Vibration of the Support Tube:Luminosity Lost and Found

Alan Fisher, Jim Turner, Franz-Josef Decker, Mike Sullivan, Clive Field

2007-04-12

10 Hz on BPM Y FFTs

Decker has seen 10 Hz on BPMs since last summer, often at much higher levels than this picture taken today.

9.75 and 19.5 Hz on Luminosity

0 Hz 100 Hz

Peaks at 9.75 Hz and its second and third harmonics, plus sidebands at ±9.75 Hz around 60 Hz.What is moving?

Vertical Geophone Tests: Backward Q5

Backward Q4

Backward Q4, Shaking the Raft

Backward Q2

Forward Q5

Forward Q4

Forward Support-Tube Bracket

Finally: A strong peak at ~9.75 Hz

Repeat Support-Tube Bracket

9.75 again, but a second peak just over 10 too.Second peak doesn’t appear in luminosity spectrum.

Fast Luminosity Signal

Put Clive’s high-bandwidth luminosity signal (designed for fast dither) onto scope.

During Y dithers, this slows to ~10 Hz, larger amplitude.

These are clearly lumi dips when using DC coupling.

With 1-µm dithers, the lumi dips just recover to baseline at top of each 10-Hz period.

Y dither lowers lumi, vibration restores it.

Hash is ~20 Hz. 2nd harmonic as vibration drops

lumi on both sides of peak.

1 s/div

Fast lumi signal is AC coupled

2 µm dithers in Y

Correcting the Vibration

Support tube and beams appear to be vibrating vertically at one sharp frequency, close to 9.7 Hz.

Can we cancel noise by moving the beams? Ideally, drive Q1 air-core trim corrector at same frequency

and opposite phase. Need a clean signal locked to vibration. Adjust amplitude to null lumi component at this frequency. Lock-in amplifier can measure and produce a drive signal. Kepco BOP supply, same as used for fast dither.

Proof of principle yesterday: Used Y fast dither set close to vibration frequency. Beats in and out of phase with vibration, making lumi better and

worse: 4% lumi gain found right away.

Canceling Vibration with Y Fast Dither

t=50: Fast dither starts, 9.75 Hz, 0.6 µm

t=275: Change to 9.70 Hz, 1 µm

Magnitude of lock-in signal is strong during luminosity dips.