1. Intra-Pulse Beam-Beam Scans at the NLC IP
Update date
Multibunch (ATF) results)
Chip status
Delete costs
Get better drawing of cavity
More details of cavity BPM scheme
Steve Smith
Snowmass 2001

2. Beam-Beam Scans Ultimate collider diagnostic Some Limitations:
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) 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 from 2 s initial offset

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