Speaker
Description
Significant progress has been made in the performance of the LCLS-II photoinjector and its photocathode subsystem. Cs₂Te photocathodes are now routinely produced with uniform, high initial quantum efficiency (QE) and Coulomb-level lifetimes. However, unexpected photocathode dynamics have emerged as an operational challenge. Over longer timescales, initially uniform QE develops spatial non-uniformities, including a persistent central damage feature. On shorter timescales, QE shows strong sensitivity to changes in laser illumination, with rapid variability that complicates machine tuning and contributes to fluctuations in FEL output. These observations point to a strong coupling between the photoemitting surface, laser illumination conditions, underlying emission physics, and downstream FEL performance. To address these issues, current efforts are prioritizing QE stability and spatial uniformity over peak QE. This includes fabricating and characterizing photocathodes under a range of growth conditions and substrate choices. Recent work has focused on high-repetition-rate laser damage studies, along with expanded post-mortem SEM/EDS analysis to better understand the underlying damage mechanisms. The overall goal is to improve photocathode robustness and enable stable, long-term FEL operation at high repetition rates.
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