[论文解读] rf Breakdown with and without External Magnetic Fields

R.B. Palmer, R. Fernow+3 more|ArXiv.org|Sep 9, 2008

Particle accelerators and beam dynamics参考文献 5被引用 10

一句话总结

本文提出了一种射频腔中依赖磁场的击穿模型，其中场致发射电子在轴向磁场作用下被聚焦，导致腔体表面熔化，进而引发汽化和电离，触发击穿。该模型合理解释了805 MHz下的实验数据，并预测在201 MHz时击穿梯度显著低于中微子工厂和缪子对撞机设计所需水平，因此需要采用磁绝缘方案以抑制场致发射损伤。

ABSTRACT

Neutrino Factories and Muon Colliders' cooling lattices require both high gradient rf and strong focusing solenoids. Experiments have shown that there may be serious problems operating rf in the required magnetic fields. The use of high pressure gas to avoid these problems is discussed, including possible loss problems from electron and ion production by the passage of an ionizing beam. It is also noted that high pressure gas cannot be used in later stages of cooling for a muon collider. Experimental observations using vacuum rf cavities in magnetic fields are discussed, current published models of breakdown with and without magnetic fields are summarized, and some of their predictions compared with observations. A new theory of magnetic field dependent breakdown is presented. It is proposed that electrons emitted by field emission on asperities on one side of a cavity are focused by the magnetic field to the other side where they melt the cavity surface in small spots. Metal is then electrostatically drawn from the molten spots, becomes vaporized and ionized by field emission from the remaining damage and cause breakdown. The theory is fitted to existing 805 MHz data and predictions are made for performance at 201 MHz. The model predicts breakdown gradients significantly below those specified for either the International Scoping Study (ISS) Neutrino Factory or a Muon Collider. Possible solutions to these problems are discussed, including designs for `magnetically insulated rf' in which the cavity walls are designed to be parallel to a chosen magnetic field contour line and consequently damage from field emission is suppressed. An experimental program to study these problems and their possible solution is outlined.

研究动机与目标

• 理解并建模强轴向磁场下真空腔体中的射频击穿，该情况与中微子工厂和缪子对撞机设计相关。
• 识别在高磁场下击穿梯度降低的根本原因，特别是低频（如201 MHz）情况。
• 评估高压气体填充腔体在磁场中射频运行的可行性，尤其在电离束照射下。
• 提出并测试磁绝缘射频腔设计，以抑制场致发射及其相关损伤。
• 规划一项实验方案，用于在201 MHz、高磁场条件下验证模型并测试解决方案。

提出的方法

• 提出一种新理论模型：从腔体一侧的微凸体发射的电子在轴向磁场作用下被聚焦至另一侧，导致局部熔化，随后发生汽化和电离。
• 模拟在射频场和磁场作用下微凸体破坏与再生之间的竞争，解释频率和脉冲长度的依赖性。
• 将模型拟合至现有的805 MHz实验数据，包括带铍窗的开放式多单元腔和盒形腔的结果。
• 基于拟合模型预测201 MHz时的击穿梯度，显示其显著低于所需运行水平。
• 设计磁绝缘腔体，使腔壁与磁感线对齐，以最小化场致发射。
• 规划一种多功能测试台，采用单法兰真空罩和可访问的仪器，用于在高磁场下测试射频腔体、线圈和组件。

实验结果

研究问题

• RQ1 当真空腔体受到强轴向磁场作用时，特别是低频（如201 MHz）下，射频击穿的根本原因是什么？
• RQ2 为何高压气体填充腔体在磁场中无法抑制击穿，尽管最初有此预期？
• RQ3 电离缪子束的存在如何通过电子和离子产生影响气体填充射频腔的性能？
• RQ4 磁绝缘射频腔是否能抑制场致发射并防止高磁场下的表面损伤？
• RQ5 在3 T磁场下，201 MHz时的预测击穿梯度是多少？与中微子工厂和缪子对撞机系统所需运行水平相比如何？

主要发现

• 所提模型合理解释了现有的805 MHz实验数据，包括带铍窗和铜按钮的盒形腔的异常结果。
• 模型预测，在3 T磁场下201 MHz时的击穿梯度将显著低于中微子工厂和缪子对撞机用于相位旋转和早期冷却所需的12–15 MV/m。
• 高压气体填充腔体虽然在击穿上无磁场依赖性，但因电离和射频场引起的电子加热导致能量损耗，降低Q值。
• 对于ISS中微子工厂和低发射度缪子对撞机，由于气体填充腔体中的束流诱导损耗，最终Q值预计分别降至7.6和22。
• 模型表明，磁绝缘（即使腔壁与磁感线对齐）可抑制场致发射，防止局部熔化和电离。
• 提出一项实验计划以验证模型和解决方案，包括在3 T磁场下进行201 MHz腔体测试，以及为后续6D冷却阶段（使用10–15 T螺线管）进行组件测试。