  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 wavenumber intervals of $0.074
~\textrm{cm}^{-1}$, 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
multi-bend 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.
