High Sensitivity Tune Measurement by Direct Diode Detection

The fractional part of the betatron tune for a circular accelerator can be measured by observing beam oscillations on a position pick-up. In frequency domain the betatron frequency is seen as sidebands on either side of the revolution harmonics. Usually beam signal pulses from the pick-up are very short with respect to the revolution period, resulting in a broadband spectrum. Classical tune measurement systems filter out just one of the betatron sidebands. As a consequence, most of the betatron energy is lost and only a very small fraction remains for further processing. This paper describes a new method, referred to as Direct Diode Detection (3D). It is based on the idea of time stretching beam pulses from the pick-up in order to increase the betatron frequency content in the baseband. The 3D method was recently tested in the CERN SPS and PS, BNL RHIC and FNAL Tevatron machines. Results from all these machines [1, 2, 3, 4] show that this method can increase the betatron signal level by orders of magnitude as compared to classical systems, making it possible to observe tunes with no explicit excitation. Frequency resolution in the order of 10-5 and amplitude sensitivity in the order of 10 nm has been achieved with this very simple hardware.