Multiphysics Analysis of Cryogenically Cooled Photocathode in a CW SRF Injector cavity

TL;DR

This paper models the thermal behavior of a cryogenically cooled copper photocathode in a superconducting RF injector, predicting stable operation at high electric fields and proposing design improvements for better cooling.

Contribution

It introduces a coupled multiphysics model for the cryogenic photocathode and proposes an improved cathode plug design to enhance cooling stability.

Findings

Negligible impact of laser on cavity quality factor

Cryogenic stability limits operational performance

Stable operation predicted at 2 W laser power

Abstract

The paper evaluates the thermal regime of a cryogenically cooled copper photocathode integrated into a continuous-wave superconducting radio-frequency injector cavity with direct thermal contact. Such a photoinjector layout is being developed at DESY and has recently demonstrated a record-high 50 MV/m axial electric field in radio-frequency tests, marking an important milestone. To address the thermal effect of the picosecond excitation laser, we first develop a two-temperature model to describe the temperature of the emitting surface at cryogenic temperatures and solve it numerically. Subsequently, we present a one-temperature model of the bulk photocathode coupled with an electromagnetic model of the injector cavity. For the current injector design, we predict a negligible impact of the laser on the intrinsic quality factor of the cavity, identifying instead the cryogenic stability of the copper cathode as the primary operational limit. To overcome cooling challenges, we propose an improved configuration of the cathode plug. For the proposed geometry, the multiphysics analysis confirms stable performance at a nominal 2 W laser power, sufficient for 100 pC beams at 1 MHz under optimistic quantum efficiency assumptions. Operation at higher laser loads will benefit from further dedicated cryogenic analysis.