title: Observation of Wakefields and Resonances in Coherent Synchrotron Radiation.
abstract: We report on high resolution measurements of resonances in the spectrum of coherent synchrotron radiation (CSR) at the Canadian Light Source (CLS). The resonances permeate the spectrum at wave number intervals of 0.074  cm(-1), and are highly stable under changes in the machine setup (energy, bucket filling pattern, CSR in bursting or continuous mode). Analogous resonances were predicted long ago in an idealized theory as eigenmodes of a smooth toroidal vacuum chamber driven by a bunched beam moving on a circular orbit. A corollary of peaks in the spectrum is the presence of pulses in the wakefield of the bunch at well-defined spatial intervals. Through experiments and further calculations we elucidate the resonance and wakefield mechanisms in the CLS vacuum chamber, which has a fluted form much different from a smooth torus. The wakefield is observed directly in the 30-110 GHz range by rf diodes, and indirectly by an interferometer in the THz range. The wake pulse sequence found by diodes is less regular than in the toroidal model, and depends on the point of observation, but is accounted for in a simulation of fields in the fluted chamber. Attention is paid to polarization of the observed fields, and possible coherence of fields produced in adjacent bending magnets. Low frequency wakefield production appears to be mainly local in a single bend, but multibend effects cannot be excluded entirely, and could play a role in high frequency resonances. New simulation techniques have been developed, which should be invaluable in further work.
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: B E
authors.initials: BE
authors.lastname: Billinghurst
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: J C
authors.initials: JC
authors.lastname: Bergstrom
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Baribeau
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: T
authors.initials: T
authors.lastname: Batten
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: L
authors.initials: L
authors.lastname: Dallin
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: T E
authors.initials: TE
authors.lastname: May
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: J M
authors.initials: JM
authors.lastname: Vogt
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: W A
authors.initials: WA
authors.lastname: Wurtz
authors.affiliation: SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA and Department of Mathematics and Statistics, University of New Mexico, Albuquerque, New Mexico 87131, USA.
authors.forename: R
authors.initials: R
authors.lastname: Warnock
authors.affiliation: Department of Mathematics and Statistics, University of New Mexico, Albuquerque, New Mexico 87131, USA.
authors.forename: D A
authors.initials: DA
authors.lastname: Bizzozero
authors.affiliation: Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: S
authors.initials: S
authors.lastname: Kramer