The principle and first results of betatron tune measurement by direct diode detection

The fractional part of the tune value of a circular accelerator can be measured by observing the betatron oscillations of the beam on a position sensitive pick-up. In the frequency domain the betatron signal is seen as sidebands on the revolution harmonics. The bunches in the beam often have a very short length with respect to the revolution period, resulting in a wideband pick-up signal spectrum, containing many betatron lines. Classical tune measurement systems filter out just one or a few of these 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), which overcomes this and a few other problems. The basic idea is to time stretch the beam pulses from the pick up in order to increase the betatron frequency content in the baseband. This can be accomplished by a simple diode detector followed by an RC low pass filter, as used in the common envelope detection technique for demodulating AM signals. It will be shown that such a circuit can increase the betatron signal level by orders of magnitude compared to classical systems. The 3D method was recently tested in the CERN SPS and PS machines with prototype base band tune (BBQ) measurement systems. Results will be presented showing that the prototypes were sensitive enough to see betatron oscillations in both machines with no added external excitation. The SPS system is also shown to be capable of measuring tunes with a resolution of 10-5 with no explicit excitation. Copies of the BBQ prototype are currently being tested on BNL-RHIC and FNAL-Tevatron as part of the US LARP collaboration, with a view to installing a similar system for tune measurement at the LHC.