Development and validation of a high-bandwidth coaxial wall current monitor for sub-nanosecond beam bunch characterization

The effective commissioning and operation of high-intensity proton accelerators, such as the 20 MeV, 352 MHz low-energy high-intensity proton accelerator (LEHIPA), necessitate non-destructive diagnostics capable of precise, sub-nanosecond beam characterization. To address this need for LEHIPA's ∼100 ps RMS beam bunches, a compact, wide-bandwidth, dual-function coaxial wall current monitor (C-WCM) has been designed, fabricated, and experimentally validated. This novel, ferrite-less monitor utilizes a single coaxial structure and eight azimuthally distributed, high-bandwidth feedthroughs to achieve a good frequency response and enhanced diagnostic capabilities. C-WCM's geometry was optimized through electromagnetic simulations, which predicted an operational bandwidth of about 9 GHz. The physical device was subsequently fabricated and assembled. Laboratory RF characterization, including S-parameter analysis and time-domain reflectometry (TDR), yielded a measured bandwidth of about 6 GHz for the test assembly, showing good agreement with simulations that modeled the lab setup. Experimental tests with the 20 MeV LEHIPA proton beam successfully validated the C-WCM's design and operational capabilities. The device exhibited effective RF noise isolation and produced robust signal levels sufficient for direct analysis, accurately resolving the ∼100 ps bunch structure. Furthermore, the C-WCM's multi-feedthrough geometry enables its use as a high-performance beam position monitor. This work presents a validated, compact diagnostic that integrates high-fidelity bunch length and wide-aperture position monitoring into a single device, offering a versatile and powerful tool for accelerator operation and beam dynamics studies.