Progress and Potentials of Ultra-Fast CO2 LASERS

Igor Pogorelsky

OutlineIntroductionIntroductionSuccess story Success story

Basic principlesBasic principles

ATF ProspectsATF ProspectsNearNear--term increase of laser strength to aterm increase of laser strength to a00~4 ~4

FemtosecondFemtosecond regimeregimeMultiMulti--terawatts at terawatts at linaclinac’’ss repetition raterepetition rate

PW and highPW and high--repetition rate frontiers of COrepetition rate frontiers of CO22laser technologylaser technologyPW via power broadeningPW via power broadening

Commercial 500 Hz lasersCommercial 500 Hz lasersUsing intraUsing intra--cavity beamscavity beams

Bandwidth limited amplificationof ps CO2 laser pulses

Prehistoric ATF laser system 1990

Igor PogorelskyJim Veligdan

Alan Fisher

YAG laser

CO2 oscillatorCO2 amplifier

ATF Laser Success Story

InverseCherenkovaccelerator IFEL

accelerator

ThomsonX-ray source

HGHG

1995 2000 2005 2010

STELLAI & II

EUV source

Ion andProtonsource

ResonantPWA

SeededLWFA

LACARA

PASER

3 TW

300 GW

30 GW

3 GW

Nonlinear Thomsonscattering

Intra-cavityCompton source

Benefits of using long-wavelength (λ=10µm) CO2 laserfor non-relativistic processes:

• Combines advantages of high-quality conventional RF accelerators and high-gradient optical accelerators with λ =~1 µm

•favorable phasing•structure scaling.

Illustrated by STELLA - the first two-stage laser accelerator Optical delay

(March 2006)

• Number of laser photons per Joule of energyis proportional to λ.

Illustrated by Thompson scattering experiment – presently the brightest Thomson x-ray source.

High-Pressure CO2 Lasers

Available methods:

•Semiconductor optical switching

•Kerr effect in optically

active liquid (CS2)

•Differential frequency generation

in parametric crystals

Solid state lasers help with picosecond pulse generation at CO2

laser wavelength

Plasma Density (cm )-3

CSMW COp-polarized

ps GW YAG45-deg polarized

Polarizer

ps COs-polarized

2

2

2

ω1ω3=ω1−ω2

ω2=ω1−ω3

CO

YAG

CO

2

2

ω2

BNL/ATF CO2 laser system delivers1 TW, 5 ps pulses every 20 seconds

10 ns

200 ps

5 ps

CO2 oscillator3-atm preamplifier

10-atm regen. amplifier

10-atm final amplifier

Kerr cell

Ge switch

5 ps YAG pulse

Part of the laser system operates“micro-bunch factory” at 0.3 Hz

regenerative

4

e-beam

femtosecond

microbuncheslaser

1 2

WakeField rε

v

PASER(in active medium)

Resonance PWA(in capillary plasma)

Focusing with Parabolic Mirror

X-ray detector

IR detector

Optical detector

IR camera

Parabolic mirror

RCF

CR39

For proton acceleration

For Compton Scattering

Characterizing the laser focusproduced with f#=2 parabola

0 50 100 1500

0.5

1

X1 r( )

r0 50 100 150

0

0.5

11

0

X1 r( )

1500 r

0 50 100 1500

0.5

11

0

X1 r( )

1500 r

µm

Transmitted

energy

75 µm 100 µm 150 µm

mw µσ 7520 =≡mw µ650 =

( ) ( )⎥⎥⎦

⎤

⎢⎢⎣

⎡−⎥

⎦

⎤⎢⎣

⎡=

zw

rzw

wIzrI

2

220

02exp),(Gaussian approximation

Conclusions:

• w0=65 µm – best fit

•1.5x1016 W/cm2

@ 1 TW

• a0=1 @ λ=10 µm

no pinhole

r

The importance of condition a0=1and importance of λ=10 µm for reaching this

condition:

•Dimensionless laser amplitude

ao=eA/mc2

•Electron quiver energy

E=a02 x mc2/2

•Electron motion becomes relativistic when

a0=1 I λ2=1.37x1018 W µm2/cm2

Proof of attaining a0=1 in ATF experiments:

Nonlinear Compton scattering

Ponderomotive force

a0=1 when Φpond=mc2

An electron experiences a force, called the ponderomotive force, which is proportional to the gradient in the amplitude of the wave-field.

Ponderomotive force generates plasma waves

Ponderomotive forcepushes electrons from foilfor ion accelerationIon acceleration

The ponderomotive energy of the electron in the optical field that controls plasma wake generation, ion acceleration and other strong-field phenomena is proportional to λ2.

