Cathodes and Shape Modification of Cavity for DESY Superconducting Photoinjector

TL;DR

This paper reports on the development and testing of superconducting RF photoinjector cavities at DESY, highlighting high-gradient performance, cathode options, and cavity shape modifications to improve electron source quality.

Contribution

It introduces cavity shape modifications and cathode configurations that enhance performance and reduce electric field stress in superconducting RF photoinjectors.

Findings

Achieved peak gradients above 55MV/m in prototypes

Demonstrated operation with different cathode materials

Proposed cavity shape modifications to reduce electric field near cathode

Abstract

Four DESY prototypes of the L-band superconducting RF photoinjector cavity demonstrated on-axis peak gradients above 55MV/m during multiple vertical cryogenic tests. Two of these prototypes,16G09 and 16G10, achieved these gradients with both superconducting and normal-conducting metallic cathodes, fabricated from either high-purity niobium or lower-purity copper. The DESY photoinjector, under development for over two decades as a continuous-wave electron source for FELs, differs from other SRF injectors in that its metallic cathode plug is attached directly to the cavity backplate, exposing the emitting surface to the high electric field within the cavity. This design obviates the need for a choke filter or load-lock system. The initial 1.6-cell cavity geometry was derived from the Low-Loss design developed for the CEBAF 12GeV upgrade, scaled from 1.5GHz to 1.3GHz. This shape was later replaced with the current High Gradient TESLA profile. In this report, we discuss current cathode options and present modifications to the cavity shape aimed at significantly reducing the electric field near the cathode opening