Abstract
We present a study of higher order mode (HOM) damping in the first multicell superconducting radio-frequency (SRF) cavity with a photonic band gap (PBG) coupler cell. Achieving higher average beam currents is particularly desirable for future light sources and particle colliders based on SRF energy-recovery linacs (ERLs). Beam current in ERLs is limited by the beam breakup instability, caused by parasitic HOMs interacting with the beam in accelerating cavities. A PBG cell incorporated in an accelerating cavity can reduce the negative effect of HOMs by providing a frequency selective damping mechanism, thus allowing significantly higher beam currents. The five-cell cavity with a PBG cell was designed and optimized for HOM damping. Monopole and dipole HOMs were simulated. The SRF cavity was fabricated and tuned. External quality factors for some HOMs were measured in a cold test. The measurements agreed well with the simulations.
Highlights
Particle colliders and light sources are among the most interesting applications for future accelerator science and technology
The results showed that high accelerating gradients (18 MV=m at the temperature of 4 K) and high cavity Q factors (4 × 109 at the temperature of 1.9 K) could be achieved in the photonic band gap (PBG) cells [27,28]
Wakefield simulations were performed in Computer Simulation Technology (CST) Particle Studio [33] to identify the most dangerous dipole modes and compute the corresponding deflecting voltages induced by a single bunch
Summary
Particle colliders and light sources are among the most interesting applications for future accelerator science and technology. The TESLA cavity does not have HOM damping ability adequate for the high-current ERL projects [15] Modifications of this design have been suggested to improve HOM damping by altering beam pipe and iris radii, somewhat sacrificing accelerating properties. The first multicell PBG accelerating structure was designed and tested with an electron beam at MIT a decade ago [22] This was a room-temperature traveling wave structure operating at the frequency of 17.14 GHz. Recently, it has been experimentally shown that a similar PBG module at 11.7 GHz has reduced HOM wakefields [23]. The presented design is well suited for high-current operation where strong HOM damping is required and, at the same time, has accelerating properties and surface fields similar to the more conventional cavities that use only elliptical cells.
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