Near-term plans for a0 enhancementwill bring ATF laser on line for such popular

applications as LWFA or ion acceleration

• Using twice shorter focal length parabola (F=75 mm) gives factor of x2 a0~2

• Shortening the pulse length to 1 ps gives another factor of x2 a0~2

• Cumulative x4 a0~4

•Shortening pulse length has an additional benefit for LWFA as it allows using x25 higher resonance plasma density increasing maximum accelerating gradient proportionally.

Pulse shortening will be achieved with a femtosecond fibre laser

SpatialShaping

Osc. Yb:Fiber Pre-Amp

PartiallyCompress

Pulse energy = 1.2 nJPulse duration = 100 fsPeak power = 12 kW

Repetition rate = 81.6 MHzAverage power = 100 mW

2*0.5 nJ200 ps2.5 W

1 kHz1 µW

2*100 µJ200 ps500 kW

1 kHz200 mW

Yb:S-FAPAmp

10 mJ200 ps50 MW

120 Hz1.2 W

PC chop

PC gate

2*80 µJ200 ps400 kW

1 kHz160 mW

500 µJ5 ps100 MW

120 Hz60 mW

5 mJ5 ps1 GW

120 Hz600 mW

50 µJ200 ps250 kW

120 Hz8.4 mW

4ω

PC gate

100 µJ5 ps20 MW

120 Hz12 mW

YB:S-FAP Amp

30 µJ200 ps150 kW

1 Hz30 µW

PC gate

40 µJ200 ps200 kW

1 kHz40 mW

5 mJ500 fs10 GW

1 Hz5 mW

Yb:GlassAmps

30 µJ200 ps150 kW

10 Hz300 µW

PC gate

40 µJ200 ps200 kW

1 kHz40 mW

600 mJ200 ps3 GW

10 Hz6 W

VacuumCompressor

300 mJ300 fs1 TW

80 MHz100 mW

To High FieldExperiments

To CO2

To Gun

Pulse Stretcher

0.6 nJ200 ps3 W

81.6MHz50 mW

10 mJ200 ps50 MW

1 Hz10 mW

Compress

2ω

1 mJ5 ps200 MW

120 Hz120 mW

Delay

Trombone

Delay

Spectral Shaping

Directions for ultra-fast CO2 laser improvement:

•Femtosecond pulses (few cycles)

• Higher energy per pulse

• Higher repetition rate

• Higher average power

CO2 femtosecond pulse generation and amplification

• Ultra-fast slicing

• Amplification in multi-isotope mixture

• Pulse chirping and dispersive compression

• Raman backscattering

• Power broadening

Ultra-fast slicing and amplificationin multi-isotope mixture

Direct amplification in a 4-atm CO2 amplifier containing a mixture of molecular isotopes with 12C, 13C, 14C, 16O, 18O. Gain bandwidth 7 THz sufficient for 150 fs pulse amplification.

Paul Corkum demonstrated in 1986 semiconductor slicing of 130 fs CO2 pulses

Pulse Chirping and Compression UCLA

Laser-induced ionization shifts the phase of the wave resulting in a chirpand subsequent pulse compression

Plasmashutter

Amplifier250 ps

40 ps

0.00

500.00

1000.00

1500.00

2000.00

0

0.5

1

1.5

2

2.5

28200 28240 28280 28320 28360 28400 28440 28480

Sig

nal A

mpl

itude

(arb

.uni

ts)

Gai

n (m

-1)

Frequency (GHz)

10P(24) 10P(18) 10P(16)10P(20)10P(22)

( ) ( ) cre ntxntx ,1, −=η

( )dxxtntn

ne

cr

e ,0

∫∂∂

=∆λπω

Can be used to compress 1 ps to 100 fsIn dispersive optical element such as ZnSe window

Measured blue shift 40 GHzcorresponds to ne= 3x1017cm-3

Gas pipe ∆η/∆ω<01 ps

200 fs

Raman backscattering

Raman backscattering of a 9.6-µm nanosecond pump into the counter-propagating femtosecond10.6-µm seed pulse in a resonance plasma ωp=∆ω.

Proposed by G. Shvets

g L0 10 20 30

0

1 TW

1 PW

Time (ps)

0 20 40 60 80 100 120

Time (ps)

0 20 40 60 80 100 120Time (ps)

0 20 40 60 80 100 120

Hypothetical combination of PITER-I with MARS (UCLA) provides Petawatt capability @ 1 ps

10 MW (3ps)input pulse

0.8 PW outputfrom MARS

1 TW outputfrom PITER

GHzR 37=∆νh/ER µν =∆Use power or Stark broadening in laser field

, at 1010 W/cm2

Rutherford Appleton Laboratory'sCentral Laser FacilityNd:glass Petawatt VULCAN laser I=1020 W/cm2; a0~10

…but λ=1 µm permits tighter focusing (assuming w0~λ) !

