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

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi mathvariant="script">O</a:mi> <a:mo stretchy="false">(</a:mo> <a:mn>1</a:mn> <a:mrow> <a:mtext> </a:mtext> <a:mtext> </a:mtext> </a:mrow> <a:mi>ns</a:mi> <a:mo stretchy="false">)</a:mo> </a:math> rf pulses can mitigate breakdown limitations, resulting in higher gradients. For example, an electric field of nearly <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mrow> <f:mn>400</f:mn> <f:mtext> </f:mtext> <f:mtext> </f:mtext> <f:mi>MV</f:mi> <f:mo>/</f:mo> <f:mi mathvariant="normal">m</f:mi> </f:mrow> </f:math> was achieved in an <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>X</i:mi> </i:math> -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 <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>X</k:mi> </k:math> -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.