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<p>Several prototype S-band pulsed klystrons for use at the Pohang Accelerator Laboratory (PAL) have been developed by a domestic company in collaboration with PAL. These klystrons successfully achieved the required RF output power at a low repetition rate (&lt; 30 Hz) but suffered from gun arcing when operated at 60 Hz, which is the repetition rate required for the PAL-XFEL. Post-mortem inspection revealed that the arcing spots on the focusing electrode (Wehnelt) were not located at the regions of maximum electric field. Particle trajectory simulations in the electron gun region suggest that the initial arcing is triggered by electron emission and followed by clump-induced breakdown processes. In addition, vacuum breakdown can be initiated from the so-called triple point (TP) at the lower end of the ceramic insulator, where a negative potential is applied during high-voltage (HV) operation. Further numerical simulations confirmed that electrons emitted from this TP can travel along the ceramic surface, undergo multiplication, and eventually cause breakdown near the anode housing. Based on these findings, we propose design improvements—most notably enhanced cleaning and polishing procedures as well as an added electric field shield near the TP—to suppress vacuum breakdown at higher repetition rates (e.g., 60 Hz). Initial high-voltage (HV) processing data show that the improved gun design significantly reduces arcing events, enabling stable klystron operation.</p>

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Design improvements of electron gun for PAL klystron

  • 期刊论文

  • 发表日期：

    2025年3月10日

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  • S. J. Park,
  • C. K. Min,
  • Y. G. Park,
  • S. D. Jang,
  • G. Y. Jang,
  • H. S. Shin

摘要

Several prototype S-band pulsed klystrons for use at the Pohang Accelerator Laboratory (PAL) have been developed by a domestic company in collaboration with PAL. These klystrons successfully achieved the required RF output power at a low repetition rate (< 30 Hz) but suffered from gun arcing when operated at 60 Hz, which is the repetition rate required for the PAL-XFEL. Post-mortem inspection revealed that the arcing spots on the focusing electrode (Wehnelt) were not located at the regions of maximum electric field. Particle trajectory simulations in the electron gun region suggest that the initial arcing is triggered by electron emission and followed by clump-induced breakdown processes. In addition, vacuum breakdown can be initiated from the so-called triple point (TP) at the lower end of the ceramic insulator, where a negative potential is applied during high-voltage (HV) operation. Further numerical simulations confirmed that electrons emitted from this TP can travel along the ceramic surface, undergo multiplication, and eventually cause breakdown near the anode housing. Based on these findings, we propose design improvements—most notably enhanced cleaning and polishing procedures as well as an added electric field shield near the TP—to suppress vacuum breakdown at higher repetition rates (e.g., 60 Hz). Initial high-voltage (HV) processing data show that the improved gun design significantly reduces arcing events, enabling stable klystron operation.