The Linear Collider Alignment and Survey (LiCAS) Project Richard Bingham, Edward Botcherby, Paul...

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

7 March 2003

LiCAS Project: UCL Seminar 2

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

7 March 2003


Why do we need another collider ?

What’s wrong with the LHC ?• It’s 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

7 March 2003


Physics with a (Linear) Lepton Collider

LHC: Can see 120 GeV Higgs

LC: Can see 120 GeV Higgs more clearly

MH = 120 GeV, 3•104 pb-

1

S/B=3.6 (5.0 105 pb-1)

7 March 2003


Why do we need a Linear Collider ?

Can’t we build a Super-LEP ?• Synchrotron Radiation

• For 1% Synchrotron radiation loss

4

/ RevE E

LEP II Super-LEP

Energy 180 GeV 500 GeV

E / Rev 1.5 GeV 5 GeV

Radius 4.3 km 255.8 km

Beam Energy

Bend Radius

7 March 2003


LEP

Synchrotron radiation loss sets the size of a Super-LEPLet’s try a Linear Particle Accelerator

The Super-LEP

7 March 2003


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)

You need to line-up your accelerator VERY precisely

7 March 2003


33km33km

200m over 600m

X-F

EL

Proposed Linear Collider: TESLA

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

Collider Alignment & Survey:

7 March 2003


Why is this hard ?

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

200m over 600m

Light gets bent by air refractionT

7 March 2003


Ground Motion: Effect on Luminosity

2s 20s1week

Time to reset collider

7 March 2003


Extra Survey Constraints

Confined space (also used as emergency escape)

Collider has mixture of straight and curved sections

Electrically noisy environment

7 March 2003


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 200m over 600m precision

Accelerator Diagnostics• Check accelerator maintains alignment (& correct it)• Find out what went wrong

7 March 2003


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

7 March 2003


Solutions: Hydrostatic Levelling Systems

Traditional method to measure vertical alignment

But water only follows local geoid…some parts of TESLA don’t

….while NLC does not at all

Measured Vertical Height

NLC

7 March 2003


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 Earth’s geoid. So cannot be used for TESLA

7 March 2003


Survey Procedure Two-step Survey procedure

1. Survey equidistant tunnel wall markers via multiple

overlapping measurements: LiCAS Job2. 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

7 March 2003


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

7 March 2003


Each carriage measures the position of a reference marker in its own co-ordinates

Q: How to tie reference marker co-ordinates together

Survey Train: External Measurements

Carriage 1

Carriage 2

Marker 1 at (x1,y1) Marker 2 at (x2,y2)

1D FSI Length Measurements

7 March 2003


Use internal system to relative positions of carriages

Internal systems ties the external measurements together

Survey Train: Internal Measurements

Carriage 1Carriage 2

(xc2,yc2)

Marker 1 at (x1,y1) Marker 2 at (x2,y2)

1D FSI Length MeasurementsSM Measurements

7 March 2003


Survey Train: LiCAS Systems

An Optical metrology system for survey of a linear Collider• Fast, automated

high precision system

• Can operate in tight spaces

Rails attach to tunnel wall

zx

y

Vacuum tube

5m Internal FSI Lines

SM Beam

0.5m External FSI Lines

7 March 2003


collider component

Tunnel Wall

Reconstructed tunnel shapes(relative co-ordinates)

wall markers internal FSI external FSISM beam

LiCAS technologyalso applicable to second instrument !

Survey Implementation

7 March 2003


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

Tunable Laser

Reference Interferometer: L

Measurement Interferometer: D

Change of phase: GLI

Change of phase: Ref

time

IRef

time

IGLI

(Grid Line Interferometer (GLI))

Ref

GLI

L

D

7 March 2003


FSI: Length MeasurementGLI

Ref

GLI

Ref

GradientD

L

7 March 2003


FSI: Thermal Drift Cancellation

Thermal effects add subtle systematic errors to FSI− Nanometre movements can contribute micron errors (

Use two lasers tuning in opposite directions to cancel thermal drift

Expansion ofInterferometer

I

I

7 March 2003


FSI: Thermal Drift CancellationGLI

Ref

True Gradient

Measured G

radient with

Laser Tuning U

p

Measured Gradient with Laser Tuning Down

7 March 2003


FSI: 2-Laser Thermal Drift Cancellation

m mRange

20 30 40 50 60

417.0

417.4

Laser 1 Laser 2 Com bined

Est

imat

ed G

LI le

ngth

/ m

m

Tim e / h r

2 0 3 0 4 0 5 0 6 0

-4 0

-2 0

0

2 0

4 0

6 0

8 0

Tim e / hr

Warm ing

Coo ling

dT

/dt

(k

s)

-1

7 March 2003


FSI: ATLAS Implementation

7 March 2003


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.

