The Linear Collider Alignment and Survey (LiCAS) Project

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The Linear Collider Alignment and Survey (LiCAS)
Project

• Richard Bingham, Edward Botcherby, Paul Coe, John
  Green,
• Grzegorz Grzelak, Ankush Mitra, John Nixon, Armin
  Reichold
• University of Oxford
• Andreas Herty, Wolfgang Liebl, Johannes Prenting
• Applied Geodesy Group, DESY

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Contents

• Introduction
• Survey and Alignment of a Linear Collider
• Survey Concept
• LiCAS System Overview
• Frequency Scanning Interferometry (FSI)
• Straightness Monitors (SM)
• Simulation of LiCAS performance
• Summary

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Why do we need another collider ?

• Whats wrong with the LHC ?
• Its a high energy, high luminosity hadron
  collider
• Good as a discovery machine eg Higgs Hunting
• But hadron colliders are messy
• Difficult to make precision measurements
• Cannot determine quantum numbers of initial state

NEED A LEPTON COLLIDER
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Physics with a (Linear) Lepton Collider
MH 120 GeV, 3104 pb-1 S/?B3.6 (5.0 105 pb-1)

• LHC Can see 120 GeV Higgs
• LC Can see 120 GeV Higgs more clearly

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Why do we need a Linear Collider ?

• Cant we build a Super-LEP ?
• Synchrotron Radiation
• For 1 Synchrotron radiation loss

LEP II Super-LEP
Energy 180 GeV 500 GeV
DE / Rev 1.5 GeV 5 GeV
Radius 4.3 km 255.8 km
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The Super-LEP
LEP
Synchrotron radiation loss sets the size of a
Super-LEP Lets try a Linear Particle Accelerator
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Requirements for a Linear Collider

• To study interesting physics, LC must be
• High Energy to create massive particles
• High Luminosity to create large numbers of
  particles
• LC must have
• Large accelerating gradients
• VERY small beam cross-sections at IP O(nm)

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Proposed Linear Collider TESLA
X-FEL

• Collider Length 33km
• Beam Energy 500 GeV
• Beam Luminosity 1034 cm-2 s-1
• Beam Alignment at IP O(nm)
• Collider Alignment Survey

200mm over 600m
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Why is this hard ?
200mm over 600m

• Temperature pressure gradients inside collider
  tunnel affect open-air measurements
• A 600m line of sight can be bent by 4.5mm for
  0.1oC/m temperature gradient
• Ground motion will misalign collider so survey
  must be quick

Light gets bent by air refraction
T
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Ground Motion Effect on Luminosity
Time to reset collider
1week
2s
20s
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Extra Survey Constraints

• Confined space (also used as emergency escape)
• Collider has mixture of straight and curved
  sections
• Electrically noisy environment

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When to Survey Accelerator

• Tunnel Construction
• Check tunnel has stopped settling
• Accelerator Installation
• Check component positions ( correct them)
• Accelerator Maintenance
• If a component is replaced the accelerator will
  be re-surveyed
• Each step has to achieve 200mm over 600m
  precision
• Accelerator Diagnostics
• Check accelerator maintains alignment ( correct
  it)
• Find out what went wrong

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Traditional Accelerator Surveys

• A team of surveyors using theodolites, laser
  trackers, etc
• Make precision measurements of accelerator site
  and accelerator
• A survey takes months to complete and requires a
  large team of people.
• But this approach is not suited to LC because
• Cannot achieve required accuracy
• Slow
• Manual
• Large space required

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Solutions Hydrostatic Levelling Systems

• Traditional method to measure vertical alignment

• But water only follows local geoidsome parts of
  TESLA dont
• .while NLC does not at all

NLC
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Other Solutions

• Use a long stretched wire
• The wire will sag under gravity Only good for
  horizontal alignment
• Use a laser to align accelerator
• In open-air, it will be refracted by temperature
  gradients
• TESLA follows Earths geoid. So cannot be used
  for TESLA

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Survey Procedure

• Two-step Survey procedure
• Survey equidistant tunnel wall markers via
  multiple overlapping measurements LiCAS Job
• Measure collider components against wall makers
  
• Advantage
• The same procedure is employed during tunnel
  construction, collider installation, operation
  and maintenance

Accelerator wall
Survey Train
Accelerator
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Survey Train

• A survey train is used to perform the first step
• Mechanical concept developed by DESY Geodesy
  Group
• LiCAS provides an optical metrology for the train
  
• Survey Train carries two systems
• Frequency Scanning Interferometry
• Makes 1D Length Measurements
• Laser Straightness Monitors
• Measures transverse displacements and rotations

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Survey Train External Measurements

• Each carriage measures the position of a
  reference marker in its own co-ordinates
• Q How to tie reference marker co-ordinates
  together

Marker 1 at (x1,y1)
Marker 2 at (x2,y2)
1D FSI Length Measurements
Carriage 2
Carriage 1
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Survey Train Internal Measurements

• Use internal system to relative positions of
  carriages
• Internal systems ties the external measurements
  together

Marker 1 at (x1,y1)
Marker 2 at (x2,y2)
1D FSI Length Measurements
SM Measurements
Carriage 1
Carriage 2 (xc2,yc2)
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Survey Train LiCAS Systems

• An Optical metrology system for survey of a
  linear Collider
• Fast, automated high precision system
• Can operate in tight spaces

Vacuum tube
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Survey Implementation
Tunnel Wall
Reconstructed tunnel shapes (relative
co-ordinates)
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Frequency Scanning Interferometry

• Interferometric length measurement technique
• Require precision of 1mm over 5m
• Originally developed for online alignment of the
  ATLAS SCT tracker

