Observation of Wakefields and Resonances in Coherent Synchrotron Radiation

TL;DR

This study investigates high-resolution resonances in the spectrum of coherent synchrotron radiation at the CLS, revealing stable spectral features and wakefield pulses, with experimental and simulation insights into their mechanisms in a complex chamber geometry.

Contribution

It provides the first detailed experimental and simulation analysis of CSR resonances and wakefields in a fluted vacuum chamber, extending theoretical models to realistic geometries.

Findings

Resonances occur at regular wavenumber intervals and are stable under various machine conditions.

Wakefield pulses are observed directly and indirectly, showing dependence on chamber geometry and observation point.

New simulation techniques accurately model wakefield behavior in complex chamber geometries.

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 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.