Beam Instrumentation Challenges at the International Linear Collider

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Beam Instrumentation Challenges at the
International Linear Collider

• 2006 Beam Instrumentation Workshop
• Fermilab

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ILC Schematic
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How the ILC Works
Every 200 msec

• Sources DRs deliver 2820 bunches of 2 x 1010
  over a period of 900 µsec
• 307 nsec bunch spacing
• Normalized emittances 8 µm x 20 nm (4001 aspect
  ratio)
• RMS bunch length 6-9 mm
• 5 GeV energy at DR exit
• Bunches are collimated, spin-rotated, compressed,
  accelerated, focused, collided
• RMS bunch length _at_ IP 300 µm
• RMS beam size _at_ IP 650 nm x 5.7 nm
• Normalized emittance _at_ IP 10 µm x 40 nm
• Beam energy _at_ IP 250 GeV
• Beam power _at_ IP 11.2 MW per beam

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Challenges of the ILC

• High-intensity collisions _at_ nanometer scale
• Emittance generation and preservation
• Luminosity, Energy, Polarization (LEP)
  measurements of the collision
• Machine Protection

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Beam Position Monitors

• Main instrument for emittance control in the ILC
• Damping Rings 500 x 3
• Ring to Main Linac 318 x 2
• Main Linac 314 x 2
• Beam Delivery System 300 total
• Plus additional BPMs in injectors upstream of
  DRs, e- linac wiggler insertion (e production),
  etc etc etc

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BPM Resolution
RMS Beam size in Ring to Main Linac (RTML) 1-10
µm is typical
Example of main linac emittance tuning
performance as a function of BPM resolution
(single bunch of 2 x 1010) Figure courtesy K.
Ranjan, FNAL
Emittance performance and jitter
detection/tracking both argue for resolution in
the sub-micrometer regime for most BPMs
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BPM Center Stability and XY Coupling
Beam Aspect Ratio in RTML
BPM Offset change of a few µm between global
emittance tunings is probably OK
XY coupling of BPM readings should be 0.1
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BPMs Collisions
Strong beam-beam interaction ? luminosity
extremely sensitive to small offsets
But a small offset ? a large kick, which can be
measured by BPMs with µm resolution
Implies that IR BPMs, and some others, must have
enough bandwidth to do bunch by bunch
measurements, and have good behavior independent
of fill pattern (µm level)
Figures courtesy G. White (SLAC) and I. Reyzl
(DESY)
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ILC BPMs Additional Requirements

• Environment
• Many of the BPMs need to be installed near SC RF
  cavities
• Some are in high-radiation environment of
  detector and/or dumpline
• Fill Patterns
• Most orbit tuning will be performed with single
  bunches
• Luminosity is produced by bunch trains
• Therefore, BPM offsets cant vary much when
  operations switches from single bunches to trains
• Behavior with varying intensity
• BPMs need to operate with charge as low as 1 x
  108 (pilot bunch operation)
• Reduced resolution okay at these charges (whew!)
• How linear do the BPMs need to be over this
  operating range? How much tuning will be done
  with very low bunch intensities?
• Near-IP BPMs
• IR with 2 mrad crossing angle
• Both beams pass through each IR BPM
• Spent (ie, less-interesting) beam has large
  offset relative to incoming (ie, more
  interesting) beam
• In general, near IP BPMs are very important!
• They play a crucial role in keeping the beams in
  collision!

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BPM Technology Choice

• Standard ILC BPM RF cavity
• Naturally high resolution
• Naturally stable centering
• Can be made relatively free of common-mode signal
• Compatible with ultra-clean cryogenic RF systems
• Tend to be fussy
• Some areas striplines
• Large apertures
• Directional
• Not great on orbit stability
• Though FFTB did pretty well!
• Good for extraction lines and 2 mrad IR

Measurement of noise in RF-BPM triplet yields 20
nm single-pulse resolution
Figure courtesy M. Ross and G. White, SLAC
Error signal from FFTB asymmetric BPM Triplet
test. The signal corresponds to 2.7 µm RMS
center variation over a period of 1 week.
BPM Error mm
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Transverse Profile Monitors

• Requirements
• Good resolution for small spots (a few µm
  vertical RMS)
• Acceptable systematics for large xy aspect ratio
  (15-25)
• Capacity to endure high charge density of beam
• Turnkey operation
• No solid materials meet all these criteria
• Liquid and gas targets have their own problems
• For ILC Photons!
• AKA Laser Wire

Survival of SLC wire scanners as a function of
particle density. ILC beams are off the scale to
the right by at least a factor of 10. Figure
courtesy C. Field, SLAC.
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Laser Wires

• Used successfully at a number of locations
• SLC IP (45.6 GeV)
• KEK-ATF (1.28 GeV)
• PETRA (7 GeV)
• Challenges
• Signal strength and system design to extract
  signal
• Wide variation in beam energies ? variation in
  signal strength and form
• Injector 1 GeV
• RTML 5-15 GeV
• Linac 15-250 GeV
• BDS 250 GeV
• Matching laser time structure to electron beam
• Can we perform an entire scan in 1 train by
  moving the light within the train?
• Slow scans in 1 bunch / 5 Hz mode
• Large aspect ratio
• Systematic overestimation of y beam size due to
  finite Rayleigh range
• Limited dynamic range
• Cant reliably scan below 1 µm RMS size

Images courtesy K. Balewski, M. Ross, H. Sakai
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Optical Transition Radiation (OTR) Profile
Monitors

• Demonstrated capacity to measure 5 µm RMS sizes
  for gt 1 GeV beam energy
• Single-shot measurement of beam ellipse
• Invasive to luminosity production
• Single-bunch only
• Susceptible to damage
• Used in a few locations
• Relatively large beams
• Diagnostic of last resort
• Measuring streaked beams (more on this next
  slide!)

