Phase II Considerations: Diode Pumped Solid State Laser (DPSSL) Driver for Inertial Fusion Energy Steve Payne, Camille Bibeau, Ray Beach, and Andy Bayramian National Ignition Facility Directorate Lawrence Livermore National Laboratory Livermore, California HAPL Review February 6, 2004 Atlanta, GA

Outline Comparison of DPSSL with NIF - Requirements - Technologies Critical Phase II science and technology issues - Beam energy - Nonlinear beam propagation - Stimulated Raman scattering - Crystal growth - Diode cost - Frequency conversion - Beam bundling ROM cost and schedule

Energy Pulse shape Smoothness Wavelength Target Gain Efficiency Reliability Diode cost Repetition rate IFE Power Plant Fusion laser architectures are predicated on meeting target physics and power plant system-level requirements Target requirements similar to NIF Additional system-level requirements imposed on IFE lasers

Gain Medium NIF (stockpile stewardship) IFE (energy) Integrated Research Exp. (scaling) Mercury (prototype) Energy 2 MJ 192 beams x 10kJ 2 MJ 700 beams x 3kJ 6kJ 2 beams x 3kJ 100 J 1/30 aperture Pulse shape, wavelength 3 ns at <0.4  m Smoothness  < 0.1 % in 1 ns (beams overlapped)  < 3 % in 1 nsec  < 10 % in 1 nsec Efficiency 0.8%, no utilities %, wall-plug %, no utilities Cost $1000/J; Flash lamps used $500/J - laser; $0.05/W - diodes $40k/J - laser; $1/W - diodes $400k/J - laser; $5/W - diodes Rep-rate Hz 10 Hz Reliability 10 4 shots for diodes 10 8 for optics diodes optics diodes optics NIF / IFE are same Enhancements needed Solid state laser driver requirements for Inertial Confinement Fusion

Amplifiers Flashlamps Telescope Mirror Diodes Reflectors Our new architectural layout of optics and amplifiers Collinear diode pumping and beam path extraction - improves gain uniformity and pump efficiency - integrates spatial filter and pump cavity Closely-spaced slabs and lenses in compact amplifier cavity - reduces “B-integral” or beam intensity modulations - optics located where damage probability is lowest Our new architectural layout of optics and amplifiers Collinear diode pumping and beam path extraction - improves gain uniformity and pump efficiency - integrates spatial filter and pump cavity Closely-spaced slabs and lenses in compact amplifier cavity - reduces “B-integral” or beam intensity modulations - optics located where damage probability is lowest Comparison of NIF and Mercury amplifiers Gas cooled

Eff. (%)NIFHgIFE Power Pump Xport Absorption4090 Quant Def6086 Emission6780 ASE67N/A67 Extraction Fill85 92 Xport93N/A95 Freq Conv6075 Total (%) Efficiency comparison NIF and Mercury-like architectures (estimates) Higher efficiency of DPSSL is achieved through many enhancements Radiative cooling Convection Nd:glass Frequency conversion Radiative cooling Yb:S-FAP Frequency conversion Reflector Yb:S-FAP Turbulent cooling Mercury

Gain medium deployed in solid state laser has fundamental consequences on cost and performance Energy Levels Storage time determines diode cost Gain Saturation fluence is F SAT = h /  G 2 MJ laser and 5¢/W diodes C diode ($B) = 0.5 /  ST (ms) Peak fluence: F PEAK = 4.5 F SAT Bandwidth for smoothing:  G Beam Energy Balances amplified spontaneous emssion (ASE) and nonlinear ripple growth E beam = (  EXT / 12 F SAT ) (3  P / 4  ) 2 nonlinear indexextraction efficiency laser pulse widthSaturation fluence Ripple growth Laser slab ASE losses

Yb:S-FAP laser material offers advantages over Nd:glass for IFE Gain Medium Diode Cost 0.5 /  st Damage 4.5 F sat Beam Energy, E beam (3 nsec pulse) 1  Band Width,  NIF (Nd:glass) $1.25B (hypothetically diode-pumped) 24 J/cm kJ (10 kJ with higher ASE losses) 1 THz IFE (Yb:S-FAP) $0.45B14 J/cm kJ0.3 THz  Comparison of Nd:glass and Yb:S-FAP gain media in fusion lasers Longer lifetime reduces cost Lower fluence reduces damage Beam energies are similar Yb:S-FAP has 2.5x greater thermal conductivity than Nd:glass  better for rep-rated operation However, crystals are more difficult to produce in large size Yb:S-FAP has 2.5x greater thermal conductivity than Nd:glass  better for rep-rated operation However, crystals are more difficult to produce in large size Bandwidth is adequate

Outline Comparison of DPSSL with NIF - Requirements - Technologies Critical science and technology issues - #1 - Beam energy / amplified spontaneous emission - #2 - Nonlinear beam propagation / optical damage - #3 - Stimulated Raman scattering - #4 - Crystal growth - #5 - Diode cost - #6 - Frequency conversion - #7 - Beam bundling ROM cost and schedule

