Harmonically resonant cavity as a bunch-length monitor

• Main Authors: B. Roberts, F. Hannon, M. M. Ali, E. Forman, J. Grames, R. Kazimi, W. Moore, M. Pablo, M. Poelker, A. Sanchez, D. Speirs
• Format: Article
• Language: English
• Published: American Physical Society 2016-05-01
• Series: Physical Review Accelerators and Beams
• Online Access: http://doi.org/10.1103/PhysRevAccelBeams.19.052801

A compact, harmonically resonant cavity with fundamental resonant frequency 1497 MHz was used to evaluate the temporal characteristics of electron bunches produced by a 130 kV dc high voltage spin-polarized photoelectron source at the Continuous Electron Beam Accelerator Facility (CEBAF) photoinjector, delivered at 249.5 and 499 MHz repetition rates and ranging in width from 45 to 150 picoseconds (FWHM). A cavity antenna attached directly to a sampling oscilloscope detected the electron bunches as they passed through the cavity bore with a sensitivity of ∼1  mV/μA. The oscilloscope waveforms are a superposition of the harmonic modes excited by the beam, with each cavity mode representing a term of the Fourier series of the electron bunch train. Relatively straightforward post-processing of the waveforms provided a near-real time representation of the electron bunches revealing bunch-length and the relative phasing of interleaved beams. The noninvasive measurements from the harmonically resonant cavity were compared to measurements obtained using an invasive RF-deflector-cavity technique and to predictions from particle tracking simulations.