Nanometre precision interferometers for the international linear collider

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Nov 15, 2013 (3 years and 4 months ago)

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Nanometre pre
cision interferometers for the
international linear c
ollider

P

A Coe, D Urner and A Reichold

Dept of Physics, University of Oxford, Keble Rd, Oxford OX1 3RH UK

p.coe1@physics.ox.ac.uk

Abstract
.
Optics is playing a greater role in High Energy p
article physics. Larger

accelerators and more precise detectors are required to study higher particle energies
with greater precision, so greater demands

are placed on monitoring systems. The
international linear collider will

provide electron/positron b
eams wi
th precisely tunable
energies from 40

to
25
0 GeV,

to investigate in detail, phenomena discovered at the
Large Hadron Collider.
Final focus

quadrupole
magnets
,

either side of the collision

point, focus the beams to an elliptical profile, a few nanome
tres by hundreds of

nanometres. Their
relative position
s

need
monitoring to nanometres across a

separation of 10 m. The
MONALISA project

is

developing i
nterferometers
to be
built
into a
monitoring
network

around
the
final focus quadrupoles. The system wi
ll
multiplex fixed frequency lasers with frequency

scanning lasers in the same
interferometer paths. The former will provide

sub
-
wavelength resolution over an
unambiguous range of half a wavelength, the

latter can achieve fractions of a
wavelength resolut
ion

over several metres
. The aim is to demonstrate an evacuated
interferometer which delivers nanometre resolution over a range of 10 metres at

a
readout rate of 100 Hz. The possibilities for using phase
extraction
techniques wil
l be
discussed. The most
recent

results

will be presented
.

1.
Challenges offered by the ILC

Particle physicists are currently planning for the construction of the International Linear
Collider (ILC) an electron


positron machine capable of

centre of mass

energies up to 500
GeV in th
e first instance
.

Th
e ILC

will require unprecedented stabilisation and alignment of
the accelerator magnets to achieve
its
design performance.
The MONALISA project at
Oxford
is to investigate

the deployment of interferometer technologies to monitor the r
elative
motions of key components in the ILC accelerator
;

down to the nanometre scale,
so as
to
drastically improve machine performance over
that of
an unmonitored acc
e
lerator.

The particle beams at the ILC will be stored in a damping ring, accelerated fo
r 14 km
along a linear section and injected into the final
few
km of
beam line
, referred to as the beam
delivery section (BDS). The aim of the BDS is to
prepare the beam for collisions by position
and energy collimation, to
monitor key diagnostics

includi
ng
the
beam energy

and
bunch
profile
s
. U
ltimately t
he bunches
will be

focused down to an elliptical profile, a few nm
high

(by several hundred nm

wide
)
and brought into collision
with
oncoming bunches, equally
tightly

focused, with the same profile
.







T
he
greatest alignment challenge is presented by the
final focus quadrupole magnets
which lie
either side of the collision

point
. They

lie roughly

10m apart and will need to be
maintained in relative
vertical

alignment to around 1 nanometre.

On the timesc
ale of a few
hundred seconds
, ground motions typical for any site considered for hosting the ILC, will
generate misalignments of order
1 nm
or greater,
caus
ing
the
beam
s to miss each other

sharply reducing

the
colli
der

luminosity.

Monitoring movements o
f the magnets will allow
corrections to be applied in real time, to maintain alignment and hence collider luminosity.


2.
Interferometers for stability monitoring

One key aim of MONALISA is to provide an interferometric system which can monitor the
final focu
s quadrupoles u
sing a network of
interferometers and straightness
monitors,
deployed as necessary, subject to the availability of direct
lines of sight. The most likely
scenario involves projecting out from the quadrupole magnets
, using absolute distance
measurement interferometers, to reference objects which have clear lines of sight between
them. One such concept is shown in
Figure
1
, alternative

configurations

will be presented.

