Laser Beam Transport and Integration

AWAKE Collaboration meeting

Mikhail MartyanovChristoph Hessler CERN, EN-STI-LPValentin Fedosseev

CERN, 04-06.12.2013

M.Martyanov, CERN 2

Overview

• Short intense laser pulse is needed for:– to create a 100% ionized plasma– moving ionization front is a source of perturbation for proton-laser

instability (micro-bunching and wake-field with a stable phase)

• Plan for the Laser system:– First it is delivered to MPP Munich for plasma experiments - mid 2014– Then it goes to CERN - end 2015 ?

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connection tunnel 400mmto be drilled…

Access tunnel

e-gun roomlaser room

AWAKE gallery

p-tunnel

Lasere-gun

laser beam 1

proton beam

electron beam

laser beam 2

- doors with central access control

- doors to laser room, local access control

AWAKE Area: Zones

- safety “shutters” with central control

plasma chamber

laser SAS

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Overview• Laser system comprises:

- laser with 2 beams (for plasma and for the e-gun)- delay line is possible in either one of these beams- optical compressor- focusing telescope- small optical compressor and 3rd harmonics generator for e-gun

• Laser parameters for plasma:- energy 450 mJ- pulse duration 120 fs after compression- beam diameter 40 mm (smoothed flat-top)

Only reflective opticson the way

Rule of thumb (B<1):I[GW/cm2]L[cm]<36

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Laser system base-line• Laser, Compressor and Telescope are in the laser room• Focusing down to 35 meters to the center of the plasma• Question is if this possible?

• Back solution: Compressor and Telescope are next to merging point in the proton tunnel

• Focusing down to 25 meters to the center of the plasma• Question is if this possible?

Crucial points are:• Focusability of the laser beam down to 25 or 35 meters• No detailed information on the laser system yet (beam quality)• The placement of the optical compressor and the focusing telescope has an impact on

the position of the anew drilled connection tunnel• Availability of vacuum components for the compressor and telescope is under study.• 10-6 Torr “easily” achievable. Pellicle or differential pumping as an option to go better

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Base Line: Merging point Laser + Protons

Some measurements of laser room with respect to merging point

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Merging point

Protons and laser towards plasma

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M.Martyanov, CERN 705.12.2013

Horizontal connection tunnel 400mm

Thanks to integration team for pictures

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Merging point in details

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1400500

500

750

1400

500p-beam

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HV volumelast mirror

10002000

• p-beam height about 1 m• HV volume (10-6 Torr) can be “easily” achieved in the laser pipes• UHV volume (10-8 Torr) is supposed to be in the p-beam line

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Merging point in details

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• Distance from p-beam envelope to optical axis is 14 mm• Assuming laser beam 10 to 16 mm• Gap between beams is 6 to 9 mm• Tough but manageable

• Possible issue: mirror charging and destruction

Thanks to Chiara Braccoproton beam

laser beam

gap 6 9 mm

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Vacuum components

Company Product Comments

ARUM Microelectronics Stepping motors, translators 1e-10Torr

Phytron Stepping motors 1e-11Torr, 10MJ/kg rad.resist.

Princeton Research Instruments

Stepping motors, translation and rotation stages

1e-09Torr, 4e-10Torr achieved with 190 l/s pumping

Tectra Stepping motors 1e-10Torr

NewFocus Picomotors 1e-09Torr

SmarAct Picomotors, mirror mounts,translators

1e-09Torr, 2900 Eur per mount

Standa Everything… but Mounts 1e-09Torr,Motorized 1e-06Torr 1800Eur

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Focusing of a 430 mJ flat-top beam 35 m downstream to the middle of the plasma.At the ideal Gaussian waist Wmax = 6.84 J/cm2 and FWHM = 2.35 mm.Flat-top beam focusing has been optimized to obtain the same maximum fluence somewhere in the plasma and equal fluence on both sides. Flat-top beam d=14 mm , f=52 m looks like a Gaussian beam and considered as an optimum.

35 m, FWHM=2mm, Wmax=6.6J/cm210 m - last mirror, beam size 16 mm, no peak in the middle for reasonably smooth beams, Wmax ~ 0.5J/cm2

Compressor and Telescope are in the laser roomFlat-top beam focusing profiles

0 m, FWHM=14mm, Wmax=0.32J/cm2 cm

Compressor and Telescope are at the merging pointFlat-top beam focusing profiles

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25 m, FWHM = 1.9mm, Wmax=6.8J/cm220 m, FWHM = 1.6mm, Wmax=5.9J/cm2

Focusing of a 430 mJ flat-top beam 25 m downstream to the middle of the plasma.At the ideal Gaussian waist Wmax = 6.84 J/cm2 and FWHM = 2.35 mm.Flat-top beam focusing has been optimized to obtain the same maximum fluence somewhere in the plasma and equal fluence on both sides. Flat-top beam d=10.6 mm , f=47 m looks like a Gaussian beam and considered as an optimum.

