The compression of a laser pulse by amplification of an ultra short pulse beam Which seeds the stimulated Raman scatter of the first beam has been long been discussed in the context of solid and gas media. We investigate the possibility of using intersecting beams in a plasma to compress nanosecond pulses to picosecond duration by scattering from driven electron waves. Recent theoretical studies have shown the possibility of efficient compression With large amplitude, non-linear Langmuir waves driven either by SRS or non-resonantly. We describe experiments in which a plasma suitable for pulse compression is created , and amplification of an ultra short pulse beam is demonstrated

Berger, R L

Charman, A E

Dewald, E

Fisch, N J

Kirkwood, R K

Landen, O L

Lindberg, R

Malkin, V M

Meezan, N

Niemann, C

Wilks, S C

Wurtele, J

UNT Digital Library

UCRL-CONF-221778Observation of amplification of a 1pspulse by SRS of a 1 ns pulse in a plasmawith conditions relevant to pulsecompressionR. K. Kirkwood, E. Dewald, S. C. Wilks, N. Meezan, C.Niemann, R. L. Berger, O. L. Landen, J. Wurtele, A. E.Charman, R. Lindberg, N. J. Fisch, V. M. MalkinJune 2, 200636th Anomalous Absorption ConferenceJackson Hole, WY, United StatesJune 4, 2006 through June 9, 2006This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  Updated October 14, 2003 Observation of amplification of a 1ps pulse by SRS of a 1 ns pulse in a plasma with conditions relevant to pulse compressionR. K. Kirkwood, E. Dewald,  S. C. Wilks, N. Meezan, C. Niemann,R. L. Berger, O. L. Landen,LLNLJ. Wurtele, A. E. Charman, R. Lindberg,UC Berkeley/LBL,N. J. Fisch, V. M. Malkin,Princeton University36th Annual Anomalous AbsorptionJackson Hole, WyomingJune 2006This work is performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.RKKIntroductionThe compression of a laser pulse by amplification of an ultra short pulse beamWhich seeds the stimulated Raman scatter of the first beam has been long beendiscussed in the context of solid and gas media.We investigate the possibility of using intersecting beams in a plasma  to compress nanosecond pulses to picosecond duration by scattering  fromdriven electron waves.Recent theoretical studies have shown the possibility of efficient compressionWith large amplitude, non-linear Langmuir waves driven either by SRS [1] or non-resonantly [2].We describe experiments in which a plasma suitable for pulse compression is created , and amplification of an ultra short pulse beam is demonstrated.[1] V. M. Malkin G. Shvets, and N. J. Fisch, PRL 82, 4448 (1999).[2] G. Shvets, N. J. Fisch, A. Pukhov, and J. Meyer-ter-Vehn,  PRL 81, 4879 (1998).RKKTwo Beams in a Plasma Will Stimulate Plasma Waves That Scatter Energy Between the Beamsstimulatedwave1 ps probe beam 1 ns pump beamtransmitted seedbeam and scatteredpump beamA beat intensity profile is produced by two beams that have spatial and temporalfrequency component determined by the difference in the two beams.to the other.kepww1, k11 w2, k2kplasma = k1 - k2wplasma = w1 - w2The plasma wave forms a 3D  Bragg cell grating that scatters power from one beamWith the right plasma conditions the wave is resonant and grows to large amplitudeRKKIf k lD is Small A Counter-Propagating Probe Pulse, Amplified by SRS, Can Deplete the Pump in a 15 cm plasma From Malkin et. al.Counter propagating beams allow an ultra short pulse to extract all the energyOf a long pulse beam, causing pulse compression when SRS gain is high.The interaction distance and plasma size are given by x = (1/2) c tlong pulse or ~ 15 cm of plasma per ns of compressed pulse.RKKCritical Issues for Pulse Compression by SRS in a Plasma1) The ponderomotive force of the two beams must be able to drive a largeenough wave amplitude to scatter nearly 100% of the pump power.a) The strongly non-linear wave response must  be verified by by simulations and proof of principle experiments.b) The necessary coherence of the scattered light must also be verified by experiments.2) Desirable plasma properties