Abstract:
Background The C-band photocathode radio frequency (RF) electron gun, with an ultra-high accelerating gradient exceeding 150 MV/m, is a key technology for generating high-brightness electron beams in fourth-generation light sources. However, its output beam features picosecond-scale ultrashort pulses, a wide charge dynamic range of 50 pC to 2500 pC, and an ultra-low transverse emittance of 0.18 mm·mrad@100 pC. Existing measurement methods developed for L/S-band systems can hardly meet the stringent requirements on measurement accuracy and bandwidth for such beams.
Purpose This study aims to develop a high-precision beam measurement system adapted to the characteristics of the C-band electron gun, based on the test platform of the South Advanced Light Source (SAPS).
Methods Firstly, a flange-mounted active integrating charge transformer (Active-ICT) was independently developed to address the challenge of narrow-pulse charge measurement, and a cross-calibration method based on a set of commercial high-sensitivity ICT and terminator was proposed, achieving a measurement linearity better than ±1% full scale (FS). Secondly, to mitigate the significant influence of space charge force in ultra-low emittance measurement, the slit parameters and drift length of the double-slit emittance meter were optimized via Astra simulation, confining the systematic error within 10% in the emittance range of 0.15－0.25 mm·mrad. Thirdly, an optical path combining double-slit collimation and a sector dipole magnet was designed to suppress noise floor interference in energy spread measurement.
Results Preliminary beam experiments were conducted with the established system. The results show that the measured photocurrent and dark current are in good agreement with Faraday cup measurements, and the beam energy curves obtained under different accelerating gradients are highly consistent with beam dynamics simulation results, verifying the reliability and measurement accuracy of the system.
Conclusions This work solves the key beam diagnostics technical bottlenecks in the commissioning of domestic C-band photocathode RF electron guns, and provides core technical support for the engineering development of similar high-gradient injectors.
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