Bunch Tiltmeter

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Bunch Tiltmeter
Steve Smith SLAC
Snowmass July 16, 2001
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BPM Sensitive to Bunch Tilt

• Observation at ASSET
• BPM Cavity power vs. beam position has minimum
  which depends on bunch tilt

• There is a small tilt signal in quadrature with
  position signal
• Add to other signals

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Response of Cavity BPMto Point Charge
Q
d
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Response of BPM to Tilted BunchCentered in Cavity
q
q/2
Treat as pair of macroparticles
d/2
d/2
st
q/2
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Tilted bunch

• Point charge offset by d
• Centered, extended bunch tilted at slope d/st
• Tilt signal is in quadrature to displacement
• The amplitude due to a tilt of d/s is down by a
  factor of
• with respect to that of a displacement of d
• (bunch length / Cavity Period )

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Example

• Bunch length st 200 mm/c 0.67 ps
• Tilt tolerance d 200 nm
• Cavity Frequency F 11.424 GHz
• Ratio of tilt to position sensitivity ½?f?t
  0.012
• A bunch tilt of 200 nm / 200 mm yields as much
  signal as a beam offset of 0.012 200 nm 2.4nm
• Need BPM resolution of 2 nm to measure this
  tilt
• Challenging!
• Getting resolution
• Separating tilt from position
• Use higher cavity frequency?

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Cavity BPM Parameters
nm
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Cavity Comments

• Signal adequate
• Wakefields OK
• Beam centering tolerance 1/10th of sturctures
• May want higher frequency
• Better position resolution
• More favorable tilt / position sensitivity ratio

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Position-Tilt Discrimination

• Phase-sensitive detection
• Position jitter or dithering measures phase of
  position signal
• Quadrature part of signal is tilt background
• One phase of residual common mode
• RF interference/leakage
• The higher the frequency the better!

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Whats Next?

• Simulate signal, background
• Optimize cavity
• Does this sensitivity to bunch tilt lead to
  increased sensitivity in an RF bunch length
  monitor?
• Transverse cavity upstream of tilitmeter
• Titlemeter measures slopebunch length
• Transverse cavity, detuned by maybe 8 MHz,
  provides internal calibration of tiltmeter.

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Cavity BPM Calculation