The LCLS BC1 Bunch Length Monitor System

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The LCLSBC1 Bunch Length Monitor System
Eric Bong1 Mike Dunning2 Paul Emma1 Patrick
Krejcik1 Tim Montagne1 Jamie Rosenzweig2 Gil
Travish2 Juhao Wu1
1SLAC 2UCLA
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Intro The Problem

• The LCLS demands tight beam parameters
• Longitudinal feedback systems needed (along with
  other diagnostics and feedback systems)
• Bunch length
• Energy
• This is a critical diagnostic
• Machine wont work without this
• Has to operate 24-7
• Need to start addressing safety and approval
  issues
• Time is short

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Intro The Approach

• UCLA to build bunch length monitor system
• System will consist of two grating
  polychromators, one at each bunch compressor
• Simulations to determine start-end response of
  monitor.
• SLAC to integrate into beamline
• Working on testing at and with ANL-APS

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Introduction
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Relevant Parameters
1.0 nC
0.2 nC
Nominal electron energy, BC1 0.25 0.25 GeV
Nominal electron energy, BC2 4.3 4.3 GeV
Peak current 3400 2500 A
Nominal RMS bunch length, BC1 200 60 µm
Nominal RMS bunch length, BC2 20 8 µm
Nominal RMS bunch duration, BC1 670 200 fs
Nominal RMS bunch duration, BC2 67 27 fs
Max single bunch repetition rate 120 120 Hz
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Intro Possible Solutions

• Streak Camera
• Interferometer
• Electro-Optic Techniques
• RF Deflecting Cavity
• Polychromator (Spectrometer)
• ______(fill in your favorite method here)

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Intro System Requirements

• Only relative bunch length needed- not absolute
  bunch length
• Need two bunch length monitors- one at each bunch
  compressor
• Single-shot
• Non-invasive
• Maintenance free for several days
• Maximum repetition rate 120 Hz
• Measurement resolution 1-2 of nominal bunch
  length(sub-femtosecond)
• Long term signal drift lt2 over 24 hours

LCLS PRD.
J. Wu et al., SLAC-PUB-11276, May 2005.
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Intro Phase Feedback
Feedback model studied by Wu, et al.,
SLAC-PUB-11276, May 2005.
Conceptual Schematic of single loop

• Observables
• - Bunch length sz
• Energy E
• Controllables
• - Linac voltage Vrf
• - Linac phase frf

LCLS longitudinal feedback has 2 bunch length
loops
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Possible Solutions

• Streak Cameras
• Single-shot
• Wide dynamic range
• - Limited temporal resolution(200 fs at best)
• - Trigger jitter

Hamamatsu "FESCA-200 (Femtosecond Streak
Camera). Temporal resolution 200 fs.
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Possible Solutions

• Interferometers
• Can be single-shot
• ? Sufficient temporal (frequency) resolution
• Compact
• - Narrow dynamic range
• - Complex

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Possible Solutions

• Electro-Optic Methods
• Single-shot
• ? Non-invasive
• ? Temporal resolution
• - Not yet mature
• - Require expensive
• femtosecond lasers

P. Bolton et al., SLAC-PUB-9529. Transverse probe
geometry produces a spatial image of the bunch.
Also see http//www.rijnh.nl/users/berden/ebunch
.html
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Possible Solutions

• RF Deflecting Cavities
• Single shot
• ? Temporal resolution
• - May require separate RF system
• - Invasive (destroy measured shot)

The UCLA 9-cell X-band standing wave deflecting
cavity. Courtesy Joel England.
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Possible Solutions

• Polychromators
• Single-shot
• Temporal resolution
• Robust
• Require relatively
• expensive detector vacuum system

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BC1 Single-Shot Spectrometer

• After 4th magnet
• CSR port
• Narrow angle
• Magnet justupstream

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Single-Shot Spectrometer
Use CSR/CER from bunch compressor chicane
magnets ? Vacuum port window ? Focusing/turning
mirror ? Entrance slit ? Grating ? Off-axis
parabola (line focus) ? Multichannel detector
Basic Design
Cryostat Bolometers
Line focus mirror
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The Selection Polychromator

• Spectrums are very reliable no dependence on
  amplitude (charge) or intricacies of pulse shape.
• Speed, bandwidth and phase noise not an issue.
• Robust (0-2 moving parts)
• Proven
• Simple
• Easy to model and easy to fix

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Single-Shot SpectrometerBunch Distributions
BC1
BC2
Courtesy P. Emma
Courtesy P. Emma

• Smooth parabolic distribution
• Simple CSR spectrum

• Wake-induced double-horn
• - Complicated CSR spectrum

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UCLA SimulationsTREDI Post-processor FieldEye

• Calculate far field radiation pattern using
  Lenard-Wiechert algorithm
• Fourier analysis
• Angular spectrum
• Dependent on TREDI
• TREDI time intensive
• Flexibility in post-processing
• Other applications
• Coherent Transition Radiation
• Coherent Cerenkov radiation

Sample CSR spectrum calculation using FieldEye
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Related CER Work (at BNL - ATF)
UCLA built ATF compressor.
Brookhaven Si Bolometerfor CER detection.
FieldEye calculated spectrum for 20 micron long
beam, 1000 particles, edge radiation in chicane
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Challenge Beamline Integration

• Low-loss vacuum port window over desired
  frequency range (Diamond)
• Cryostats liquid helium nitrogen
• Helium hold time (weeks?)
• Closed-cycle nitrogen system (Sterling Engine?)
• Room temperature alternative?
• Windowless enclosure for detector system

