fast and precise beam energy measurement at the international linear collider michele viti

Fast and Precise Beam Energy Measurement at the

International Linear Collider

Michele Viti

04 November 2009 Michele Viti 2

Outline

• ILC overview• Beam energy measurement • An overview of my work and results

– 4-magnet chicane spectrometer• Magnetic measurements• Relative beam energy resolution

– Laser Compton energy spectrometer

• Conclusions


ILC

•30 km electrons/positrons linear accelerator

•Center-of-mass energy 500 GeV (upgradeable to 1 TeV)

•High luminosity (2*10^34 /cm^2*s)

•A machine for precise measurements


Precise measurements

• Well understood background, clean experimental environment

Precise measurements.• “Input” parameters well controlled, e.g. the center of mass

energy at the interaction point (IP) . • Direct measurement of is very difficult

'2' bEs 'bE


Precise measurements

Solution: – Measurement the beam energy upstream ( ) and downstream

of the IP for both beams plus a slow monitoring of . – Combine with the measurement of .

• Fast (bunch-to-bunch, good resolution), precise and non-destructive monitor for .

• Accuracy required for

• Similarly for the resolution.• From now on as beam energy we refer to beam

energy upstream the IP for electrons as well positrons.

44 10)MeV50(103

b

b

t

t

b

b

E

E

m

m

E

E

bEbEs 2

bE

bE

bEL d/d

bE


Magnetic Chicane Energy Spectrometer

field magnetic B andmagnet theinsideparticletheoftrajectorywith

,

dl

Bdld

LEb

•At ILC baseline method for measurement is a 4-magnet chicane.

•Offset dd measured the by Beam Position Monitors, BPM, together with the B-field integrals of (11) and (22) give access to .

•Method well tested at LEP with an accuracy of .

4107.1

offset d

magnets

L

BPM BPM

BPM

11

22 33

44

bE

bE


Experiment T474/491 (SLAC)

• At End Station A (ESA) a 4-magnet energy spectrometer commissioned in 2006/2007 (experiment T474/491).

• Demonstrate the feasibility of the system (mainly BPMs and magnets).


End Station A

Parameter ILC-500 SLAC ESA

Repetition Rate 5 Hz 10 Hz

Energy 250 GeV 28.5 GeV

Bunch Charge 2.0 x 1010 2.0 x 1010

Bunch Length 300 m 300-500 m

Energy Spread 0.1% 0.2%

Bunches per train 2820 1

Beam Parameters at SLAC ESA and ILCBeam Parameters at SLAC ESA and ILC

• Prototype components of the Beam Delivery System and Interaction Region.• Characteristic:

–Parasitic with PEP II operation

–10 Hz train repetition and = 28.5 GeV

–Bunch charge, bunch length, energy spread similar to ILCbE


Experiment T474/491• Institutes involved: SLAC, U.C. Berkeley, Notre Dame,

Dubna, DESY, RHUL, UCL, Cambridge • 2006, experiment T474:

– April (2 weeks): Commission of cavity BPMS.– July (2 weeks): Commission of interferometer.

• 2007, experiment T491:– March (3 weeks): Commission and installation of magnets: first

chicane data.– July (2 weeks): Additional new BPM in the centre of the chicane.

Magnetic measurements



• B-field integral, , essential parameter for beam energy measurement.

• Need to be measured with an accuracy of 50 ppm to obtain

Bdl

410

b

b

E

E



• Nov 2006 – Feb 2007 measurements performed in the SLAC laboratories (DESY, Dubna, SLAC).

• Purpose of the measurements:– General understanding of the magnets

• Stability of the B-field and B-field integral.

• Monitoring of the residual B-field.

• B-field map.

• Temperature coefficient for B-field and B-field integral.

– Development and tests of a procedure to monitor the B-field integral in ESA.



Important restriction:• Monitor of the B-field integral: in ESA no device to

measure directly this quantity.• Solution: measure the B-field in one point and from that

determine the integral.– Basic assumption BdlB

When the field is changing in one point, it changes everywhere by the same amount. The field is assumed to be scaled

B

Z


Magnetic measurementsSome results:• B-field measured by NMR probe.• In the lab:

– Flip coil technique to measure B-field integral.

– Calibration of the NMR

– Comparison of the B-field integral calculated with the measurement.

• Error = mean + rms.• Values close to the requirement.• Not all the error sources visible in

the figure (like calibration and alignment error for the flip coil).

01 pBpBdl



• The total error of the B-field integral using the one-point B-field measurement was

• Main contributions are alignment errors of the devices (flip coil).

• Several suggestions were proposed to improve the results.