include:a) hot enough to minimize inverse bremsstrahlung absorptionof the beams and the collisional damping of the Langmuir waves.b) cold and dense enough to allow weak Landau damping.Our Experiment tests 1) under conditions determined by 2)RKK0.0010.010.1110100100010 18 10 19 10 20 10 21tau.datkld = 0.05kld = 0.1kld = 0.15kld = 0.2kld = 0.25kld = 0.3kld = 0.35kld = 0.4kld = 0.45kld = 0.5Optical Depth(< 1 needed)Density (cm -3 )The minimum plasma density fluctuation to scatter 100% of the pump powerInto the probe indicates average plasma densities of > ~ 2-3 x 1018 are needed.Langmuir wave growth requires low klD while good beam transmission requires low inverse bremstrahlung, which leads to greatest effects at lowest densities.Our experiments begin with2.5 x 1018  and 1 x 1019 cm-3As the resonant densityAnd produce temperatures In the triangular region ofInterest.Optimization of Plasma Conditions for Compressing 1 nsPulses into 1 ps Pulses with 1 micron Wave Lengthsdn/n> 1kl d > 0.3absorption too highdensityrangestudiedRKKAn Single Beam Produces a Uniform Density Plasma,Simulations Confirm Little Time Variation During 1 nsInterferometric measurements witha single (unsmoothed) beamsHave confirmed uniform initial gasand plasma density over the rangeof parameters.(axial posn.)527 nm Interferometer (2 ns gate)Hydra simulations of RPP smoothed beam confirm little variation of density plateau during the 1 ns pulse, and low temperatures at 2.5 x 1018/cm3 density.  (Experiments have shown the simulated temperature may be an overestimate due to a local transport model, Gregori PRL 2004).T < 270 eVK lDebye < 0.45RKK0501001502002503000 0.005 0.01 0.015 0.020.5 mm length0.25 mm lengthAmplification of 1 ps Seedn/ncriticalPIC Simulations of Experimental Conditions Show Substantial Amplification Under These ConditionsE transmittedTime (w0-1)probe input levelTransmitted Probe Ez vs.  time Power Amplification Vs. DensityPIC simulations confirm significant amplification of a 1ps pulse under theseConditions, and show the amplified pulse length ~ 1ps, and the maximumAmplification is near the resonant value of density.resonantdensityRKKExperiments with Janus/Comet Laser Systems Test USP Pulse Amplification in a Plasma Suitable for 1ns Pulse Compression• Experiments with Janus/Comet Beams show amplification of the 1ps Comet beam by the 1ns Janus beam.• A gas jet produces a He plasma about 1-3 mm wide.• High intensity interaction across the entire plasma could deplete 10-20 ps slice of pump beam and demonstrate pulse compression• Available intensities (< 1.1 x 1015/cm-2) are well above what is needed if plasma response is linear, so the non-linear response has been studied.The back scattergeometry has the beamsnearly parallel.(beams perpendicular to jet)The 1ps beam is converted to1124 nm and 1200 nmby a H2 gas Raman CellMaximum energy is Exchanged between beamswhen density is adjustedto resonant value.Transmission MeasurementInteraction beamsPumpBeam1 ns1054 nm.ProbeBeam1 ps1124 nmAnd 1200 nmlines Gas jetplasmaRKKUnseeded (single beam) SRS spectra from Low Density He Plasmas Shows Rapid Increase of Scattering, and the Expected Increase ofWavelength, with Plasma Density (or Gas Pressure)10010001041051061071100 1120 1140 1160 1180 1200 1220 1240300 psi700 psi400 psi200 psi240 psi1180 psiSRS spectral density (arb.)wavelength (nm)300 psi700 psi400 psi200 psi240 psi1180 psiSRS is seen to beMuch more intense andAt longer wavelength As plasma density is Increased, as expected.  