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Vacuum Port WindowMaterials Issues

• Central Issue Flat response curve over THz
• Materials
• Z-cut crystalline quartz
• Silicon
• HDPE
• PTX (polymethylpentene)
• Diamond
• References
• http//tesla.desy.de/rasmus/projects/autocorrelat
  or/plan.pdf
• SLAC-PUB-11249

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Diamond Vacuum Port WindowAbsorption Curve
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Diamond Vacuum Port WindowVendors
Harris International http//209.123.148.104/windo
ws.asp Fraunhofer http//www.iaf.frau
nhofer.de/eng/gf/cvd-prod-strah.htm EOC (Electro
Optical Components) http//www.eoc-inc.com/diamon
d_optics.htm
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Mirrors
OAP focusing/turning mirror
OAP line focus mirror
100 mm

• Conventional gold coated copper
• Here could be CNC aluminum

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MirrorsVendors

• Kugler of America
• http//www.kuglerofamerica.com/optics.htm
• Reynard Corp.
• http//www.reynardcorp.com/index.php
• Janos Tech
• http//www.janostech.com/index.html
• CVI Optical Components
• http//www.ocioptics.com/product.html

Janos Tech
Kugler of America
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Grating
150 mm

• Machined Copper
• Blazed
• Groove width 1 mm
• To be optimized

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Challenge Detectors

• BC1
• Frequency range
• 150-500 GHz
• 20 channels
• Easy, but big
• large vacuum chamber
• large optics
• InSb hot electron bolometers

• BC2
• Frequency range
• 1-4 THz
• 20 channels
• More challenging than BC1
• Needs special filtering
• Thermal composite bolometers?
• Need to research more

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DetectorsBolometers

• Speed
• Sensitivity
• Cryo-cooling
• Cryostat hold time
• Expensive
• Vendors
• QMC Instruments http//www.terahertz.co.uk/QMCI/q
  mc.html
• IR Labs http//www.irlabs.com/irlabs20pages/irla
  bs_frameset.html

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DetectorsPyroelectric

• Relatively inexpensive
• Speed
• - Sensitivity
• - Resonances
• Vendors
• EOC
• http//www.eoc-inc.com/pyroelectric_detectors.htm
  
• Fuji Co.
• http//www.fuji-piezo.com/prodpyro.htm

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DetectorsGolay Cells

• Less expensive than bolometers
• Speed (25 Hz)
• Noise sensitivity
• Size
• Vendors
• QMC Instruments http//www.terahertz.co.uk/QMCI/q
  mc.html
• Tydex http//www.tydex.ru/about.html

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DetectorsDiodes

• Inexpensive
• Speed
• Frequency response
• Sensitivity
• Vendors
• Virginia Diodes http//www.virginiadiodes.com/ind
  ex.htm
• Advanced Control Components http//www.advanced-c
  ontrol.com/products-detectors.php

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BC1 Detector Assembly

• InSb hot-electron bolometers
• 10 liter cryostat
• Helium hold time 4-6 weeks!

Detector blocks with Winston Cones and Filters
20-channel linear array of InSb hot-electron
bolometers,courtesy QMC Instruments.
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Testing

• LCLS BC1 is most similar to the APS LEUTL Bunch
  Compressor.
• Propose collaboration with ANL on testing.
• Rapid installation of window and turning mirror
  (site specific) during proximate access.
• Installation of polychrometer as assembly.

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What will we test

• Beam energy, energy spread, charge and bunch
  length can mimic LCLS design.
• Simulations can be confirmed for a range of bunch
  lengths.
• Noise, dynamic range in amplitude (charge) and
  bunch length.
• Hard to test feedback

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Project Start
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Project Overview
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Schedule Issues

• Theory behind on simulations
• Acquisition driven by detector
• Testing can you install and test anything in 2
  months?
• Delivery November 2006
• Post delivery beamline integration, safety
  issues, etc.

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Conclusions

• Simulations behind schedule
• Subcomponent selection proceeding well
• Challenging schedule can be addresses with APS
  collaboration, better coordination with SLAC, and
  timely funding.
• Hardware cost and schedule driven by detector
• Beamline integration needs to be considered now

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End
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Possible SolutionsSummary
Single-shot Non-Invasive Sufficient Temporal Resolution Maintenance Free
Streak Camera Y Y N Y
Interferometer Y Y ? N
Electro-Optic Y ? ? ?
RF Deflector Y N ? N
Polychromator Y Y Y Y
PBPL
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Single-Shot SpectrometerChallenge BC2 CSR
Spectrum

• Double-horn distribution complicates CSR
  spectrum
• Similar to Gaussian below 4 THz
• Wu Stay below 4 THz

CSR energy spectrum after BC2. Black curve
double-horn distribution Blue curve Gaussian
distribution Red curve step function From J.
Wu, et al., SLAC-PUB-11275, May 2005.
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Workplan

• Simulate CR exiting vacuum ports of BC1, BC2
  arriving at detector
• TREDI/FieldEye simulations
• Choose detector type
• Finalize bolometer evaluations
• Continue to study system
• Formation length (source to detector distance)
• Dynamic range (grating, in situ tuning)
• Calibration methods
• Mechanical design beamline integration with
  SLAC
• CAD design work
• Finalized by SLAC
• Test system (SPPS or APS Linac)