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Analytical and numerical studies of dark current in radiofrequency structures for short-pulse high-gradient acceleration

Gaurab Rijal, Michael Shapiro, and Xueying Lu*,†

  • *Contact author: xylu@niu.edu
  • †Also at Argonne National Laboratory, Lemont, Illinois 60439, USA.
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Phys. Rev. Accel. Beams 28, 111301 – Published 21 November, 2025

DOI: https://doi.org/10.1103/w9mh-3klh

Abstract

High-gradient acceleration is a key research area that could enable compact linear accelerators for future colliders, light sources, and other applications. In the pursuit of high-gradient operation, rf breakdown limits the attainable accelerating gradient in normal-conducting rf structures. Recent experiments at the Argonne Wakefield Accelerator suggest a promising approach: using short rf pulses with durations of a few nanoseconds. Experimental studies show that these O(1  ns) rf pulses can mitigate breakdown limitations, resulting in higher gradients. For example, an electric field of nearly 400  MV/m was achieved in an X-band photoemission gun driven by 6-ns-long rf pulses, with rapid rf conditioning and low dark current observed. Despite these promising results, the short-pulse regime remains an underexplored parameter space, and rf breakdown physics under nanosecond-long pulses requires further investigation. In this paper, we present analytical and numerical simulations of dark current dynamics in accelerating cavities operating in the short-pulse regime. We study breakdown-associated processes spanning different time scales, including field emission, multipacting, and plasma formation, using simulations of the X-band photogun cavities. The results reveal the advantages of using short rf pulses to reduce dark current and mitigate rf breakdown, offering a path toward a new class of compact accelerators with enhanced performance and reduced susceptibility to breakdown.

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Phys. Rev. Accel. Beams 28, 111301– Published 21 November, 2025
  • Received 5 July 2025
  • Accepted 29 October 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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