Detector
Quadrupole
Quadrupole
10 m
Detector
Detector
Detector
Project
Out
Reference
Objects
Direct line of sight
No Direct line of sight
Reference
Objects

Figure
1

Any system for monitoring the ILC final focus quadrupoles will
involve projecting out from the magnets to a clear line of sight.


There are several crit
i
cal magnets
in the ILC, particularly in the BDS,
which are
highly
po
sition and alignment sensitive
. Performance of the accelerator would benefit greatly from
monitoring the stability of these key components. One example is the

components of the

energy measurement chicanes, where the electron beam is deliberately deflected to
measure the momentum and he
nce the energy of the particles.
Another example is the

optical diagnostic tools
used to
monitor electron bunch profiles

(for example Shintake

monitors
[1]
)

which
also
need to maintain stable positions relative to the particle
beam
.
These and similar case
s offer

further opportunities for interferometric
alignment techniques to
be applied
.

2.1.
Demonstration systems at Oxford

At the time of writing
,
s
e
veral potential interferometer designs are under construction.


The
ultimate aim i
s to combine the absolute distance interferometry technique, Frequency
Scanning Interferometry (FSI)
[2]

with Fixed Frequency Interferometry (FFI) to measure to
sub
-
micron precision with the former and to nanomet
re precision within the half wavelength






dynamic range of the latter, with a readout rate for the FFI somewhere between 100

Hz and
1 kHz.


We use tunable lasers operating at telecomms wavelengths around 1500nm to 1560 nm
allowing multiple interferometers t
o be fed from the same source, with the power boosted
using EDFAs where necessary. The latest status of interferometer testing will be reported in
this presentation.

2.2.


Proposed deployment at KEK Japan
.

The accelerator test

facility (ATF) at the KEK labor
atory in Ts
u
kuba, Japan

offers a realistic
test bed for interferometers monitoring
the
relative stability of acclerator components. Two
beam position monitors,
(BPM)s
, 5

m apart in the ATF set
-
up.
We plan
[3]

to install a netw
ork

of
21

distance meter interferometers at the ATF with an arrangement shown in
Figure
2
.

The
network is designed to
measure the relative vertical displacement of one BPM with respect to
the other, by constraining each BPM relat
ive to

the six
position and orientation

degrees of
freedom of the
the reference triangle, formed between

two ceiling girder nodes and
one

floor
node.


SLAC
BPM
KEK
BPM
Interferometer
Lines
Floor
Node
Ceiling
Girder

Figure
2

Proposed network of interferometric distance meters for monitoring
th
e relative stability of two beam position monitors (BPM)s
.


An interferometer design for
measuring along
each

network lines
consists of
a fibre
coupled
,

launch and receive head at one end of the interferometer,
aimed at

a l
ow mass

retroreflector at the far

end. A vacuum tube will be placed around each line to allow the
effects of gas refractivity to be supressed.

The interferometer launch heads should be

stable
to a nanometre and
offer
point
ing

stability at the 100

microradian level or better.

Potential
designs for such interferometers will be discussed.

Acknowledgements

This work is supported by by the PPARC LC
-
ABD collaboration and by the Commission of the
European Communities under the 6
th

Framework Programme ``Structuring the European Research
Area'',

contract number RIDS
-
011899.








References

[1]

T Shintake, "Proposal of a nanometer beam size monitor for e+e
-

linear colliders", Nucl.
Instrum. and Meth. in Physics Research A311 (1992) 453
-
464.
doi:10.1016/0168
-
9002(92)90641
-
G

[2]

PA Coe, DF Howell and RB
Nickerson, Meas. Sci.

Technol. 15 (2004)
2175
-
21
87.

doi:10.1088/0957
-
0233/15/11/001

[3]

PA Coe, D Urner and A Reichold, “
Stabilization of the ILC Final Focus Using Interferometers
”,
proce
edings of European
P
article
A
ccelerator
C
onference

EPAC06
, Edinburgh 2006.