NLC - The Next Linear Collider Project
Next Linear ColliderBeam Position Monitors
Steve SmithSLAC
Interaction PointBeam Instrumentation Study
June 26, 2002
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
NLC Linac BPMs
• “Quad” BPM (QBPM) – In every quadrupole (Quantity ~3000)– Function: align quads to straight line– Measures average position of bunch train– Resolution required: 300 nm rms in a single shot
• Structure Position Monitor (SPM)– Measure phase and amplitude of HOMs in accelerating cavities– Minimize transverse wakefields– Align each RF structure to the beam– Few 104 devices in the two linacs– Not used in Interaction Region
• “Multi-Bunch” BPM (MBBPM) – Measure bunch-to-bunch transverse displacement – Compensate residual wakefields– Measure every bunch, 1.4 ns apart– Requires high bandwidth (300 MHz), high resolution (300 nm)– Line up entire bunch train by steering, compensating kickers
• Intra-Train Feedback BPM
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
QBPM Requirements
Parameter Value Conditions Resolution 300 nm rms @ 1010 e- single bunch
Position Stability 1 m over 24 hours (!)
Position Accuracy 200 m With respect to quad magnetic center
Position Range 2 mm
Charge Range 0.5109 to 7.5109 e- per bunch
Number of bunches
1 - 190
Bunch spacing 1.4 ns
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
• Dipole frequency: 11.424 GHz• Dipole mode: TM11• Coupling to waveguide: magnetic• Beam x-offset couple to “y” port
• Sensitivity: 1.6mV/nC/m (1.6109V/C/mm)
• Couple to dipole (TM11) only
• Does not couple to TM01
• Compact
• Low wakefields
Port to coax
BPM Cavitywith TM110 Couplers
Zenghai Li
Author NameDate
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Cavity BPM Parameters
Parameter Value Comments
Dipole frequency 11.4 GHz
Monopole frequency 7.66 GHz
Cavity Radius 16 mm
Wall Q ~4000 Ignoring beam duct, etc
Cavity coupling = 3
Loaded Q 1000
Bandwidth 11 MHz
Beam aperture radius 6 mm
Sensitivity 7 mV/nC/m (too much signal!)
Bunch charge 0.7 x 1010 e- Per bunch
Signal power @ 1m - 29 dBm Peak power
Decay time 28 ns
Required resolution = 200 nm
Required Noise Figure 57 dB For = 100 nm, thermal only
Wakefield Kick 0.3 volt/pC/mm Long range
Structure wakefield kick ~2 volt/pC/mm Per structure
Short-range wakefield ~1/200th of structure
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Antenna Scan
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Antenna Scan - Residuals
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Multi-Bunch BPMs
• Stripline pickups
• Report position of every bunch in bunch train
• Used to program broadband kickers to straighten out bunch train
Parameter Value Conditions & Comments
Resolution 300 nm rms At 0.6 x 1010 e- / bunch
for bunch-bunch diplacement frequencies below 300 MHz
Position Range 2 mm
Bunch spacing 1.4 ns
Number of Bunches 1 - 190 @ 1.4 ns
Beam current dynamic range
1109 to 1.4 1010 Particles / bunch
Number of BPMs 278
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
ATF Bunch Current
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Energy from BPMs
• Dispersion:– How energy deviation translates into transverse beam motion
x pp – Where is the dispersion ~ 20 – 80 mm
• BPM stability of ~ 1 m energy measurement of ~ 10-5
• But BPM measures mean position of charge of bunch
• Transverse tails?
• Energy tails?– 1% of charge off energy by 1% error of 10-4.
• Need luminosity weighted energy– Get charge-weighted energy
Author NameDate
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Luminosity from BPMs
• BPMs measure beam-beam deflection angle • Deflection angle depends on beam size, charge, beam offsets,
angles, bunch length,…
• But luminosity is given by x, y:
L = • Can be difficult to extract true L from deflection.
• Really need luminosity monitor that measures collision processes.
• Deflection measurement is crucial for optimization, feedback, spot size measurement, etc.
yx
beam
cm
D PN
E
H
4
2
Author NameDate
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Beam-Beam Deflection(electron microscope!)
Deflection coefficient
(for small offsets)
25 radian / nm At Interaction Point
Displacement 100 m / nm At Position Monitor
dq
e+
e-
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Beam-Beam Deflection“Guinea Pig” simulation provided by A. Seryi
Initi
al S
lope
Slope at Large O
ffset
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Beam-Beam Deflection Slope
40 m focal length!
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Power of Deflection Measurement
• Beams see 40 m focal length lens (at < 1 y offset)
• Image distance 4 meters• Magnification of 4m/40 m = 105
• BPM resolution of 1 m – beam offset resolution of 1 m / 105 = 10 pm (!)
• Confounded by – Tails
– Angle jitter
– Rotation
• Dilution of 100 beam–beam offset resolution ~ 1nm
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Beam-Beam Scans
• Important 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
• i.e. low frequency noise, ground motion, damping ring jitter.
• 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– Assuming existence of Intra-train IP feedback.
Author NameDate
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
IP Intra-Train Feedback BPM
• Interaction Point Intra-Train Feedback BPM– IP only
– Much like Multibunch BPM
– Critical difference:
• Very short propagation delay
• Close loop in ~ 40 ns < 250 ns train passage time.
– Other differences:
• Half the bandwidth
• Don’t care about bunch-bunch motion
– feedback only makes bunch-bunch motion worse!
Author NameDate
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Intra-Train Feedback
• Vertical spot size comparable to ground motion
• Small beam size luminosity sensitive to beam jitter
• We can measure extremely small inter-beam offsets via beam-beam deflection
• Can we make a feedback fast enough to correct beam-beam mis-alignment within the 265 ns bunch-crossing time?
• System consists of a fast position monitor, kicker, and feedback regulator.
• Old-fashioned analog feedback.
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Intra-Pulse Feedback
Amp
BPMProcessor
IP Round Trip Delay
+
Amp
14 ns
14 ns
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Intra-Train Feedback
• Fix interaction point jitter within the crossing time of a single bunch train (265 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 is complicated by:– round-trip propagation delay to interaction point in the feedback
loop
– Response non-linearity of beam-beam defelction for flat beams
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Limits to Beam-Beam Feedback
• Must close loop fast– Propagation delays are painful
• Beam-Beam deflection is non-linear– Feedback gain drops like 1/dfor large offsets
– Feedback converges too slowly beyond ~ 30 to make a recover luminosity
• Can fix misalignments of 100 nm with modest kicker power
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Intra-Train Feedback
• Setup critical:– IT Feedback holds outgoing beam to a position setpoint
– Other hardware required to establish/maintain proper setpoint
– Multibunch BPM readout of IT feedback, symmetric pickups
• Higher bandwidth
• Better resolution
• Better stability
• (SLOW)
• Response of beam to FB drive contains diagnostic information– Need deflection vs. bunch digitized and logged
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
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.
• Noise is all at low frequency
• Use to
• Establish collisions
• Measure IP spot size
• Waist scans
• What else?
Author NameDate
Slide #
Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Intra-Pulse Feedback(with Beam-Beam Scan & Diagnostics)
Amp
BPMProcessor
IP Round Trip Delay
Digitizer
Beam-Beam Scan& Diagnostics
Ramp
+
Amp
Author NameDate
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
Beam-Beam Scan
Beam bunches at IP: blue points
BPM analog response: green line
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Next Linear Next Linear ColliderCollider
Steve Smith - 6/26/02
IP Beam Position Monitors
• Stabilize collisions
• Measure spot sizes
• Diagnostics