nanoBPMs at ATF

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nanoBPMs at ATF Steve Smith ATF2
Workshop January 6, 2005 SLAC
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Cavity BPMs

• C-Band Cavities
• Livermore Spaceframe
• Hexapods
• flexure legs
• Dual Downconversion
• First IF at 476 MHz
• Second IF at 25 MHz
• Digitize at 100 MSamples/sec

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Algorithm

• Digital Downconversion
• Multiply digital waveform by complex local
  oscillator eiwt
• Low-pass filter (currently 2.5 MHz B/W)
• Sample complex amplitude of position cavity at
  peak
• Divide by complex amplitude from reference cavity
• Scale by calibration constants
• Refine calibration with linear least-squares fit
  to other BPM measurements
• Removes rotations, calibration errors.

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Data Raw Demodulated
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Calibration

• Move BPM more than beam jitters
• Estimate scale, phase
• Doesnt use information from other BPMs

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Does calibration work in presence of beam jitter?

• Look at same calibration data
• Use other BPMs to remove beam jitter

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Fits to Calibration Data, All BPMs
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Measurement

• Predict Y2
• Linear least-squares fit to (x, y, x, y) at
  BPMs 13

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Preliminary Resolution

• s 20 nm
• Individual BPM resolution is better, this is
  measurement prediction from 2 other BPMs
• Calibration scale is clearly off by 20

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Move BPM in 1 mm Steps
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X Resolution
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Anticipated Improvements

• Analysis improvements
• Adjust cavity parameters
• Frequency
• Decay constant
• Optimize algorithm
• Filter bandwidth may be reduced gt improve
  statistical power
• Optimize measurement sample time
• Investigate handling of saturated pulses
• Is saturation handled properly in this algorithm?
• Are there non-linearities apparent at large
  amplitudes?
• Potential Physical improvements
• Lock Local Oscillators to accelerator RF
• Improve understanding, operation of movers

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Exercise BPM Movers

• Y2 is a little low
• Move Y2 up 1 mm
• Oops, wrong way
• Go up 2 mm
• What happens during the move?
• 40 mm excursions(!) in process of making 1 mm
  move
• Why is resolution worse after the move?
• s40 nm, was 20nm
• BPM rotated during move? No

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Conclusions

• Resolution is not difficult
• 20 nm, should get better with optimized analysis
• Demonstrating resolution is hard
• Must beat beam jitter, drifts
• Stability looks good, but is poorly studied so
  far.
• Implemented system should be able to prove BPM
  capabilities
• Redundancy?
• Movers?

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