Laserwire, a tool to measure the beam size at the ILC a tool to measure the beam size at the ILC...Laserwire, a tool to measure the beam size at the ILC ... Now working on other projects

Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 1/41

Laserwire, a tool to measure the beam size at the ILC

➢Emittance issues at the ILC ➢Laserwire principles➢Laserwire challenges➢Laserwire prototypes➢Results➢Outlook


Laserbased beam diagnostics (laserwire) collaboration

Present and past collaboratorsUniversity of Oxford: Laura Corner, Nicolas Delerue, Sudhir Dixit(*), Brian Foster, Fred Gannaway(*), David Howell, Myriam Newman, Armin Reichold, Rohan Senanayake, Roman Walczak+ Design Office and Mechanical workshop

Royal Holloway, University of London (RHUL): Grahame Blair, Stewart Boogert, Gary Boorman, Alessio Bosco, Lawrence Deacon, Chafik Driouichi(*), Pavel Karataev, Michael Price

BESSY: Thorsten Kamps

DESY: Klaus Balewski, HansChristoph Lewin, Freddy Poirier, Siegfried Schreiber, Kay Wittenburg

FNAL: Marc Ross

KEK: Alexander Aryshev, Nobuhiro Terunuma, Junji Urakawa

SLAC: Joe Frisch, Douglas McCormick

(*) Now working on other projects.


The next e+ e collider● LEP has been very successful but

its maximal energy was limited by synchrotron radiation.

● Building an accelerator with a much bigger radius is unrealistic.

● The next e+ e collider will have to be linear

● RDR for the ILC (including costing) released last week.


Achieving the highest luminosity

● In a circular collider such as LEP the same electron bunch collide many times:=> the best integrated luminosity is achieved by balancing beam lifetime (and refill time) and high instantaneous luminosity.

● In a linear collider bunches collide only once and thus the higher the instantaneous luminosity, the higher the integrated luminosity.=> Beam must be strongly focused


Experience gained from SLC

● Stanford Linear Collider was the first Linear Collider ever built.

● Several factors limited the initial performances:emittance dilution, stability, beam optimisation, background control.

● A careful monitoring of the emittance growth along the accelerator helped increase the luminosity.


Beam focusing● At the ILC the beam size will be only 5nm high

and 100nm wide at the interaction point.● Achieving such small size requires very

complicated focusing optics.● These optics will perform as required only if the

beam emittance is kept as low as possible.

14mrad optics, Nosochkov et al., SLAC_PUB_11591


Emittance & Emittance control

● Emittance is the 6D volume of the beam in the phase space● According to Liouville's theorem, emittance is conserved in a

magnetic field. Normalised emittance is conserved when the beam is accelerated.=> The best emittance must be achieved in the damping ring and its growth must be monitored along the accelerator

● Around 70 laserwires will be needed along the ILC from the source to the beam delivery section to monitor the emittance

● Increasing the Laserwire resolution allows to decrease the length of the diagnostic sections.


Emittance measurement● To measure the emittance of a beam one needs to know

its position (x) and transverse momentum (px) at a given location.

● This can be deduced from a measurement of the beam size as a function of the strength of a magnet

● With a known lattice (beta function), one can measure the beam size at 4 different locations and use these measurements to fit the best beam parametres.

Okugi et al.

LINAC98 TU4063


Traditional emittance measurement tools

● Traditional emittance measurement tools (such as screens, slits,...) block the beam and thus are not suitable for a continuous monitoring of the beam quality.

● At SLC thin wires (few micrometres) were swept across the beam to measure the beam profile and hence its emittance.

● Only a small fraction of the beam was affected at a time and thus could be used for a regular monitoring of the emittance.


Wirescanners limitations

The energy/charge deposited by the ILC beam in a thin wire would be too high and this would cause the wire to break (this was already experienced and studied at SLC and at the KEK ATF).

An example of scan where a wire broke


Laserwire at SLC

● At SLC a method of replacing the mechanical (tungsten) wires with a nondestructible wire was studied. This nondestructible wire was created by a laser => Laserwire

● The laserwire R&D at SLC confirmed that a beam size measurement could be obtained with this method.


How does a laserwire work?

● When the laser beam “hits” the electron beam, High energy Compton photons are produced.

● The number of Compton photons is proportional to the laser power and the electron bunch width where the beam was hit by the laser.

● By varying the position at which the laser beam hits the electron bunch, it is possible to reconstruct the bunch profile.


How does a laserwire work?


Other Laserwires

● The focus of the UK laserwire groups is to develop a laserwire suitable for the ILC linacs (ie: single pass).

● A “damping ring” laserwire to measure the beam size in a ring (multiple passes) has been developed at the KEK Accelerator Test Facility

● SNS uses a laserwire to monitor its H beams=> beams are different.

