Phase II Considerations

1
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 94550
HAPL Review February 6, 2004 Atlanta, GA
2
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

3
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

4
Solid state laser driver requirements
for Inertial Confinement Fusion
NIF / IFE are same
Enhancements needed
5
Comparison of NIF and Mercury amplifiers

• 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

Mirror
Telescope
Amplifiers
Flashlamps
Reflectors
Diodes
Gas cooled
6
Efficiency comparison NIF and Mercury-like
architectures (estimates)
Convection
Ndglass
Radiative
Radiative
Frequency
cooling
cooling
conversion
Reflector
Yb
S
-
FAP
Yb
S
-
FAP
Turbulent cooling
Frequency conversion
Mercury
Higher efficiency of DPSSL is achieved through
many enhancements
7
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 FSAT hn / sG
2 MJ laser and 5/W diodes Cdiode (B) 0.5 /
tST (ms)
Peak fluence FPEAK 4.5 FSAT Bandwidth for
smoothing DnG
Beam Energy Balances amplified spontaneous
emssion (ASE) and nonlinear ripple growth
laser pulse width
Saturation fluence
Ebeam (hEXT / 12 FSAT) (3 l tP / 4g)2
nonlinear index
extraction efficiency
8
YbS-FAP laser material offers advantages over
Ndglass for IFE
Comparison of Ndglass and YbS-FAP gain media in
fusion lasers
Longer lifetime reduces cost
Lower fluence reduces damage
Beam energies are similar
Bandwidth is adequate

• YbS-FAP has 2.5x greater thermal conductivity
  than Ndglass
• ? better for rep-rated operation
• However, crystals are more difficult to produce
  in large size

9
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

10
ST issue 1 Models indicate that
multi-kilojoule output is feasible from a single
coherent aperture

• 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 3w-conversion of
  75 to be included
• Reduced losses and higher diode efficiency
  possible

11
ST issue 2 Mercury closely-spaced slab
architecture has reduced nonlinear beam breakup
relative to widely-spaced (NIF-like)
architecture
Focal spots
Widely-spaced slabs have more intensity on pinhole
B 3.8 radians
Mercury Closely-spaced slabs
B 3.8 radians

• Optical damage risk is mitigated in Mercury
  architecture two ways
• Closely-spaced-slab architecture reduces
  nonlinear ripple growth
• Lower saturation fluence of YbS-FAP vs.
  Ndglass reduces average fluence

12
ST issue 3 Stimulated Raman Scattering (SRS)
in S-FAP, or unwanted nonlinear frequency
conversion, must be controlled in the IRE
Gain lowers with angle between laser and SRS
SRS is predicted for the IRE based on gain
TmYAG absorber suppresses SRS
Quantitative modeling yields - Aperture
limit is gt20x30 cm2 at 3 GW/cm2 - Longitudinal
SRS is controlled by - inserting TmYAG
absorber in amps - adding a small wedge
to the slabs
13
ST issue 4 Combination of bonding and large
diameter growth provides pathway to 20x30 cm2
YbS-FAP slabs
3.5 cm boules (standard)
6.5 cm boules (last year)
10 cm boules needed for IRE
Bonding choices
Schott - glue bonding
Onyx - high temperature
Approximately 10 cm boules will be needed to bond
three parts together for each 20x30 cm2 slab
14
ST issue 5 Learning curve analysis suggests
that diode bar prices will drop as the market
grows
Low duty cycle diode bars
- High production rate ? reduced cost - Higher
efficiency diodes are desired
Heatsinks
Diode laser bars
Backplanes
15
ST issue 6 Average power frequency conversion
with gt80 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

16
ST 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
Management of high average power likely to be
very challenging
Clusters of bundles
17
Phase I resolves most issues associated with
component design and functionality

• 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
• YbS-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)

18
Cost Breakdown for Phase II DPPSL
Timeline for DPSSL- IRE (6 kJ) development and
operation (rough estimate)
Construct Procure 135M
Laser Design 12M
Laser Activation 22M
Vendor readiness 22M
Integrated experiments Laser36M Chamber10M
Construct Procure 6M
Chamber Design 0.5M
Chamber Activation 9.5M
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)
19
Rep-rated high-energy solid-state laser
initiatives have sprung up around the world,
which is likely to accelerate progress