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Beam transient studies in high-intensity linacs

Bruce Yee-Rendon*, Yasuhiro Kondo, Jun Tamura, Shin-ichiro Meigo, and Fujio Maekawa

  • *Contact author: byee@post.j-parc.jp
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Phys. Rev. Accel. Beams 29, 103201 – Published 14 October, 2026

DOI: https://doi.org/10.1103/shnn-b3rn

Abstract

Space charge is one of the main challenges for high-intensity linear accelerators (linacs). To overcome this, most linacs have adopted the space charge compensation (SCC) scheme at the low-energy beam transport (LEBT). However, SCC is a buildup process that produces beam transients, i.e., nonoptimal beam conditions, which can lead to significant beam degradation and losses. Furthermore, most LEBTs are equipped with a chopper to provide a beam structure intended for the downstream sections and for the gradual ramp-up of power. However, operation of the chopper will destroy the SCC, leading to additional beam transients. SCC studies on beam transients focus primarily on the LEBT section, resulting in insufficient analysis of the downstream section, where beam transients can cause significant losses. This work addresses this gap by analyzing beam transients generated in the LEBT and their propagation throughout the linac to assess beam losses. This analysis challenges conventional notions by revealing that the worst-case scenario does not occur when the SCC is zero, but rather within twice the compensation time (τ*). This is because, although mismatch and emittance growth are at their maximum at the beginning, 80% and 450%, respectively, the current transmitted to the superconducting linear accelerator is minimal (30%); consequently, beam instabilities caused by the mismatch are less severe. Most nonoptimal beams were intercepted by the cone collimator and the radio-frequency quadrupole at energies below the activation threshold (2.16 MeV), with a peak power loss density that remained within cooling capacity limits. Furthermore, any residual risk was safely eliminated by the collimators in the medium-energy beam transport. As a result, transient losses in the superconducting linac were estimated to be below 1  W/m and were confined to the low-energy section.

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Phys. Rev. Accel. Beams 29, 103201– Published 14 October, 2026
  • Received 24 March 2026
  • Accepted 29 September 2026
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