Diagnostic and Detectors for Charging and Damage of Dielectrics in High-gradient Accelerators

TL;DR

This paper develops a microwave-based detector to analyze charging, conductivity, and damage in dielectric materials used in high-gradient accelerators, addressing issues of beam-induced effects and material degradation.

Contribution

It introduces a novel microwave resonant cavity method for real-time detection of dielectric charging and damage in accelerator applications, with improved sensitivity and signal processing techniques.

Findings

Capable of detecting 0.1% changes in dielectric constant

Able to identify charge scraping of 0.3nC from dielectric walls

Demonstrated effectiveness in a plasma test case

Abstract

The research is aimed to address issues of analysis and mitigation of high repetition rate effects in Dielectric Wakefield Accelerators, and more specifically, to study charging rate and charge distribution in a thin walled dielectric wakefield accelerator from a passing charge bunch and the physics of conductivity and discharge phenomena in dielectric materials useful for such accelerator applications. The issue is the role played by the beam halo and intense wakefields in charging of the dielectric, possibly leading to undesired deflection of charge bunches and degradation of the dielectric material. The detector that was developed is based on measurement of the complex electrical conductivity, which would appear as a transient phenomenon accompanying the passage of one or more charge bunches, by observing the change of complex admittance of a resonant microwave cavity that is fitted around the dielectric tubing. The detector also can detect permanent damage to the material. During initial stage of development, microwave apparatus was built and signal processing was developed for observing time-dependent electronic charging of dielectric surfaces and/or plasmas located on or near the inner surface of a thin-wall hollow dielectric tube. Three frequencies were employed to improve the data handling rate and the signal-to-noise. The test and performance results for a plasma test case will be presented; in particular, the performance of the test unit showed capability to detect small changes ~ 0.1% of a dielectric constant, which would correspond to the scraping-off of only 0.3nC to the walls of the dielectric liner inside the cavity from the passing charge bunch.