4108.1

Bdl

Bdl

Relative beam energy resolution


Relative Beam Energy Resolution

• At ESA, NMR probes in magnet 11 and 33 damaged.• A complementary method to cross-check the absolute

energy measurement was not implemented.• Only relative energy measurements possible at ESA.

offset d

magnets

L

BPM BPM

BPM

11

22 33

44



• The offset dd = XbXb - X0X0• XbXb measured by BPMs the X0X0 by extrapolation using

BPMs upstream and downstream of the chicane.• dd set to 5 mm, resolution required < 500 nm (in order to

have )

Beam direction

410/ bb EE



• The BPMs measure the beam transverse position (X and Y) and angle (tilt) in the X-Z and Y-Z plane (X’ and Y’).

• X0X0 can be written as

• For zero current XbXb=X0X0 , the BPM measures directly X0X0.• The coefficients (i=1,…,N and j=1,…,4) determined

by a minimization procedure.

NNNNNNNN ycycxcxc

ycycxcxccX

''

''0

)4()3()2()1(

1)4(

11)3(

11)2(

11)1(

10

)( jic

i

i

i

i

y

x

y

x

'

'

Position,

respectively,

tilt of the

monitor i

upstream or

downstream of

the chicane



Fundamental condition: the magnetic chicane must work

symmetrically, i.e. the upstream path must be restored

downstream in order to use the BPMs downstream for X0X0

determination.

Beam direction

Ideal trajectory

Wrong trajectory



• In ESA 4-magnet chicane not symmetric. • For a given current the B-fields were different up

to ~3%.• BPMs downstream could not be used to

determine X0X0.

Worse resolution for dd.



• A resolution of 24 MeV was found for

• Relative resolution of

• Largest contribution

comes from the resolution on d d (>2 microns).

4105.8/ bb EE

GeV 5.28bE

Laser Compton Energy Spectrometer



• At LEP it was possible to have redundant beam energy measurements cross check

• At ILC so far, complementary methods for upstream beam energy measurements not foreseen.

• We studied the feasibility of an upstream energy spectrometer based on Compton backscattering (CBS) events.



• Compton process with initial electron not at rest.

• Energy spectrum for electrons (photons) with sharp cut-off (Compton edge):

• Scattered particles strongly collimated in forward region.

2

min, 41

m

EEE

Eb

be



New approach

Measure 3 positions of particles: • , the center of gravity of the scattered photons, or,

equivalently, one end point of the SR fan.• , position of beam, possible to measure with BPMs• , position of the scattered electrons with minimum

energy.edgeXbeamX

0X

0

2

4 XX

XX

E

mE

beam

beamedgeb

ly.respective

electron,theofmasstheand

laserphoton initialtheof

energy theareandmE



Detailed GEANT4 Simulation:• Beam parameters

– Beam energies 50-500 GeV (250 GeV default value).

– Beam size in x (y) 20-50 (2-5) microns.• Geometrical parameters

– Drift distance 25-50 m.– B field 0.28 T, magnet length 3 m.

• Laser parameters– Smaller wavelength (e.g. green laser).– Pulsed laser with 3 MHz frequency.– Laser spot size 50-100 microns.– Laser pulse energy must ensure 10^6

scatters e.g. 30 mJ for green laser.– Crossing angle ~8 mrad.

Accuracy required for

– < 1-2 microns– < 1-2 microns– < 20 microns

0X

edgeXbeamX

410/ bb EE



In practice• Beam position measured with a cavity BPM (very well

know and precise technique).• Edge position

– Diamond strip detector,– Quartz fiber detector,– Basic simulation shows that both are feasible.

• Photon detection, 2 possibilities– Center-of-gravity of backscattered photons, – One edge of the synchrotron radiation photons.



In particular,• Number of backscattered

photons 4 orders of magnitude less than SR photons

• <Energy> ~100 GeV, <energy> SR photons ~3 MeV.

• 1° option– thick absorber in front of the

position detector – measure the profile of shower – signal from dominant – quartz fiber detector suitable.

)( Compt

Compt

Compt



• 2° option:– No absorber.

– Measure one end point of the SR fan.

– SR photons dominant.

– Novel detector under development in DUBNA (Xenon gas detector).

• Main problem for both configurations: very high radiation dose (10-100 GGy per year).

Simulations demonstrate feasibility.


Conclusions I

• ILC represents the next generation of electron/positron collider, providing a unique environment for precise measurements.

• Beam energy essential information for precise measurements (e.g. top quark mass).

• Baseline method for upstream beam energy at ILC is BPM-based spectrometer.

• In the years 2006/2007 a prototype of such device was commissioned in the End Station A (experiment T474/491).


Conclusions II

In the thesis an essential contribution was given• In the experiment T474/491:

– Monitor the B-field integral. An accuracy was found (ESA-SLAC note and PAC poster).

– Determination of the resolution of the 4-magnet chicane. A value of was found (to be published…).

• A novel method based on Laser Compton scattering was studied and its feasibility demonstrated (NIM

publication). – A proof-of-principle experiment is under study; proposal in

preparation.

4105.8/ bb EE

4108.1 BdlBdl

fast and precise beam energy measurement at the international linear collider michele viti

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