Measuring devices at BESSY for stored beam currents ranging from 0.8 pA to 1 A

Abstract

The electron storage ring BESSY is a primary radiometric standard where the photon flux can be adjusted within a dynamic range of 12 orders of magnitude via the stored beam current IR. We describe the three different devices installed at BESSY to perform the measurement of the absolute beam current within the full range. Two toroidal dc beam current transformers are used in the range 0.5 mA<IR<1 A. The measurement of lower beam currents is based on monitoring the emitted white synchrotron radiation flux. If less than 1000 electrons are stored (IR<0.8 nA) then electron counting is performed. In the intermediate range (0.8 nA<IR<1 mA), IR is determined via windowless photodiodes which have been calibrated against the transformers and against the single electron measurements which allow a check of the linearity of the detectors. We report on the calibration procedure, on influences of changes of the source, and on the resultant uncertainties.