Beam Stability in a 6 GeV Synchrotron Light Source

Future synchrotron radiation sources designed to produce low emittance electron beams for wigglers and undulators will present beam position control problems essentially similar to those encountered by users of existing accelerators,1,2 however tolerances will be tighter due to: 1) the small emittance (7× 10-9 mrad) proposed for the electron beam and the correspondingly small emittances (sizes) of secondary photon beams, 2) the sensitivity of the electron beam closed orbit to quadrupole motion and dipole roll, 3) the high power levels associated with undulator and wiggler beams which will permit (and probably require) high precision and stability of the photon beam position measurements, in addition, 4) the large number of users on the roughly sixty beam lines will demand beams capable of producing the best experimental results. For the present paper, we assume the accelerator control function, which would initially involve making and coordinating all changes, would eventually evolve to setting and verifying the limits of user control: within these limits the beam position would be controlled by users. This paper describes the effects of motion of beam components (quads, rf cavities and dipoles) on the beam and considers the properties of a compensation system from the perspective of users. The system departs from standard practice in considering active perturbation of the electron beam to verify beam corrections. The effects of local closed orbit perturbations to direct undulator beams at different experimental setups are also considered.