Image of a tilted 10 µm x 13 µm RMS spot at
KEK-ATF via a beryllium OTR profile monitor.
Image courtesy M. Ross.
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Bunch Length Measurement

• RMS bunch lengths of 150-300 µm
• Or 0.5-1.0 psec
• Use S-band dipole-mode cavity on zero-crossing
• dV/dz streaks beam in vertical
• Image streaked beam on a downstream profile
  monitor
• Typically OTR
• Occasionally laser wire
• Used at SPPS, TTF2
• Will be used at LCLS, XFEL
• Voltage determined by beam energy, RF wavelength,
  ratio of bunch length to vertical angular
  divergence
• ILC needs are modest because of small vertical
  divergence

TTF beam at 450 MeV which has been vertically
streaked by a 20 MV dipole-mode cavity at 2856
MHz. Image courtesy M. Huning.
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Bunch Length Measurement (2)

• ILC dipole mode cavities will be short (0.4 m)
• Fill time 150 nsec
• Can fill in the inter-bunch interval becomes a
  single-bunch device
• Limits long-range wakefield buildup
• Relatively power hungry
• Other diagnostic uses besides sz
• Ez correlation
• Requires dispersion at readout point
• Yz correlation
• Requires horizontal streak

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Bunch Length Measurement (3)

• RF Detector method
• Measures beam power in high-frequency band
• Poor mans hardware FFT aka xylophone
• Non-invasive, cheap and simple
• Only a relative measurement
• Needs dipole-mode cavity as calibration

Measurement of several RF diodes at the end of
SLAC as a function of bunch compressor parameters
(voltage and phase of the compressor RF). Figure
courtesy M. Woods.
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Luminosity Monitor

• ILC operates in regime of intense beam-beam
  interaction
• Studies have shown that for realistic bunch
  shapes, optimum luminosity does not occur at the
  point where the beam-beam deflections are zeroed
• Only way to optimize is to zero deflections, then
  vary beam-beam offset to maximize luminosity
• Has to be done within 1 train
• Optimum varies from train to train
• Requires a luminosity monitor which can provide
  useful bunch by bunch information for dither
  feedback controller

Simulation of optimizing luminosity as a function
of collision offset and angle within 1 train.
Necessary because the max lumi, head-on
collisions, and zero deflection all occur at
different points in the parameter space!
Figures courtesy G. White, SLAC
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LuminosityEnergyPolarization Measurements

• Beam polarization crucial part of ILCs power as
  a physics instrument
• Precision measurement of energy similarly crucial
• Strong beam-beam results in a long tail on
  luminosity spectrum, depolarization during
  collision
• Precision measurement of luminosity (at
  collision), energy (pre- and post), polarization
  (pre- and post) necessary to understand
  luminosity spectrum

Image courtesy International Linear Collider
Technical Review Committee
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Machine Protection

• Damage from beam impact at normal incidence
• Niobium threshold is around 5x1014 e-/cm2
• Copper is about the same (collimators)
• Titanium can take 15x higher density
• At glancing incidence
• High-z materials about the same as normal
  incidence
• Low-z factor of a few higher density can be
  tolerated
• For ß 100 m, 2e10 e-/bunch, single bunch
  density
• 8x1013 _at_ 5 GeV
• 4x1015 _at_ 250 GeV
• Even at low energy, a few low-emittance bunches
  will damage anything they hit!

Electron micrograph of 1.4 mm thick Cu target
with silhouette of passage of 30 GeV electron
beam. Image courtesy D. McCormick, SLAC.
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Machine Protection (2)

• A significant amount of ILC commissioning will
  need to be done with low-density pilot bunches
• Lower charge and/or larger emittance
• Instrumentation must be adequately responsive for
  low-intensity tuning
• Current baseline design includes a pilot bunch on
  every accelerator cycle
• 10 µsec ahead of luminosity bunches
• Additional protection for some areas, including
  detector
• Need dedicated MPS-linked beam instruments which
  detect the pilot bunch, or radiation generated
  when it hits something, reliably determine that
  it did/didnt made it to the dump
• Need to monitor trajectories of main beam bunches
  as well something could go wrong between pilot
  bunch and end of lumi bunches
• System from MPS instruments to MPS response
  devices (abort kickers, DR extraction kicker)
  needs very low latency travel times for signals
  will consume most of the pilot bunch head start

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Conclusions

• ILC will break new ground in terms of beam
  quality requirements
• which implies breaking new ground in terms of
  both quality and quantity of beam instruments
• Cant make a new accelerator, with innovative
  requirements, entirely out of COTS instruments!
• Heavy reliance on high-precision, stable BPMs for
  emittance tuning, feedback, feedforward
• Transverse emittance measurement based on
  substantial number of laser wire scanners
• Two bunch-length monitoring technologies dipole
  mode cavities and RF spectrum analysis
• Fast luminosity monitor required for collision
  orbit tuning on every accelerator cycle
• Energy and polarization measurements needed to
  decolvolve luminosity spectrum
• Machine protection will require some beam
  instrumentation designed solely for this purpose

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Acknowledgements
There are many people who have worked for a
decade and more on linear collider beam
instrumentation, either directly (hardware
prototyping and experiments) or indirectly
(studies of requirements and usage of the
instrumentation suite). There are too many
people in this category to list, but they know
who they are and I thank them.