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
• LiCAS Phase I
• Frequency Scanning Interferometry (FSI)
• Straightness Monitors (SM)
• LiCAS Phase II Final Focus Stabilisation
• Summary

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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
• Ground motion will misalign collider so survey
  must be quick
• These precludes conventional open-air surveying
  techniques

Light gets bent by air refraction
T
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Survey Procedure

• Two-step Survey procedure
• Survey regularly spaced tunnel wall markers via
  multiple overlapping measurements LiCAS Job
• Measure collider components against wall makers
  
• Can be done manually with simple instrument
• Advantage
• The same procedure is employed during tunnel
  construction, collider installation, operation
  and maintenance

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Survey Implementation
Tunnel Wall
Reconstructed tunnel shapes (relative
co-ordinates)
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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

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First Survey Train Carriage

• This carriage forms the mechanical body to hold
  the LiCAS sensors (FSISM).
• The carriage is able to move the sensors into
  position

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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 Interferometer
Glass BeamSplitter
Tunable Laser
Fibre
Collimator
Retroreflector
Photodiode
Quill
Fibre Splitter

• Common path optical-fibre based interferometer
• Optical fibre allows remote delivery and
  measurement
• Allows interferometer heads to be small compact

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FSI Implementation
½ Sphere mounted retroreflector
Fibre Collimator
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FSI Results

• A retroreflector was moved perpendicular to FSI
  measurement interferometer
• Error on each measurement is 4mm
• Good for first attempt but still need to achieve
  1mm

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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 Apparatus
Retroreflector
Beam Splitter
Linear Stage
Rotation Stage
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SM Results Linear Translation
CCD 1
The beam spots move in the same direction for
translation
CCD 0
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SM Rotation
CCD 1
The beam spots move in the opposite directions
for rotation
CCD 0
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Simulations of Train over 600m
Error on positions lt 200mm after 600m
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End of LiCAS Phase I ..Onto LiCAS Phase II
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LiCAS Phase II Final Focus Stabilisation

• Final Focus magnets of LC need to be stabilised
  to nanometres
• FSI can provide micron resolution absolute
  measurements.
• Interferometer fed with light from a tunable
  laser
• Michelson interferometry gives differential
  nanometre resolution
• Add fixed frequency laser to interferometer
• Combine to get best of both techniques

Tunable Laser
Photodiode
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LiCAS Final Focus Stabilisation
Final Focus Magnets
e
Detector
Machine Components

• M-FSI can measure absolute and relative lengths
• Position is not dependent on beam
• Light fed by fibres
• No complex geometry for light path
• Can follow same route as DAQ cables out of
  detector
• Grid can measure all degrees of freedom

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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
• General collider alignment survey is a good
  test-bed for LiCAS technology
• Plans to extend LiCAS technology for Final Focus
  stabilisation

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Final Focus Stabilisation Optical Anchor

• Use Michelson interferometers to monitor nm
  movement of magnets
• Complex geometry for light path
• Not all degrees of freedom can be measured
• Can only make relative measurements
• Position data lost when beam is lost

Optical Anchor
Final Focus Magnets
e
Detector
Optical Anchor
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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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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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FSI Installation

• Installation of first components of ATLAS FSI
  system into first carbon fibre barrel.
• Just completed on Tuesday (19/08/2003)

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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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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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Getting FSI to nanometres
Tunable Laser
Photodiode

• FSI can provide micron resolution absolute
  measurements.
• Interferometer fed with light from a tunable
  laser
• Michelson interferometry gives differential
  nanometre resolution
• Add fixed frequency laser to interferometer
• Combine to get best of both techniques