Absolute Calibration of a Ppm-Precise HV Divider for the Electron Cooler of the Ion Storage Ring CRYRING@ESR

Abstract

The heavy ion storage ring CRYRING at GSI provides a unique possibility to test atomic structure calculations with slow exotic ion beams. In order to cool the ions and thus achieve a low momentum spread of the stored beam, CRYRING features an electron cooler, where the ion beam is superimposed with a mono-energetic electron beam. In high-precision experiments at ion storage rings the velocity of the ions is a critical quantity. For measurements at CRYRING the electron cooler determines the ion velocity and momentum spread of the ions. Consequently, a precise knowledge of the acceleration voltage of the electron beam is essential for the accuracy of the experiments.

To measure the acceleration voltage a high-precision HV divider for voltages up to 35 kV has been constructed. The design of this divider is similar to the ultrahigh-precision voltage dividers which have been developed and built in Münster in cooperation with PTB for use at the KATRIN experiment. It could be demonstrated that the precision of the divider is in the low ppm range.

In order to determine the absolute values and the stability of the scale factors of the high-voltage divider, regular calibration measurements with ppm-precision are essential. A new absolute calibration method based on a differential voltage measurement at high voltage has been developed. Here a voltage of 1 kV is added to a base high-voltage potential. This additional voltage and the high voltage divider output change are traceable to reference systems. The base voltage is monitored with a reference measuring system, which does not require ppm-precision. First results have demonstrated an absolute calibration with expanded measurement uncertainties of about 1 ppm.