NDR Tune Coupling Measurement

The tunes are brought close together moving the xtune knob. Both tunes are then measured on the spectrum analyzer and plotted as a function of the knob setting. The closest separation of the tunes gives a measure of the coupling in the ring.

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Tune measurement

The beam is transversely excited through the damping ring "growler" system and is driven from the
output of the network analyzer after first being mixed down to the 0-mode betatron sideband
frequency. The amplitude response of the beam from a BPM stripline is detected at the lower
betatron sideband of the 2nd revolution harmonic.

The two measured tunes are plotted as function of the xtune knob.

The difference between the two measured tunes is plotted as a function of the xtune knob.  The minimum of this curve is the closest tune approach and is a measure of the coupling in the ring.

Coupling Reduction

Since there are no adjustable skew quadrupoles in the ring, we use vertical orbit bumps in the sextupoles to change the coupling.  Initially we put the tunes at the point of closest approach, then as the coupling is reduced, the tunes move closer together on the spectrum analyzer.  This is a convenient technique for monitoring the coupling while different closed orbit bumps are tried.   In the past we have measured the vertical emittance of the extracted beam to see if the coupling had been reduced, which takes much longer.  We were able to reduce the tune separation from 188 KHz to 81 KHz but beam injection losses increased.  Some anomalies in the orbit near the extraction region have now been resolved and this issue will be revisited..
If you have comments or suggestions, email Roslind Pennacchi at shelter@slac.stanford.edu