Nanosecond Radio-Frequency Pulse Driven Photogun for Very Hard X-ray Free-electron Laser

TL;DR

This paper proposes a nanosecond rf pulse-driven photogun, CUPID, capable of producing high-brightness electron beams for very hard X-ray free-electron lasers by using high power rf pulse compression to prevent breakdowns.

Contribution

Introduction of the CUPID photogun design and demonstration of its potential for high-brightness electron beams and hard X-ray photon production.

Findings

CUPID can achieve 300 MW rf power at 20 ns pulses.

The photogun can produce electron beams with 60 nm emittance.

Simulation shows potential for mJ pulse energy at 40 keV X-ray photons.

Abstract

One pathway to producing high brightness electron beams is to use a radio-frequency (rf) driven high field photogun to rapidly accelerate photoemitted electrons to the relativistic regime and preserve the brightness. However, the highest attainable field is limited by rf breakdowns of materials used in a photogun. Shortening rf pulse duration feeding into a photogun provides a viable pathway to achieve high field and prevent rf breakdowns. Here we propose and investigate Compressed Ultrashort Pulse Injector Demonstrator (CUPID), a nanosecond rf pulses driven photogun powered by a klystron and rf pulse compression system capable of achieving 300 MW at 20 ns duration, to produce bright electron beams with high electric field. We first introduce the design of the CUPID photogun and its expected rf performance at 500 MV/m driven by high power nanosecond rf pulses, followed by beam dynamics studies showing its capability for producing bright electron beams with 60 nm emittance when forming a photoinjector with a superconducting solenoid and downstream accelerating structures. Finally, we show a proof-of-concept start-to-end simulation of the CUPID photoinjector paired with the existing Linac Coherent Light Source (LCLS) copper accelerator free-electron laser (FEL) to demonstrate achievable mJ pulse energy very hard x-ray photons at 40 keV or higher.