The Linear Collider Alignment and Survey (LiCAS) ProjectRichard Bingham*, Edward Botcherby*, Paul Coe*,

Grzegorz Grzelak*, Ankush Mitra*, Johannes Prenting, Armin Reichold*

*LiCAS Group, Particle Physics sub-department, University of Oxford, UKApplied Geodesy Group, DESY, Germany

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Contents

• Introduction

• LiCAS Project Overview

• Description of Measurement System

• Summary

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Introduction

• Next generation of Linear Colliders will require precision alignment

(eg:- TESLA: 33km Tunnel, Alignment: 200m over 600m)

• The problem– Current open air survey techniques are not sufficient– Ground motion will bring accelerator out of alignment

• Solution: Proposed DESY survey train

with LiCAS : A novel optical system

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Accelerator Alignment

Accelerator Component

Tunnel Wall

Reference Markers

• Place reference markers along tunnel wall • Measure accelerator component with respect to reference markers

How are the reference marker positions measured ?

Tunnel Rail

Use a survey train

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LiCAS Project

Internal Measurement System (FSI+SM) inside a vacuum pipe

Tunnel Wall

Tunnel Rail

External Measurement System (FSI)

Measurement system for the alignment and survey of a Linear Collider

An optical metrology system for the survey train

− Vacuum enclosure between cars– Frequency Scanning Interferometry (FSI) to measure absolute distance – Laser Straightness Monitors (SM) to measure transverse displacements

4.5 m

22.5 m

0.5m

z

x

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Survey Train in ActionTunnel Wall

Tunnel Rail

Internal FSI Lines and SMReference Wall Markers External FSI Lines

Reconstructed Tunnel Shape

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Laser Straightness Monitor (SM)

• Measures inter-car transverse displacements (and rotations)

• Provides single reference straight line for entire train

• Require measurement precision of 1m over length of train (2x22.5m)

CCD Camera Translation:

Both spots move to the right

Rotation:The spots move in opposite directions

Beam-Splitter

Retroreflector

z

y

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SM: Two Beams

Use two parallel beams to measure rotation about z-axis

z

y

x

y CCD Image

SM beams coming out of the screen

Image of beam spots observed on CCD Camera

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Frequency Scanning Interferometry

Tunable Laser

•Interferometric length measurement system

•Aim for measurement precision of 1m over 5m

Reference Interferometer: L

Measurement Interferometer: D

Change of phase: GLI

Ref

GLI

L

D

Change of phase: Ref

time

IRef

time

IGLI(Grid Line Interferometer (GLI))

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FSI in LiCAS

Delivery Fibre

Return Fibre

LiCAS Collimated Quill

Fibre Splitter

fibre end

Collimation Optics

Retroreflector

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Extensions of FSI for LiCAS

Move to Telecoms wavelength (1510 nm-1640 nm)

Exploit cheap high quality tunable lasers Reduce drift errors

– Greater continuous tuning range (130 nm)– Faster tuning rate (40 nm/sec)

Easier separation of laser signals – Use amplitude modulation

Use Erbium Doped Fibre Amplifiers (EDFA) Modular power distribution

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LiCAS Survey Train

FSI

Laser Straightness Monitor

Rails attach to tunnel wall

Measures 3D position of accelerator wall reference markers

Vacuum tube

z

x

y

Internal FSI+SM measures train’s internal positions

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LiCAS Survey Train: 2D

z

x

Tunnel Wall

z

y

reference markers for accelerator(Ball-mounted retroreflectors)View from above

Side on view

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Survey Train Simulation

•Simulations of train performance using “SIMULGEO”

•Simulation includes whole system: Internal & External FSI + SM

•Studies have highlighted areas for improvements in the design

•Eg: Train sensitivity to rotation about z axis • Consider addition of tilt sensors to cars and/or 2 rows of wall markers

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Summary

The LiCAS group has proposed an automated optical measurement system for the survey and alignment of any future Linear Collider

The current design for TESLA alignment is under development at Oxford