Workshop on Coherent Synchrotron Radiation in Storage Rings

about the possibility of broadband coherent synchrotron radiation (CSR) as a source of intense radiation in the traditionally difficult far-infrared/terahertz wavelength region. One of the most interesting features of CSR is that the emitted power scales quadratically with the number of electrons involved. Since the typical number of electrons in a bunch is a billion or more, the potential for gain is huge. To discuss recent advances in exploiting this novel source, the Workshop on Coherent Synchrotron Radiation in Storage Rings was held in Napa, California, on October 28–29, 2002. The workshop was sponsored by the Advanced Light Source (ALS) and BESSY-II and was chaired by John Byrd (ALS). The possibility of generating broadband CSR was raised over half a century ago with the advent of the first high-energy electron accelerators. Coherent emission occurs when the length of the electron bunch (or any structure on the bunch) is comparable to the wavelength of the emitted radiation. The longest emitted wavelength is limited by a waveguide cutoff condition determined by the vacuum chamber height and the radiation opening angle and typically is between a few millimeters and few hundred microns in third generation rings. Therefore, bunch lengths less than a few millimeters and vacuum chambers large enough to transmit the radiation are necessary for observing CSR. The first observations of stable CSR in a storage ring, reported at the workshop in presentations by Gode Wustefeld (BESSY-II) and by Peter Kuske (BESSY-II), were achieved via reduction of the bunch length at low currents (~10 μA/bunch) by reducing the lattice momentum compaction and controlling higher order effects with harmonic sextupoles. The BESSY group observed a coherent enhancement at wavelengths as short as a few hundred microns. Fernando Sannibale (ALS) showed that the emission at shorter wavelengths than expected can be explained by a static distortion of the electron bunch due to its interaction with the radiation. However, the gain from the CSR is not unlimited. The interaction of the radiation with the electron beam can also drive a self-amplified instability, resulting in periodic bursts of CSR. Such bursts of far IR synchrotron radiation have been observed at a number of rings. A model of this instability was presented by Gennady Stupakov (Stanford Linear Accelerator Center) and the first experimental results showing agreement of the instability threshold MEETING REPORTS