● Shintakesensei has developed at SLAC a device that creates an interferometric pattern to measure the beam size.=> Better resolution but does work only with very small beams (100s of nanometres)


ILC Laserwire challenges● Laser: High power, good stability/quality,

must match the accelerator frequency/pulses.

● Delivery optics: must bring the laser light from the laser to the accelerator (several metres) without loss of quality

● Scanning system: must deflect the beam in a few hundred nanoseconds

● Focusing lens: must reduce the beam size from millimetres to micrometres.

● Interaction chamber: must be UH Vacuum tight and preserve the laser beam quality

● Beam optics: Should not create background noise

● Calorimeter: must detect the Compton photons


Laser issues● High power lasers are often unreliable and/or

complex to operate (from a particle physicist point of view).

● Beam quality is critical: laser beam size added in quadrature with electron beam size.

● At the ILC the intratrain repetition rate is high (~6 MHz during 1ms) but with long interval (199ms) between trains => Unconventional for laser systems

Myriam NewmanLaura Corner

Flattop profile of the ATF EXT LW


Laser R&D● Fibre laser (Ytterbium doped fibre) offer several advantages

over more established solidstate lasers for our application: Better beam quality Easier operation (“turn key”) Less environmental constraints (solid state lasers are very sensitive to vibrations, dust,...) Cheaper

● But this is a less established technology.=> No commercially available laser offers what we want without significant R&D

● No UK company ready to collaborate on this


Reaching high powers with a laser: Chirped amplification

● High powers can damage amplification crystals (either fibre or bulk crystals; threshold is higher for some bulk crystals than for fibre)

● Most applications requiring high power require short pulses => low energy(Laserwire: 50MW during 1ps = 50 microjoules)

● A laser pulse can be stretched in time, amplified (at a lower power) and then compressed.


Chirped pulse amplification

Source: Wikipedia


Photonic crystal fibres (PCF)● Ytterbium doped conventional optical fibre have a limited

amplification potential. Pulses of 1 microjoule seems to be the limit.

● Beyond 1 microjoule, one needs to use Photonic crystal fibre. In such fibre the core has specific properties that increases the damage threshold.(eg: Hollow core,...)

● To reach 50 microjoules we are likely to need such fibres. ● Field is recent and finding where to source the right fibre is

an issue...


Laseraccelerator synchronisation

● The ILC bunches will be 1ps (x c) long.

● To minimize the power required, our laser pulse will have a similar length.

● The frequency at which the laser fires must be precisely adjusted (0.5ps jitter =>12% signal loss)

● A phaselocking system must adjust the length of the cavity.=> Easy to do with freespace cavities, more difficult with a fibre cavity!

● Phase locking electronics must have very little noise!(~100dBc/Hz at 100Hz from carrier)

Roy Wastie & ND


Fast scanning system● Long term goal: allow to perform a position scan

over several micrometres during a bunch train.● Uses an electrooptic crystal to reach scanning

rates beyond 100kHz.● Strategy: use an electrooptic prism driven by a

high voltage pulser● Aberrations are compensated by a second prism

in opposite direction.Alessio Bosco,

Gary Boorman et al.


First scanner prototypeExperimental tests with 2 kV applied voltage

Alessio Bosco,Gary Boorman et al.


V = 0

V = 1 kV

V = 2 kV

input spot = 2 mm

2.5X beam exp after the EO prism

waist = 26 µm (52 diameter)

shift = 31 µm

deflection from prism = 0.25 mrad

deflection after beam = exp 0.1 mrad

scan range @ focus S applied voltage

First scanner prototypeExperimental results

Alessio Bosco,Gary Boorman et al.


Focusing lens

● Creating a 1 micrometre spot with a green laser (532nm) is challenging!

● This requires very large aperture optics.● Light must enter the accelerator vacuum without

loss of quality● Laser constraints:

some glasses can not sustain high power ghost reflections must be avoided


Mechanical constraints● To get the smallest spot size one need to use a

very wide lens located very close from the focus(f#=focal length/aperture)

● In an accelerator some clearance (~25mm) must be left for the beam in case of steering error.

● One of the elements of the lens must be tightly sealed to allow the laser light to enter the vacuum. This element should be thick enough so that it does not bend under the pressure difference=> 12.7mm fused silica window

● An indium seal has been developed in collaboration with the workshop to meet the specified vacuum requirements.

S. Yang & DFHdisp.=0.27um

DFH


Bandwidth issues● The refractive index of any glass varies with the light's

wavelength.● For diffraction limited optical systems such as ours a

change of 2nm in wavelength can blow the spot at the IP by 100%.

● Nd:YAG laser have a very narrow bandwidth (<<1nm), but fibre laser can have very wide bandwidth.

● Chromatic effects can be compensated by using different glasses but the choice of glasses supporting high laser power and irradiation is limited.


