Thermal Emittance Isolation by Cathode Retraction

TL;DR

This paper introduces a novel method combining cathode retraction and two-slit emittance measurement to isolate and directly measure thermal emittance in electron beams, demonstrated on a state-of-the-art SRF gun.

Contribution

It presents a new technique for isolating thermal emittance using cathode retraction and two-slit measurement, enabling high-resolution phase-space mapping beyond traditional methods.

Findings

High-resolution phase-space distribution maps achieved.

Cathode retraction effectively compensates RF defocusing.

Technique applicable for optimizing photoinjector performance.

Abstract

In this work, a combination of cathode retraction and two-slit emittance measurement technique is proposed as an advanced means to individually modify emittance growth components, specifically, rf injector fringe fields, to isolate and directly measure the thermal emittance, the fundamental beam emittance metric for an electron beam. A case study of the LCLS-II-HE Low Emittance Injector (LEI), a state-of-the-art superconducting radiofrequency (SRF) gun, designed for LCLS-II HE upgrade is used to showcase the power of the two-slit technique. Particularly, it is demonstrated that generating a high resolution phase-space distribution map, dominated by the intrinsic emittance of the electron bunch, is possible. This result goes beyond the normal single-parameter distribution characterizations (e.g. RMS emittance and Twiss parameters) provided by the solenoid scan. One key feature making this technique work (and in the end practically useful) is the ability to retract the cathode, because it provides the ability to compensate for radiofrequency (rf) de-focusing. It is demonstrated how the cathode retraction can serve as an additional optimisation tool for tailoring the routine performance of the photoinjector. We posit that a variable position cathode may be a useful method for optimizing photoinjector performance across multiple parameters regimes.