7 March 2003


FSI: ATLAS Resolution

7 March 2003


1m

FSI: ATLAS Resolution

Stage is kept stationary

RMS 3D Scatter

< 1 m

Retro Reflector

ATLAS FSI SystemLaser 1

Laser 2

Reference Interferometer

piezodetector

C-Band Amplifier (1520-1570 nm)

L-Band Amplifier (1572-1630 nm)

Splitter Tree

LiCAS FSI System

1m GLI

Uncollimated Quill

APD

Collimated Quill

5m GLIADC

+AMPS

RAM To PC

f1

f2

Amplitude Modulation @ f1


Detectors

Demodulator

@ f1 , 1

Demodulator

@ f2 , 2

Demodulator

@ fn , n

7 March 2003


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 ?

Decay

Signal~1550nm

Pump980nm

4I15/2

4I11/2

4I13/2

Incoming Single Photon

Outgoing Photons

fluor

esce

nce

Wavelength / nm 1530 1610

Single Telecoms Channel

7 March 2003


Quill Collimation Refractive

Reflective

Quill end

RetroreflectorCollimation lens

Retroreflector

Reflective, off-axis paraboloid

Quill

7 March 2003


Laser 1

M1

M2

DetectorLaser 2

Demodulator

@ f1 , 1

Demodulator

@ f2 , 1

wa

vele

ng

th

time

1

2

wa

vele

ng

th

time

0

2

Vo

lts

time

Vo

lts

time

t0 t1

t0 t1



f1

f2

Two Laser AM Demodulation

Need 2 lasers for drift cancellation Have both lasers present & use

AM demodulation to electronically separate signals

7 March 2003


Vol

ts

Time

15% mod.

15% mod.

Time

Vol

ts

• Amplitude Modulation on FSI fringe

@ 40 & 80 kHz (now) 0.5 & 1MHz (later)

• FSI fringe stored as amplitude on

Carrier (à la AM radio)• Demodulation reproduces FSI Fringes

• High Pass Filter

Two Laser AM Demodulation

7 March 2003


Results of Demodulation

Demodulation of modulated laser does not effect interferometer signal

Both signals have same frequency !!

7 March 2003


Reference Interferometer Phase Extraction

Reference Interferometer is FSI’s “yard-stick”• Must measure interferometer phase precisely

Uses standard technique of Phase-Stepping

Expansion ofInterferometer

Step1: I(true-1.5)Step2: I(true-0.5)Step3: I(true+0.5)Step4: I(true+1.5)

Carré Algorithm

true

Reference Interferometer mirror moved in 4 equal sized steps

7 March 2003


Raw Data Reconstructed Interferometer Signal

Software Phase Extraction Telecoms laser tunes linearly Extract phase with software “phase-stepping”

7 March 2003


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

7 March 2003


Straightness Monitors

Used to measure carriage transverse translations and rotations

Require 1m precision over length of train

z

y

Translation:Spots move same direction

Rotation:Spots move opposite directions

CCD Camera

7 March 2003


z

y

x

y

SM beams coming out of the screen

Image of beam spots observed on CCD Camera

SM: Rotations about Z

Use two parallel beams to measure rotation about z-axis

7 March 2003


SM: Splitter Configurations

1. Single Beam Splitter + End carriage retroreflector

2. Double Beam Splitter per carriage

z

y

CCD 1

CCD 2

z

y

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

7 March 2003


SM: Low Coherence Beams

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

Beam-Splitters

The two reflected rays can interfere if coherent

The two reflected rays can interfere if coherent

CCD Chip

CCD Glass Face-plate

7 March 2003


SM: Interference Rings

Laser with long coherence length.

Interference rings observed on CCD

Laser with low coherence length

No interference structure is observed

Interference Rings Perfect Gaussian Beam

7 March 2003


SM: Demagnification Lenses

CCD cameras are ½’’ square.

A long collimated beam 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 CCD

7 March 2003


SM: Results

7 March 2003


SM: Stability Results

0.125 pixels = 1m

0.125 pixels = 1m

7 March 2003


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

7 March 2003


Without tilt meters With 1-Axis tilt meters

Simulations (of single car) FSI resolution 1m, SM resolution 1 m Weak measurement of rotation around z-axis due to

small separation between two beams on CCD Tilt meters resolution 1rad

7 March 2003


Simulations of Train over 600m

Imperative each point is known

precisely!!!

Error on positions < 200m after 600m

7 March 2003


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

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The Linear Collider Alignment and Survey (LiCAS) Project Richard Bingham, Edward Botcherby, Paul...

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