NLC Intra-Pulse Fast Feedback

1
NLC Intra-PulseFast Feedback

• Simon Jolly
• Oxford University

NLC Beam Delivery Meeting July 2001
2
Before we begin...
I have stolen parts of this talk from Glen
White, Steve Smith, PT and then some..
3
Plan of Action

• Requirements of a feedback system.
• Current design
• Physical specs.
• Signal filtering electronics.
• Simulated performance.
• Current status and planned tests.
• Track reconstruction.
• A brief word on beam jitter.
• Short term and long term plans.

4
Fast Feedback - Who needs it?

• Jitter inherent in beams and accelerating
  structures - leads to relative position offset of
  beams.
• Position offset leads to
• large luminosity loss

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Fast Feedback - System Constraints
A corrective feedback system needs to

• Recover significant amount of lost Luminosity.
• Correct offset within a single bunch train (266ns
  - hence fast...).
• Dominant time factor should be distance to IP,
  NOT speed of feedback - too fast for analytical
  electronics.
• Be unaffected by intra-train jitter..

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NLC Fast Feedback System
Use beam-beam interaction to enhance offset
measurement

• System consists of 3 components
• BPM ( BPM processor).
• bunch charge gain adjuster.

• Kicker (and kicker driver).

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Design of Feedback System

• Initial system design and proof of principle in
  Simulink simulation by Steve Smith.
• Glen White (Oxford) simulation makes a number of
  improvements
• Includes gain effects.
• Accurate beam-beam interaction model - original
  flat beyond 12s (GUINEA-PIG).
• Effects of intra-train (bunch-to-bunch) jitter
  considered.
• System currently only corrects position offset
  (no angle jitter).

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Simulink Block Diagram
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BPM Processor
Most signal conditioning executed by BPM processor
But what does it do?
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BPM Electronics
Simulink diagram for BPM processor
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BPM Signal Filtering
5
10
25
15
20
30
Time (ns)
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BPM Electronics Output
0
100
200
266
Time (ns)
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Beam Correction at IP (Simulink)
0
-2
Vertical offset (nm)
-4
-6
-8
0
100
200
266
Time (ns)
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Effect of Feedback System
Y position offset (sy)
Effect of the feedback system on the luminosity
loss (Glen White).
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What Happens Next?

• Bench test BPM electronics.
• Beam test of stripline BPM and electronics.
• Confirm design of kicker dimensions and power
  requirements - dependant upon location, train
  structure.
• Beam test of complete system (location on a need
  to know basis.).

• Reconstruction of tracks in beam test ? use PTs
  Collimator Wakefield Matlab routines.

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Collimator Wakefields (PT)

• 4 collimation slots used.
• Determination of bunch kick due to wakefield
  effects.
• To reconstruct kicks
• Measure positions of bunches (25 per step) along
  sector 2.
• Subtract reference track (100 bunches).
• Use transport matrices to reconstruct bunch
  position and angle at slot.

Collimator slot dimensions
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Reconstructed wakefield kick
Collimator slot height vs. angle deviation
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Reconstructed kicks (slot 1)
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Angular Jitter on Kick Reconstruction
4.0
Collimator slot height vs. angular jitter for
reconstructed wakefield kicks (slot 1)
3.5
3.0
2.5
2.0
1.5
RMS angular jitter (mr)
1.0
0.5
0
0
-0.5
-1
0.5
1
1.4
-1.4
Wakefield box slot y posn. (mm)
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A Quick Look at Position Jitter
2D Histogram of beam jitter
Data taken from 160 data samples over 12 days
500 x 500 mm
X
Y
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X and Y jitter on SLC e- beam
Histogram of jitter in x
Histogram of jitter in y
sx 17.65 mm
sy 14.34 mm
0
50
100
-100
-50
0
50
100
-100
-50
x distance from mean orbit posn. (mm)
y distance from mean orbit posn. (mm)
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Time Dependence of Jitter
Beam jitter in y for 3 BPMs
Chart shows beam jitter (rms deviation from mean)
for 3 BPMs during 160 runs. Each point is rms of
y position for 100 bunches (1 reference
scan). Includes 12 days worth of data.
sy (mm)
Run number
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Time Dependence of Jitter (2)
Beam jitter in y for 7 BPMs
BPM
801
631
511
411
301
146
114
0
40
80
120
160
Run number
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And Finally...

• Next step is to bench test BPM electronics.
• Start looking at possible solutions for kicker
  design.
• Longer term beam tests of BPM systems, kicker
  design and complete system.
• Very very long term install system in the NLC.