Laser Source for the  -  Collider Jim Early Lawrence Livermore National Lab Laser Science and Technology SPLAT Short Pulse Lasers, Applications & Technology Presented to:Snowmass 2001 July 6, 2001

Requirements for gamma-gamma laserLLNL Efficient conversion of electron energy to  photons requires: - laser wavelength near 1  m - laser pulse duration near 2ps to overlap electron pulse - 1J per subpulse in high quality beam for adequate photon density in conversion zone - laser pulse format matching electron accelerator Low duty factor and short pulse duration requires use of “storage” laser - solid state storage lasers used in laser fusion program give ns pulses - thermal management of high average power a challenge in solid lasers Optical compression required to avoid damage in laser amplifier - pulse must be stretched from several ps to several ns before amplifier - chirp pulse stretching and compression technique required

Laser architecture driven by electron bunch format 96 pulses 1J, 2ps, 1  m 3ns spacing 120 Hz macro-pulses 100 J macro-pulses require large “storage” laser amplifier Amplifier thermal design leads to low, 10 Hz, laser pulse rate - 12 amplifiers use simple spatial combiner to achieve 120 Hz DOE Inertial Fusion Energy program developing “Mercury” laser that meets requirements Optical design breaks 100 J macro-pulse into train of 1 J sub-pulses

 laser system architecture: CPA front end seeds 12 Mercury power amplifiers Mode-locked oscillator Spectral shaper StretcherOP-CPA preamp Mercury power amp Beam splitters J power amplifiers Optics: Combiner, splitters Grating compressor 100 J macropulse: 100X 2ps micropulses 120 Hz 0.5 J 3 ns 120 Hz

Goals: 100 J 10 Hz 10% electrical 2-10 ns The Mercury laser will utilize three key technologies: gas cooling, diodes, and Yb:S-FAP crystals vacuum relay gas-cooled amplifier head Injection and reversor Architecture: - 2 amplifier heads - angular multiplexing - 4 pass - relay imaging - wavefront correction front end DM

Pump delivery Front end Injection multi-pass spatial filter Diode pulsers Gas-cooled amplifier head

Milestone budget breakout: 1.$3030kBuild two pump delivery systems 2.$1800kFabricate Yb:S-FAP crystals 3.$825kDesign and build wedged amplifier head 4.$1025kBuild injection and reverser hardware 5.$1270kIntegrated tests and code benchmarking 6.$300kAdvanced Yb:S-FAP growth 7.$350k(LLE) Spectral sculpting experiments and evaluation of average-power frequency conversion design Mercury project FY01 funding from IFE program We are on schedule to build half Mercury in FY01. FY02 funding will be slight increase. We are on schedule to build half Mercury in FY01. FY02 funding will be slight increase.

Objective 1: Build two pump delivery systems beam diode package on split backplanes gas-cooled amplifier head vacuum enclosure pump duct and homogenizer Goal Status 80 V-BASIS 23-bar 900 nm tiles fabricated Two functioning backplanes loaded with diodes Remaining power supplies/pulsers purchased Pump delivery hardware assembled, integrated, and currently being activated

The Mercury diodes deliver the pump light to Yb:S-FAP crystals Half array is made of 5x7 =35 tiles Full array  161 kW 4 pairs of half arrays like these are required for Mercury  644 kW Each tile is made of 23 diode bars  2.3kW Diode light distribution (green) obtained in a plane normal to the optical axis 7 tiles 5 tiles

The V-BASIS packaged diode bars meet the optical specifications of the Mercury Laser System 1 full backplane array (72 Mounted tiles) complete 5% droop demonstrated 44% demonstrated Completed Fabrication of 80 tiles StatusRequirement Demonstrated 3.7 nm FWHM on tiles for one split backplane Pulse integrated Linewidth < 8.5 nm FWHM Assemble tiles on split backplane Power droop during pulse < 15% Testing is ongoing, but currently demonstrated 1.4 x 10 8 shots without problems Reliability of > 2 x 10 8 shots 45% electrical to Optical efficiency 115 W peak / 1 cm bar demonstrated with good lifetime 100 W peak /1 cm bar

