• U. Rauch, P. Forck, P. Hülsmann, P. Kowina, P. Moritz
  GSI, Darmstadt

A precise tune determination is crucial for stable operation of GSI SIS-18 synchrotron especially for intense beam conditions. In order to avoid nearby resonances in the tune diagram the fractional part of coherent betatron motion needs to be measured with a resolution of 10^-3 also during ramping mode. This is achieved using a fast digital readout system for Beam Position Monitors (BPM). The broadband BPM signal is sampled with a rate of 125 MSa/s which corresponds to an average of about 50 Sa per bunch for SIS-18 machine parameters. The signal is integrated bunch-by-bunch which minimizes thermal and digitization noise and the beam position is calculated. The tune is then determined in baseband directly by Fourier-transformation of the positions of a certain bunch typically over 2048 turns. This algorithm does not require any additional input parameter. Since particle losses due to significant emittance blow-up have to be avoided, excitation power has to be kept as low as possible. This was achieved using a digital pseudo random noise (PRN) generator for beam excitation, which produces white noise on a carrier frequency with adjustable bandwidth.