The Metrology Light Source can be operated for special PTB shifts with electron beam currents between 1 pA (one stored electron) and normal current of about 200 mA, thus enabling PTB to match the photon flux to the sensitivity of the devices to be calibrated over a dynamic range of more than 11 decades. Currents in the upper range, i. e. above 2 mA, are measured with two DC parametric current transformers. Electron currents in the lower range, i. e. below 40 pA, are determined by counting the number of stored electrons. For this, the electrons are gradually kicked out of the storage ring by a mechanical scraper that can be moved closely to the beam while measuring the step-like drop of the synchrotron radiation intensity by cooled photodiodes. Electron beam currents in the middle range, i. e. from about 10 pA up to 2 mA, are determined by three sets of windowless linear Si photodiodes with different filters that are illuminated by synchrotron radiation. The calibration factors of these photodiodes, which relate the photo current to the electron beam current, are determined by comparison with the electron beam current measured at the upper and lower end of the range as described above.
The same scheme is used at the BESSY II electron storage ring. There the electron beam current can be varied between 0.2 pA (one stored electron) up to 200 mA and more.
Electron beam current measurement by electron counting at the MLS