Amplified spontaneous emission rates are accelerated for larger slabs Greater extraction efficiency leads to higher B-integral (i.e. beam modulation) Diode efficiency of ~60% and 3  -conversion of ~75% to be included Reduced losses and higher diode efficiency possible S&T issue #1: Models indicate that multi-kilojoule output is feasible from a single coherent aperture 10 x 15 cm 2 20 x 30 cm 2 30 x 45 cm 2 Optical-Optical Efficiency B-Integral, radians (beam modulation) Quadrant of desired operation Design point 4.2 kJ 1.7 kJ 8.3 kJ Ripple growth Laser slab ASE losses

S&T issue #2: Mercury “closely-spaced slab” architecture has reduced nonlinear beam breakup relative to “widely-spaced” (NIF-like) architecture Optical damage risk is mitigated in Mercury architecture two ways: Closely-spaced-slab architecture reduces nonlinear ripple growth Lower saturation fluence of Yb:S-FAP vs. Nd:glass reduces average fluence Widely-spaced slabs have more intensity on pinhole Focal spots Mercury: Closely-spaced slabs B = 3.8 radians Fitting function: Peak-to-Ave = Static · (1 + Alpha · e B ) Widely-spaced architecture

S&T issue #3: Stimulated Raman Scattering (SRS) in S-FAP, or unwanted nonlinear frequency conversion, must be controlled in the IRE Quantitative modeling yields: - Aperture limit is >20x30 cm 2 at 3 GW/cm 2 - Longitudinal SRS is controlled by: - inserting Tm:YAG absorber in amps - adding a small wedge to the slabs Tm:YAG absorber suppresses SRS Gain lowers with angle between laser and SRS SRS is predicted for the IRE based on gain SRS Laser

3.5 cm boules (standard) Onyx - high temperature Schott - “glue” bonding Bonding choices S&T issue #4: Combination of bonding and large diameter growth provides pathway to 20x30 cm 2 Yb:S-FAP slabs Approximately 10 cm boules will be needed to bond three parts together for each 20x30 cm 2 slab 6.5 cm boules (last year) 10 cm boules needed for IRE

S&T issue #5: Learning curve analysis suggests that diode bar prices will drop as the market grows Low duty cycle diode bars Diode packaging house created from LLNL tech-transfer HeatsinksDiode laser bars Backplanes - High production rate  reduced cost - Higher efficiency diodes are desired

S&T issue #6: Average power frequency conversion with >80% efficiency can be obtained for ~ 1 THz bandwidth using BBO crystal Main challenge is to “tile” multiple BBO crystals to cover aperture of beam - Based on current technology, four crystals must be tiled for Mercury Conversion vs. 0.7 GW/cm 2 KDP, YCOB BBO Conversion vs. Intensity (thermally loaded)    He cooling BBO doubler 2.5 mm BBO tripler 4 mm He cooling

S&T issue #7: Amplifier can be integrated into bundles and clusters to simplify cooling and minimize the footprint 36 kJ bundle of 12 apertures 4 kJ beam lines Clusters of bundles Management of high average power likely to be very challenging

Phase I resolves: Yb:S-FAP performance Laser architecture and gas-cooling Pockels cell design Optical damage Diode package Diode commercialization Laser operations Beam smoothing Control system architecture Nonlinear beam propagation (#2) Frequency conversion (#6) Phase II resolves: Beam energy (#1) Stimulated Raman scattering (#3) Scale-up of crystals & bonding (#4) Mass production of diodes (#5) Beam bundling (#7) Higher diode eff., 45  60% Management of higher power Phase I resolves most issues associated with component design and functionality

Cost Breakdown for Phase II: DPPSL Vendor Readiness ($22M): - Contracts ($10), Crystal growth ($6.5), Overhead ($5.3) Design ($12M): - Personnel ($7.2), Overhead ($4.8) Procurement and Construction ($135M): - Personnel ($10) - Diodes (assumed cost $1.2 / Watt, 30 MW) ($39.6) - Crystals ($10) - Laser Hardware ($12.9) - Power Conditioning ($17) - Facilities and Utilities ($22.9) - Overhead ($22.3) Activation ($22M): - Personnel ($8.1), Crystals ($4.8), Procurements ($1.2), Overhead ($7.6) Integrated experiments ($36M): - Personnel ($12.0), Crystals ($3.6), Procurements ($1.8), Overhead ($18.6) $277M Personnel and Laser Hardware ($168M + $50M contingency) - LLNL Overhead ($59M; Assumes 30% reduction in tax base) Vendor readiness $22M Construct & Procure $135M Laser Design $12M Laser Activation $22M Integrated experiments Laser:$36M; Chamber:$10M Timeline for DPSSL- IRE (6 kJ) development and operation (rough estimate) Construct & Procure $6M Chamber Design $0.5M Chamber Activation $9.5M

Rep-rated high-energy solid-state laser initiatives have sprung up around the world, which is likely to accelerate progress