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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 2/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 3/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 4/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 5/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 6/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 7/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 8/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 9/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 10/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 11/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 12/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 13/41
How does a laserwire work?
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 14/41
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)
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 15/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 16/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 17/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 18/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 19/41
Chirped pulse amplification
Source: Wikipedia
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 20/41
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...
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 21/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 22/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 23/41
First scanner prototypeExperimental tests with 2 kV applied voltage
Alessio Bosco,Gary Boorman et al.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 24/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 25/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 26/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 27/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 28/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 29/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 30/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 31/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 32/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 33/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 34/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 35/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 36/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 37/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 38/41
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
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 40/41
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.
Nicolas Delerue, University of Oxfordhttp://wwwpnp.physics.ox.ac.uk/~delerue/ Oxford PP seminar, 13 II 2007Oxford PP seminar, 13 II 2007 41/41
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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