Interferometer measurementsshow plateau plasma densityis consistent with the SRSSpectral peak.Additional measurementsAt low beam intensityShow little change in SRS reflectivity, Consistent with stronglySaturated waves.I = 1.1 x 1015 W/cm2Pressure and Density Scan RKK01 10-52 10-53 10-54 10-51140 1160 1180 1200 1220pump + 1ps probe at t = 400 ps (amplification)probe onlypump + 1ps probe at t = 1ns (no interaction)pump onlytransmitted spectral density (J/nm)wavelength (nm)Seeding the high density plasma with a 1ps probe pulse at the resonant wavelength of 1200 nm produced a large amplification showinghigh density is favorable as expected.The observed amplification of the transmission of the seed pulse is consistent with  weak damping and saturation of the scattering Langmuir waves at this density.0.46 mJ transmittedin amplifiedcaseIpump = 3.5 x 1014 W/cm2 , 115 Jne = 1.0 x 1019 cm-3RKK05 10-50.00010.000151140 1160 1180 1200 1220pump + 1ps probe at t = 240 ps (amplification)probe onlypump + 1ps probe at t = 1ns (no interaction)transmitted spectral density (J/nm)wavelength (nm)Seeding the high density plasma with a 1ps probe pulse at the resonant wavelength of 1200 nm  at Increased Intensity produced Higher Amplification showing Scaling with Intensity.The observed scaling of the amplification of the transmission of the probe pulse with intensity is will allow a pulse compressionexperiment to be designed.Ipump = 1.1 x 1015 W/cm2ne = 1.0 x 1019 cm-33 mJ transmittedIn amplifiedcaseRKKThe 1124 nm transmission isMeasured on the same shots,With the same timing and plasma conditions and indicate4x attenuation of ‘Non-resonant’light at the time of the InteractionAttenuation is due to both inverse bremsstrahlungabsorption and spreadingof the beam outside the collection optics.This increases measured gainfollowingAmp. = (‘Pump+seed’ -‘Pump only’)(attenuated ‘seed only’)= 4.4x at low I= 16x at high IAttenuation of Non-Resonant Light (1124 nm) Shows 4x Attenuation Due to Plasma Absorption and Beam Spreading, Indicating The Real Amplification at 1200 nm is Even Greater0501001502002503003504001080 1090 1100 1110 1120 1130 1140 1150pump + 1ps probe at t = 400 ps (amplification)pump onlypump + 1ps probe at t = 1ns (no interaction)probe onlySpectral Density (arb.)wavelength (nm)RKKSeeding the low density plasma with a 1ps pulse at the resonant wavelength of 1124 nm produced less amplificationthan at high densityThe small amplification of the transmitted seed pulse relative to its vacuum valueis consistent with strongly damped or saturated Langmuir waves at this density01002003004005006007001100 1110 1120 1130 1140 1150Pump only (largest signal case)Pump + seed, t ~ 330 psPump + seed, t ~ 50 psSeed onlySeed onlycountswavelength (nm)The ‘pump+seed’ waveform isOnly slightly larger than thesum of the ‘pump only’ and‘seed only’ cases at early timePump only SRS is weak and varying adding to un-certaintyof amplification.Little attenuation of seed is seen,Or expected at this density, soAmplification is compared to Vacuum seed transmission to Give:  Amp. <~ 1.75x at high Iand low neRKKAmplification was Optimized vs. Time in Pulse and Resonant value of Plasma DensityAmplification of 1200 nm seed vs. time in 1 ns pulseAmplification of 1200 nm seed vs. densityScans of beam timing and plasma density show amplification is resonant with density and time varying during pump pulse.   (Amplification factors are relative to measured transmission of the non-resonant wavelength line).010203040500 0.2 0.4 0.6 0.8 1 1.2Ipump = 1.1 x 1015 W/cm2Ipump = 3.5 x 1014 W/cm2amplificationtime (ns)05101520250.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2amplificationdensity / resonant densityRKKPIC Simulations Predict as Little as 35 mJ of Seed EnergyBegins to Deplete the Pump Under These ConditionstimeThis “missing” energywent into the probe.IpumpIprobetimeZOHAR Simulations of the present experimental configuration with the seed energy increased ~300 x (to 35 mJ) already deplete the pump Output Seed Pulse Output Pump PulseRKKLow density and low temperature plasmas have been recognized as desirable for efficient compression of a 1 ns pulse by Stimulated Raman Scattering in a Plasma.Experiments are the first demonstration, and study of the scaling of, the amplification of a 1 ps pulse in a plasma produced by a 1ns pulse, and have demonstrated as much as 37x amplification of a short pulse with  1.0 x 1019/cm3 electron density and 1.1 x 1015 W/cm2 pump intensity.Further experiments showed a large reduction in amplification at low density (< 1.75 x  at 2.5 x 1018/cm3) and a smaller reduction at low intensity (4.4 x at 3.5 x 1014 W/cm2), suggesting favorable scaling with density and intensity.Simulations also show that pump depletion is possible with increased seed energySingle beam near backscatter measurements under these conditionsshow scattering increasing rapidly with plasma density in the density range studied, with weaker dependence on beam intensity.SummaryRKKFive ‘Comet Only’ shots (shown)Before and after the amplification Experiment show little variability.The two Janus shots with no Comet Interaction showed similarScattering resultsFurther, calorimeter measurements showed 456 mJ on the amplification shot and <~ 200 mJ (trig.level) on all other shots.These results indicate thatthe large signal on the Janus+Comet shot was due to plasmaAmplification (not laser fluctuation).Both Comet Only and Janus Only Data Showed Good Reproducibility, Supporting  the Amplification Result0501001502002503003504001160 1180 1200 1220 1240countswavelength (nm)Comet Only measurementsRKK469 nm HeII Line Indicates T - ~25 eV Near Resonant Density469 nm(HeII)532 nm(probelaser)050100150200250420 440 460 480 500 520 540 560Intensity (arb.)Wavelength (nm)Line to continuum ratio at 85psi jet pressure consistent with ~ 25eV electrontemperature (un inverted).   Spatial imaging indicates localization to beam.RKKHe Line to Continuum Ratio Gives Te vs. Position0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4253035404550radial position [mm]T TextEnde TextEnd[eV]TextEnd50psi He gas jetHWHMOf Beam~45 eV(peak) K lD = 0.25 !85psi jet pressureRKKInterferometer Measurements Show Peak Density Approximately Tracks Gas Jet Pressure, as Expected00.511.520 100 200 300 400 500 600peak frindge shift (frindges)gas jet backing pressure(psi)Error bars indicateFrindge reading uncertiantyEven with varying profiles, peak frindge shift on interferometer approximatelyfollows the gas jet pressure.  (pressure may be best indication of average density)4mm dia. nozzle1.25 frindge = 2.6 E 18/cc(for 2mm plasma length)Which is resonant with1124nm lightObservedResonance(near 410 psi)RKKThermal SRS Observed at Higher Plasma Densities Have Appropriate Wavelengths for Measured Density Profiles0102030405060701000 1050 1100 1150 1200 1250 1300 1350Intensity(arb.)wavelength (nm)-0.500.511.522.533.5-2 -1 0 1 2 3frindgesWavelengthof plateaudensityOperation of jet at higher pressure allows high density plasma to be producedand thermal SRS spectrum (un seeded) is observed.Density profileFor 4 mm dia. jet(plateau is 6.3 E 18/ccFor 2mm plasma length)SRS Spectrum Measured Frindge ProfileRKKSBS Was Found to be Very Large Even withwith Defocused (Low Intensity) Beams at High Density00.050.10.150.20 100 200 300 400 500 600 700 800Focused, High Power Focused, Low PowerDefocused, High PowerFraction SBSgas pressure (psi)Further SBS studies in this plasma in a later  talk by D. FroulaRKKDemonstration of Efficient Raman Conversion of a 1 ps Beam to 1124 nm Has Been an Essential StepAn H2 gas Raman Cell (500 psi, 1 m length) allows the 1054 nm Comet lineto be converted to 1124 nm with ~ 15% efficiency.-200002000400060008000100001080 1100 1120 1140 1160Output Intensity(arb.)Wavelength(nm)Spectral widthof input 1054 nmline.RKKShort pulse Transmission Measurement in Small Scale Plasma Gave Some Evidence of Amplification Near the  Resonant DensityPlasma wavesAmplitudes at ‘trapping threshold’ n/n <~ 10-3would produce<~ 2x amplification0.50.60.70.80.911.10 50 100 150 200 250Trans frac (2/25,26)Transmitted Fraction Gas Jet Pressure (psi)Maximum amplification observed with small scale plasma is <~ 1. 2x Expected Resonance

Observation of amplification of a 1ps pulse by SRS of a 1 ns pulse in a plasma with conditions relevant to pulse compression

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Observation of amplification of a 1ps pulse by SRS of a 1 ns pulse in a plasma with conditions relevant to pulse compression

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