Initial design: f/2 fused silica lens

● Focal length: 56mm

● One aspheric element, one flat

● Back focal length: 24mm

● f/2 BW~1nm

● All elements are made of fused silica

● No primary ghosts, one secondary ghost

● Expected spot radius ~2 micrometres


Lens studies● Test setup installed in Oxford to measure

the performances of the lens and compare them with the simulations.

● Setup benchmarked with a commercial lens.● Preliminary results confirm that the

performances are comparable to the expectations.

● This setup will be used to establish an alignment strategy to allow a quick installation of the lens. Myriam Newman

David Howell


Other optics design:SLC Laserwire

● In the SLC laserwire a mirror was used as focusing device.

● Allows a larger BW● Measuring the performances

of the system at the IP is more difficult.

SLC LW


Interaction chamber● The chamber in which the interactions

take place must allow the lens to get as close as possible from the IP.

● Very stringent requirements set by the accelerator Ultra High vacuum.

● Must allow the addition of diagnostic tools to monitor the electron & laser beams.

● ATF LW chamber Design done by D. Howell, F. Gannaway and R. Senanayake

● Built by the mechanical workshop.


Diagnostics at the IP● To demonstrate that our prototype is well

understood we want to be able to measure the laser & beam properties independently

● This requires the insertion of devices that can “measure” the electron beam (wirescanner) and the laser beam (knifeedge scanner).

● Having a screen is also very useful to find the 2D position where the 2 beams overlap.


Electron Beam optics● At the ATF the nominal beam size at our location is

rather large (~20um).● Specific optics have been designed to focus the beam to

about 1 micrometre near our IP.● Verification of the beam size is difficult without

profiling device at our IP.● During data taking the beam optics must be carefully

adjusted to minimize the background hitting our detectors.


Laserwire prototypesThe UK groups have developed 2 prototypes:● A prototype at PETRA to study fast scanning and

2D scanning● A prototype at the KEK ATF to demonstrate that

micometre resolution can be achieved.

The experience gained from these prototypes will be used to provide emittance measurements at PETRA III, at the ATF2 and at the ILC.


High PowerLASER

2D LW scanner at PETRA

Free space Beam Transport (~ 30m)

Laserrep rate = 20 Hzpulse width 6 ns Laser spotsize ~ 12 um

e spot size50 – 200 υm

Scanning device:Piezomirror

max speed 100 Hz

Post IP Imaging Systemaligned for both dimensions

for real time laser size monitoring

Automatic beam findingTranslation stages

2D scanthrough automatic

path selection

Lens f=250mm

Slide by M. Price, A Bosco et al


0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 00

1

2

3D a ta : T 1 0 0 P ro f ile _ BM o d e l: G a u s s A m p E q u a tio n : y= y0 + A *e x p (0 .5 *((x x c )/w )^2 ) W e ig h tin g :y N o w e ig h tin g C h i^2 /D o F = 0 .0 0 1 8 1R ^2 = 0 .9 9 4 4 9 y0 0 .4 3 4 1 2 ± 0 .0 0 9 0 5x c 2 8 1 .7 3 5 6 5 ± 0 .6 0 3 8 7w 4 6 .7 3 2 7 6 ± 0 .7 0 8 4 6A 1 .6 1 8 9 5 ± 0 .0 1 9 1 9

Ca

lori

me

ter

Re

ad

ou

t [V

]

D is p la c e m e n t a t IP [u m ]

Seeded (100 steps, 100 points per step) 10,000/20Hz = 500secAveraged stepbystep

Sigma ~ 47um

M. Price, A Bosco et al


ATF Extraction line laserwire

● Installed in the extraction line of the Accelerator Test Facility at KEK

● Goal: demonstrate umscale resolution in a single pass system

● System successfully installed and tested last year with a commercial lens

● Strong focusing lens will be installed this year


ATF LW results (spring 2006)● Obtaining Compton photons at the LW

IP is a 2D problem: photons and electrons must overlap in time (within 200ps) and space (vertically, within 20um)

● First collisions observed in April 2006.

● Measured beam size compatible with our expectations.

● Scan asymmetry due to lens aberrations

● Laser M2 ~ 2.9

Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 39/41Slide by Stewart Boogert


Outlook for the current prototypes

● The 2D scanning system is being tested at PETRA this week.

● Data have been taken last week with the ATF extraction line Laserwire by Myriam.

● The strong focussing lens will be installed at the ATF this spring to bring the resolution in the micrometre scale.


Beyond the ATF and PETRA

● With the conversion of PETRA into a light source (PETRA III), the laserwire system will be integrated and be part of the diagnostic available to the operators.

● The ATF extraction line will be extended (ATF2) and several laserwires will be installed to provide beam size measurements in conditions very close from those of the ILC operations.

● Effort is ongoing to understand ILC specific issues (integration into cryostats, signal extraction,...)

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Laserwire, a tool to measure the beam size at the ILC a tool to measure the beam size at the ILC...Laserwire, a tool to measure the beam size at the ILC ... Now working on other projects