Tunable Laser
Reference Interferometer L
Measurement Interferometer D
(Grid Line Interferometer (GLI))
Change of phase DFGLI
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FSI Length Measurement
DFGLI
DFRef
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FSI Thermal Drift Cancellation

• Thermal effects add subtle systematic errors to
  FSI
• Nanometre movements can contribute micron errors
  (µ (n/Dn) )
• Use two lasers tuning in opposite directions to
  cancel thermal drift

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FSI Thermal Drift Cancellation
DFGLI
DQ()
True Gradient
DQ(-)
Measured Gradient with Laser Tuning Up
Measured Gradient with Laser Tuning Down
DFRef
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FSI 2-Laser Thermal Drift Cancellation
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FSI ATLAS Implementation
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FSI ATLAS Test Grid

• 6 simultaneous length measurements made between
  four corners of the square.
• 7th interferometer to measure stage position.
• Displacements of one corner of the square can
  then be reconstructed.

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FSI ATLAS Resolution
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FSI ATLAS Resolution

• Stage is kept stationary
• RMS 3D Scatter
• lt 1 mm

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LiCAS FSI System
ATLAS FSI System
Laser 1
Laser 2
Reference Interferometer
Splitter Tree
piezo
detector
Uncollimated Quill
Collimated Quill
1m GLI
5m GLI
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Erbium Doped Fibre Amplifiers

• EDFA are optical power amplifiers
• Used to amplify low power tunable laser
• Standard equipment for Telecoms
• but will it work for FSI ?

4I11/2
Pump 980nm
Decay
Single Telecoms Channel
4I13/2
Signal 1550nm
fluorescence
4I15/2
1610
Wavelength / nm
1530
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Quill Collimation

• Refractive
• Reflective

Quill
Retroreflector
Reflective, off-axis paraboloid
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Two Laser AM Demodulation

• Need 2 lasers for drift cancellation
• Have both lasers present use AM demodulation to
  electronically separate signals

M1
t0
t1
M2
Laser 1
Laser 2
Detector
t0
t1
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Two Laser AM Demodulation

• Amplitude Modulation on FSI fringe
• _at_ 40 80 kHz (now) 0.5 1MHz (later)

• High Pass Filter

• FSI fringe stored as amplitude on
• Carrier (à la AM radio)
• Demodulation reproduces FSI Fringes

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Results of Demodulation
Both signals have same frequency !!
Demodulation of modulated laser does not effect
interferometer signal
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Reference Interferometer Phase Extraction

• Reference Interferometer is FSIs yard-stick
• Must measure interferometer phase precisely
• Uses standard technique of Phase-Stepping

Step1 I(ftrue-1.5Df) Step2 I(ftrue-0.5Df) Step3
I(ftrue0.5Df) Step4 I(ftrue1.5Df)
Reference Interferometer mirror moved in 4 equal
sized steps
ftrue
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Software Phase Extraction

• Telecoms laser tunes linearly
• Extract phase with software phase-stepping

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FSI Extensions for LiCAS

• Collimation optics for quill outputs
• Move to Telecoms wavelength (1510nm 1640nm)
• Telecoms fibres and equipment are cheaper
• Exploit cheap, high quality lasers
• Reduce drift errors
• x300 increase in continuous tuning range (0.24nm
  130nm)
• x3000 increase in tuning rate (100 GHz/min
  5THz/sec)
• New features such as Amplitude Modulation (AM)
• Use Erbium Doped Fibre Amplifiers (EDFA)
• Modular power distribution

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Straightness Monitors

• Used to measure carriage transverse translations
  and rotations
• Require 1mm precision over length of train

Rotation Spots move opposite directions
Translation Spots move same direction
CCD Camera
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SM Rotations about Z

• Use two parallel beams to measure rotation about
  z-axis

SM beams coming out of the screen
Image of beam spots observed on CCD Camera
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SM Splitter Configurations

• Single Beam Splitter End carriage
  retroreflector
• Double Beam Splitter per carriage

• Pro Measurements independent of splitter angle
• Con Retroreflector introduces unknown
• transverse walk to all carriages

• Pro No retroreflector No unknown walks
• Con The angle of each beam-splitter in each has
  
• to be determined 12 extra
  calibration
• constants

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SM Low Coherence Beams

• Low coherence length diode lasers are used to
  avoid CCD interference
• Stray reflections off surfaces can interfere if
  coherent

The two reflected rays can interfere if coherent
The two reflected rays can interfere if coherent
Beam- Splitters
CCD Chip
CCD Glass Face-plate
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SM Interference Rings

• Laser with long coherence length.
• Interference rings observed on CCD

• Laser with low coherence length
• No interference structure is observed

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SM Demagnification Lenses

• CCD cameras are ½ square.
• A long collimated beam Þ large beam
• This can be larger than the CCD
• Use of demagnification lenses increase dynamic
  range
• Lenses must be high quality to prevent beam
  distortion

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SM Results
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SM Stability Results
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SM Extensions for LiCAS

• Use of two parallel beam to measure rotations
  about z-axis
• Two beam-splitter configurations are under
  investigation
• Simple SM under test
• Low coherence length laser under test
• Demagnification lenses are being designed

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Simulations (of single car)

• FSI resolution 1mm, SM resolution 1 mm
• Weak measurement of rotation around z-axis due to
  small separation between two beams on CCD
• Tilt meters resolution 1mrad

Without tilt meters
With 1-Axis tilt meters
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Simulations of Train over 600m
Error on positions lt 200mm after 600m
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Summary

• Future linear colliders require precision survey
  and alignment
• The LiCAS group is developing optical metrology
  techniques to address this in collaboration with
  DESY
• Proposed solution is being developed for TESLA
  but can be applied to any collider
• Preliminary results have been encouraging
• LiCAS is now PPRP approved ?