IntraPulse BeamBeam Scans at the NLC IP

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Intra-PulseBeam-Beam Scansat theNLC IP

Steve Smith Snowmass 2001
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Beam-Beam Scans

• Ultimate collider diagnostic
• Spots sizes
• Alignment
• Waists
• Etc. etc. etc
• Some Limitations
• Takes lots of time at one measurement per pulse
  train
• Sensitive to drifts over length of scan
• Machine drifts
• low frequency noise
• Can we do a scan in one bunch train?
• Increase utility of diagnostic by increasing
  speed
• Reduce sensitivity to drift and low frequency
  noise
• Yes
• Leverage hardware from intra-pulse IP feedback.

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Intra-pulse Feedback

• Fix IP jitter within the crossing time of a bunch
  train (250 ns)
• BPM measures beam-beam deflection on outgoing
  beam
• Fast (few ns rise time)
• Precise (micron resolution)
• Close (4 meters from IP?)
• Kicker steers incoming beam
• Close to IP (4 meters)
• Close to BPM (minimal cable delay)
• Fast rise-time amplifier
• Feedback algorithm to converge within bunch
  train.

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Intra-Pulse Feedback
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Intra-Pulse Feedback(with Beam-Beam Scan
Diagnostics)
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Single-Pulse Beam-Beam Scan

• Fast BPM and kicker needed for interaction point
  stabilization
• Open the loop and program kicker to sweep beam
• Digitize fast BPM analog output
• Acquire beam-beam deflection curve in a single
  machine pulse
• Eliminates inter-pulse jitter from the beam-beam
  scan.
• Use to
• Establish collisions
• Measure IP spot size
• Waist scans.
• What else?

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Beam-Beam Scan
Beam bunches at IP blue points BPM analog
response green line
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Conclusions

• Given Intra-Pulse Interaction Point Feedback, we
  have tools to perform beam-beam scans within the
  bunch train.
• Speeds up beam-beam scans by an order of
  magnitude, maybe more.
• Increases the number of parameters which can be
  optimized by beam-beam scans, or the optimization
  update frequency.
• Reduces low frequency noise in beam-beam scans
• Machine drifts
• 1/f noise
• Valuable
• Cheap!

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Limits to Beam-Beam Feedback

• Must close loop fast
• Propagation delays are painful
• Beam-Beam deflection slope flattens within 1 s
• Feedback converges too slowly beyond 20 s to
  make a difference in luminosity
• May be able to fix misalignments of 100 nm with
  moderate kicker amplifiers
• Amplifier power goes like square of misalignment

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Beam-Beam Deflection
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Simulated BPM Processor Signals
BPM Pickup (blue) Bandpass filter (green) and
BPM analog output (red)
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Beam Position Monitor

• Stripline BPM
• 50 Ohm
• 6 mm radius
• 10 cm long
• 7 angular coverage
• 4 m from IP
• Process at 714 MHz
• Downconvert to baseband
• (need to phase BPM)
• Wideband 200 MHz at baseband
• Analog response with lt3ns propagation delay (plus
  cable lengths)

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Fast BPM Processor
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Stripline Kicker

• Baseband Kicker
• Parallel plate approximation Q 2eVL/pwc
• (half the kick comes from electric field, half
  from magnetic)
• 2 strips
• 75 cm long
• 50 Ohm / strip
• 6 mm half-gap
• 4 m from IP
• Deflection angle Q eVL/pwc 1 nr/volt
• Displacement at IP d 4 nm/volt
• Voltage required to move beam 1 s (5 nm) 1.25
  volts (30 mW)
• 100 nm correction requires 12.5 Watts drive per
  strip
• Drive amp needs bandwidth from 100 kHz to 100 MHz

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Capture Transient
Capture transient from 2 s initial offset
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Capture Transient
Capture transient from 10 s initial offset (gain
increased to improve large offset capture)