Design and performance of a capacitor-based burn-through monitor for high-power X-ray beams at XFEL facilities

The reliable and rapid detection of uncontrolled X-ray beam propagation is critical for machine and personnel protection at high-power X-ray free-electron lasers. We present the design, operation, and performance of the X-Ray Flux Capacitor (XRFC) burn-through monitor (BTM), a parallel-plate capacitor device implemented as a printed circuit board, intended to provide an inexpensive, scalable, and fail-safe beam containment monitoring solution when used with appropriate readout electronics. The capacitor BTM operates on the principle that focused high-power X-rays degrade the FR-4 dielectric between the capacitor layers, resulting in a high-voltage electrical short that is easily detectable. Ninety-two high-voltage prototype capacitor BTMs were tested at various beam parameters at the Linac Coherent Light Source XCS and XPP endstations, using X-ray energies between 7.17 keV and 13.1 keV. All capacitor BTMs shorted when exposed to beam conditions with sufficient energy density, with shorting thresholds ranging from 0.7 eV atom^-1 pulse^-1 to 5.8 eV atom^-1 pulse^-1 (in copper), depending on X-ray energy and beam focus. In all cases, a high-voltage short was detected before full burn-through of the capacitor BTM occurred. These results suggest that the XRFC BTM concept can be used as a reliable, fast-response diagnostic for beam integrity monitoring in high-flux environments.