Beam diagnostics at the BESSY I electron storage ring with Compton backscattered laser photons: Measurement of the electron energy and related quantities

The electron storage ring BESSY I in Berlin-Wilmersdorf is used by the Physikalisch-Technische Bundesanstalt (PTB) as a primary radiation source standard in the vacuum-UV and soft X-ray spectral range. This requires the determination of the storage ring parameters relevant to the calculation of the spectral photon flux with small relative uncertainties. Within this framework, a new set-up for measuring the electron energy is described: CO2-laser light is scattered in a head-on collision with the electron beam in the electron storage ring and the backscattered photons are measured with an energy-calibrated high-purity germanium (HPGe) detector. The exact energy and shape of the high energy edge of the measured Compton spectrum can then be used to determine the electron energy and also the electron beam energy spread and related storage ring parameters. Moreover, at BESSY I, we have the unique possibility of measuring the electron energy by the alternative method of resonant spin depolarization. Comparison of the results obtained by these two techniques shows agreement within the combined relative uncertainties of 2 x 10(-4) An overview of the current experimental set-up at the storage ring BESSY I and first experimental results are given.