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Intra-Pulse Beam-Beam Scans at the NLC IP

Intra-Pulse Beam-Beam Scans at the NLC IP. Steve Smith Snowmass 2001. 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

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Intra-Pulse Beam-Beam Scans at the NLC IP

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  1. Intra-PulseBeam-Beam Scansat theNLC IP Steve Smith Snowmass 2001

  2. 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.

  3. 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.

  4. Intra-Pulse Feedback

  5. Intra-Pulse Feedback(with Beam-Beam Scan & Diagnostics)

  6. 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?

  7. Beam-Beam Scan Beam bunches at IP: blue points BPM analog response: green line

  8. 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!

  9. 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

  10. Beam-Beam Deflection

  11. Simulated BPM Processor Signals BPM Pickup (blue) Bandpass filter (green) and BPM analog output (red)

  12. 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 <3ns propagation delay (plus cable lengths)

  13. Fast BPM Processor

  14. 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

  15. Capture Transient Capture transient from 2 s initial offset

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

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