射频击穿电弧建模
Modeling rf breakdown arcs
摘要：
我们描述了在805 MHz的射频加速器腔中的一些自洽机制的击穿。我们分为三个阶段的击穿过程：1）我们使用静电拉伸应力引起的断裂的分子动力学来模拟表面失效，2）我们使用细胞代码中的粒子来模拟负责等离子体引发和等离子体生长的中性粒子的电离，以及3）我们通过假设类似于单极电弧的过程来模拟表面损伤。我们发现，冷，密集的等离子体与表面接触产生非常小的德拜长度和非常高的电场在一个大的区域，符合单极电弧的行为，虽然单极电弧严格定义的等势边界。这些高磁场产生了强烈的侵蚀机制，主要是自溅射，与我们看到的陨石坑形成相一致。我们使用OOPIC模型来估计非常高的表面电场在稠密的等离子体和测量这些领域使用电流体力学参数与所施加的电场的表面损伤的尺寸。我们还提出了场致发射体的大增强因子的几何解释，这与暴露于高场的表面上明显没有晶须是一致的。我们得到的增强因子，当结合福勒Nordheim分析产生一致的图片的击穿和场发射从表面在7 - 10 GV/m的局部字段。我们表明，我们从OOPIC获得的等离子体的增长率是一致的腔短路电流使用X射线测量的增长率。我们相信，这里介绍的射频击穿电弧的一般情况，应该直接适用于更大类的真空电弧。从等离子体模拟的结果包括作为一个指南，该模型的实验验证。
英文摘要：
We describe breakdown in 805 MHz rf accelerator cavities in terms of a number of self consistent mechanisms. We divide the breakdown process into three stages: 1) we model surface failure using molecular dynamics of fracture caused by electrostatic tensile stress, 2) we model the ionization of neutrals responsible for plasma initiation and plasma growth using a particle in cell code, and 3) we model surface damage by assuming a process similar to unipolar arcing. We find that the cold, dense plasma in contact with the surface produces very small Debye lengths and very high electric fields over a large area, consistent with unipolar arc behavior, although unipolar arcs are strictly defined with equipotential boundaries. These high fields produce strong erosion mechanisms, primarily self sputtering, compatible with the crater formation that we see. We use OOPIC modeling to estimate very high surface electric fields in the dense plasma and measure these field using electrohydrodynamic arguments to relate the dimensions of surface damage with the applied electric field. We also present a geometrical explanation of the large enhancement factors of field emitters.This is consistent with the apparent absence of whiskers on surfaces exposed to high fields. The enhancement factors we derive, when combined with the Fowler-Nordheim analysis produce a consistent picture of breakdown and field emission from surfaces at local fields of 7 - 10 GV/m. We show that the plasma growth rates we obtain from OOPIC are consistent with growth rates of the cavity shorting currents using x ray measurements. We believe the general picture presented here for rf breakdown arcs should be directly applicable to a larger class of vacuum arcs. Results from the plasma simulation are included as a guide to experimental verification of this model.
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