Copper Heat Sinks Microchannel Cooled Bars Microchannel Cooled Monolithic Arrays CW Bars Peak Power Bars Sources:L&O Market Survey 10/93 L&O Market Survey 11/95 LLNL/USEC Survey 96 Year $/Watt Laser Focus World 2/98 Purchase Order 99 Quotation for 00 delivery D. Scifres (SDL) CLEO ‘99 Similar to other integrated circuit technology, the cost of diode arrays has been dropping even while the performance has been increasing

Gas cooled head and vanes 1/ Mach gas flow 4 atm pressure static Pressure and gas flow contributes 1/16 wave to wavefront distortion

Fabrication of Yb:S-FAP crystals Goal A full size 4x6 cm amplifier slab Two 3x5 cm slabs Status One full size bonded amplifier slab completed - awaiting polish and AR coating Two smaller slabs (usable) also completed and await finishing Processes are not completely reproducible at this point A axis 4 cm 6 cm C axis

Crystals of Yb:S-FAP are grown by using the Czochralski (CZ) Method

Appropriate spectral sculpting of the input pulse can lead to a linearly chirped gaussian output pulse (2 psec stretched output pulse case) RJB/VG 3-Oct-00 short Pulse Mercury Laser Pass 4 (output) Pass 3 Pass 2 Pass 1 Input Pulse Normalized Emission Line and Saturated Gain for Yb:S-FAP nm

Telephoto Imaging System LCM Light Valve Gratings: 1740 grooves/mm Telephoto imaging system EFL ~ 800 mm A compact spectral sculptor using a liquid-crystal modulator light valve has been demonstrated Grating

8 May 1999 New technology enables production of Terawatt to Petawatt (1000 TW) pulses

Stretcher and compressor gratings LLNL StretcherCompressor Substrate materialsilicasilica Coating materialgoldMulti-layer Grating size (cm)4 x 1530 x 84 Roof mirror size (cm)4 x 8 (flat)30 x 40 Grating separation (m)515 Lines per mm Laser beam diameter (cm)110 Cut bandwidth (nm) Exit sub-pulse duration (ps) Efficiency-single bounce (%) System efficiency (%)6080 Laser macro-pulse fluence (J/cm 2 ) Damage fluence (J/cm 2 ) Approximate cost ($K) cm aperature gold coated diffraction Multilayer dielectric dielectric grating grating used for pulse compression designs of high-index (H) and low-index (L) on the Petawatt laser layers and groove corrugations (G).

Typical high-power CPA systems use multiple Amplifier stages to obtain gains of 10 11

Optical parametric amplification is based on difference frequency generation NONLINEAR CRYSTAL signal idler PUMP LASER short wavelength pump pulse stretched long wavelength seed pulse pp ss  i =  p -  s ksks kiki kpkp kk residual pump

600 mJ pump, 532 nm, 8.5 ns 500 pJ seed, 1054 nm, 3 ns BBO preamplifier vacuum relay telescope I vacuum relay telescope II 15 % BS 90 mJ 420 mJ BBO power amp WP TFP to compressor 31 mJ 0.1 TW-scale OPCPA was demonstrated as a full replacement for regenerative amplifierLLNL

beam splitter optical delay line polarizer wave plate 100 J macro-pulse from laser converted to train of 1 J subpulses Combination of beam splitters and optical delay lines gives two beams with string of pulses Two beams combined on polarizer to give single beam - alternating linear polarization in pulses - 96 pulses ( 3 x 2 5 ) Optical Pulse Train Generation LLNL

Gamma-gamma laser system cost estimate ($01) LLNL Capital costs$M 20lasers 40diodes (at $5/W) 10optics system 20building 20 development program _40_contingency $150Mtotal Operating costs$M/y 8diodes at $5/W (5y or 10 9 shot lifetime) 4labor 4power _4_contingency $20M/y

Gamma-gamma laser summary LLNL Pulse format of 1  m  -  laser must match electron bunch format Mercury laser amplifier under development by DOE can serve as  -  laser - laser under construction with single head to be completed in FY01 New front end for Mercury laser will generate input pulse format needed Optical Compression Amplification and use of pulse string generation optics can modify Mercury pulse format to  -  requirements DOE Mercury laser project can serve as the demonstration prototype for the  -  laser project