分充电现象的出现.统计发现平均放电功率等于平均沉积功率,三阶模式的放电功率大约是一阶模式1%的水平,由此判断加速器腔体动态建场过程中高阶模式的影响较小.二次电子倍增过程中,一阶模式呈现单电子束团加速运动特性,而三阶模式则呈现加速/减速/加速及多电子束团相向运动的复杂特性;二次电子倍增进入饱和后,三阶模式的空间电荷效应明显弱于一阶模式.

Comparison between the 1st and 3rd order mode temporal characteristics of two-sided multipactor discharge in cavity

For investigating the influence of high order two-sided multipactor discharge on the accelerator field-building process, the temporal characteristics of the 3rd order two-sided multipactor discharge in oxygenfree copper cavity are studied numerically. The particle-in-cell and Monte-Carlo methods are used in the simulation and the characteristics of the 1st order mode are also studied for comparison. The numerical results can be concluded as follows. In the multipactor discharge evolution, the electron number, discharge current, deposited and discharge power increase exponentially and tend to be saturated. At the saturation stage of the 3rd order mode, the values of electron number, discharge current, deposited and discharge power are lower than at the saturation stage of the 1st order mode. Meanwhile, the rising time of waveform in the 3rd order mode is longer than in the 1st order mode. There is a time-delay phenomenon in the waveform of discharge current, which results in a partial charging process in multipactor discharge. The average value of the discharge power is equal to the average deposited power. The value of discharge power in the 3rd order mode is about 1% of that in the 1st order mode. Therefore, the 3rd order mode is not significant in accelerator field-building process compared with the 1st order mode. The characteristic of the 1st order two-sided multipactor discharge is the accelerated motion of single electron beam, while that of the 3rd order is the complex accelerated-decelerated-accelerated motion of multi-electron beams. When the multipactor discharge enters into the saturation stage, the space charge effect of the 3rd order mode is not stronger than that of 1st order mode.