Experimental verification of space-charge saturation scaling laws in high-gradient photocathode RF guns

TL;DR

This paper experimentally verifies the scaling laws of space-charge saturation in high-gradient RF photoguns, providing a predictive framework for optimizing ultrafast electron sources in advanced applications.

Contribution

It offers the first experimental verification of cigar-regime scaling laws in RF photoguns, supported by measurements, simulations, and analytic models.

Findings

Confirmed charge-field scaling laws for pancake beams

First experimental verification of cigar-regime scaling

Established a framework for optimizing high-gradient photoinjectors

Abstract

We investigate the limits of photoemission yield in a high-gradient S-band radiofrequency photoinjector in the space-charge-dominated regime. Using an RF phase-scan technique, where the emitted charge is measured as a function of the RF-field phase in the gun, we directly monitor photoemission over a range of launch fields and laser parameters, enabling quantitative characterization of space-charge saturation. Measurements, supported by simulations and analytic modeling, confirm the characteristic charge-field scaling laws for pancake beams and provide the first experimental verification of cigar-regime scaling in an RF photogun. These results establish a predictive framework for identifying the onset of space-charge saturation and guide the optimization of photoinjectors for ultrafast electron diffraction, microscopy, and high-brightness light sources operating at ultra-high gradients.