0 m, FWHM = 10.6mm, Wmax=0.57J/cm2 cm

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Compressor predesignTwo gold coated gratings 1700 lines/mm, 100x140 and 120x140 mmDamage threshold ~ 250 mJ/cm2 (in AWAKE less then 100 mJ/cm2)Efficiency per 1 reflection @ 800nm and 10deg deviation – 92%Gratings supplier – SPECTROGON, Sweden

Acceptance: compress 160 ps to 120 fs, bandwidth 24nm, beam size 50mmCompressor fits to 1200 x 400 mm footprint, 400 mm high, 2 view-ports for alignmentMax efficiency of the compressor – 70%

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Telescope predesign

Around 3-fold mirror telescope, detuned to provide 25 meter focusing, flat geometryConcave mirror R=2400mm, incident angle 2Convex mirror R=800mm, incident angle -3.54 in the same planeMirrors displacement 806mmBeam size 40mm, ray focal spot size ~100mAberrations are negligible with respect to diffraction limit (spot size ~1 mm)

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Telescope footprint is 1000 x 200 mm

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Compressor and Telescope

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Entire footprint is 2400 x 600 mm

Concave mirror 3”

Mirr

ors 2

”

Convex mirror 2”

Launch mirror 3”

System To define / To doLaser room, equipped with a big SAS

Air circulation, conditioning, humidity, filters, circuits (electrical, demineralized water, tap water, compressed air, control cables), safety (fire/smoke alarm), shutters, access etc.

Connection tunnel 40cm Drilling, coordinates of laser beam to be definedAccess to laser room and tunnel AWAKE access concept including Laser Access Modes to p-tunnel and e-

gun room, safety shuttersTi:Sa laser Arrangement in a squeezed room, max laser table width is 1m

Chillers and electronics are below the tables or in the separate ventilated rack/cabinet or in the big SAS

Vacuum pulse compressor and focusing telescope.

HV (10-6 Torr) or UHV (10-8 Torr): pellicle or differential pumping

Placement is not defined yetPlacement in the laser room is a base-lineIn case of p-tunnel everything must fit between p-beam and wall,“dirty” environment in p-tunnel is not good for compressor/telescope installation and maintenance

Transfer line to p-tunnelMerging point chamber

HVUHV (only 2 mirrors, possibly without in-vacuum motors)

Transfer line to e-gunSeparate small compressor3rd harmonic generation

In fore-vacuumNext to the gun

Laser dedicated list of “Things to Do”:Laser Installation

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System IssuesTi:Sa laser Controls and diagnostics are provided by the supplier of the laser systemPulse compressor and focusing telescope

Diagnostics are provided by the supplier

Laser beam in the p-tunnel Steady diagnostics:Focused beam spot monitor (virtual plasma, the same long distance run); near field before merging mirror; screens before and after plasma tube sensitive to “both” beams (laser, electrons, protons) also equipped with fiber-coupling for rough timimg measurementsOn demand or maintenance diagnostics:Auto-correlator, angular spectrometer, phase-front detector, …

Laser beam in the e-gun room(small compressor and 3rd harmonic generation are next to the gun)

Steady diagnostics:Virtual cathode CCD, UV energy meter, some IR signal coupled to a fiber for rough timimg measurementOn demand or maintenance diagnostics:Auto-correlator, angular spectrometer, …

Delay control between pulses: ionization and e-gun

Delay line either on one of 2 beams, proper delay simulation required.Split after RegAmp was proposed by AMPLITUDE with 2.5mJ IR output for e-gun

Laser dedicated list of “Things to Do”:Laser Operation

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OTR or laser light- Imaging (lens system and CCD)- Capture and measure with photodiode or streak-camera

(coupling to a fiber or lens system)- Other techniques

Alignment of 3 beams

plasma

p-beamlaser-beam

e-beam

BPM

BPM

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Just started …

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Alignment of 3 beams• 3 beams (protons, electrons and laser) have to be align in space and time• Transverse accuracy ~ 0.2mm• Angular accuracy ~ 0.2mm / 10 m = 20rad• Timing electrons-laser ~ 100fs – alignment by response? Rough alignment is

needed anyway• Timing protons-laser ~ 100ps – alignment with fast photodiode and scope possible,

1pJ of light is required. Streak-camera.

For robust alignment of 3 beams we need an optical signal which comes from the same screen sensitive to 3 beams (the power of laser beam can be reduced for the measurements not to damage the screen)

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Preliminary

AWAKE access modes are under discussion …

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Thank you!

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