However:♦ Interacting with e-beam you do not want to focus laser tighter than e-beam (decreases acceleration quality and x-ray yield). CO2 laser focusing is sufficient to interact with low-emittance e-beams.♦ In laser/matter interactions ten times tighter focus of the 1 µm laser results in 100 times smaller area and 1000 times smaller interaction volume where we can see an equivalent effect. This will proportionally reduce the process yield.♦ Thus, 1 PW CO2 laser in certain cases is equivalent to 100 PW solid state laser!

w0

λ= λ=

Demonstrated: Pumping Efficiency 20%, SSG 10%/cm

Another possibility is direct energy transfer via reactions F+D2=DF*+D, D+F2=DF*+F, DF* + CO2 =DF+ CO2*

10.0 10.2 10.4 10.6 10.8 11.0 11.2 mµ

C

C

O

O

+N

N

O

O

2

2

2

2

PR PR

High-pressure CO2:N2O laser optically pumped by HF chemical laser

Courtesy of M. AzarovRussian Academy of Science

High-pressure CO2 laser amplifierwith optical pumping

Courtesy of M. AzarovRussian Academy of Science

Amplifier cell

Optical pumpconverter

Capabilities of compact 1.4 liter optically pumped high-pressure CO2 amplifier:•Output energy: 30J/pulse•Repetition rate: >10 Hz (limited by a pump laser)•Many ATF laser components are compatible to this speed

200 fs, 150 TW can be achieved at ATF;this allows a0=10, potentially at linac’s repetition rate

OSCILLATOR

PITER-I AMPLIFIER

GAS PIPEOPTICALLY-PUMPEDMULTI-ISOTOPE AMPLIFIER

REGENERATIVE AMPLIFIER

Kerr cell

Pockels cell

10 TW 150 TW

300-fs fiber laser

10 ns 100 ns

200 ps

1 ps

200 fs

100 GW

1 MW

1 MW

10 MW

PREAMLIFIER

Commercially Available High-Pressure High Repetition-Rate CO2 Lasers

Repetition Rate 20 -500 HzPulse Energy 1.5 JBeam Size 13 x 13 mm2

Average Power 750 W

Current Positron Source Proposals for ILC

Conventional non-polarized positron source

Polarized gamma sourceby Compton scattering

Polarized gamma sourceby spontaneous wiggler radiation

150 GeV linace-gun

200-m wiggler

target

Polarized γ source requirementsParameter Symbol Value UnitPulse repetition rate frep 150 Hz

e- per bunch ne 6x1011

Number of γ per bunch NγxNlaser 6x1012

Bunches per pulse Nb 100Bunch Spacing ∆tb 12 nsLaser energy Elaser 1 J

Number of γ per electron Nγ/ ne 1

Size at focus σlaser 40 µmLaser pulse length tlaser 5 ps

Number of lasers Nlaser 10

Needed: 15 kHz, 15kW, picosecond, sub-terawatt CO2 laser

CO2 Laser system

for ILC PPS

intra-cavity pulse circulation :– pulse length 5 ps– energy per pulse 1 J– period inside pulse train 12 ns– total train duration 1.2 µs– train repetition rate 150 Hz

– Cumulative rep. rate 15 kHz– Cumulative average power 15 kW

Kerr generator

IP#1 IP#5

2x30mJ

γ

CO2 oscillator

2 pulses 10mJ 5ps from YAG laser

2 x 200ps

2 x 1mJ5ps

2x10mJ5ps 2x300mJ 5 ps

TFPPC PC

150nsGe

2 x1J

e-

Test setup

3-atmCO2 amplifier

parabolic mirrors

vacuum cell

detector

YAG (14 ps)200 ns

200 ps

Ge

3% over 1 µs

Observations:•Optical gain over 4 µs•Single seed pulse amplification continues to the end

• CO2 laser offers fundamental advantages due to the λ2-proportional ponderomotive potential and λ-proportional number of photons per 1J.

• 5-ps, 5-J, 1-TW CO2 laser has been demonstrated and used in ATF user’s experiments.

• Demonstration of a0~1 in nonlinear Compton scattering and ion acceleration experiments.

• Near-term possibilities for a0~4 by tighter focusing and shortening to 1 ps with a new fiber laser.

• Available resources for 200 fs CO2 pulse generation and amplification: ultra-fast slicing and amplification in multi-isotope mixture; pulse chirping and dispersive compression; Raman backscattering.

• Power broadening allows to reach petawatt power in a big medium-pressure CO2 amplifier such as UCLA MARS.

• Multi-terawatt, femtosecond CO2 laser with optical pumping can operate at the linac’s and higher repetition rate.

• Commercially available high-pressure CO2 lasers can provide 1 TW output at up to 500Hz and 0.75 kW average power.

• Non-destructive intra-cavity process, such as Compton scattering, allows to utilize a terawatt laser beam more efficiently at multi-kHz rate and >10 kW average power (potential application for ILC).

Conclusions