New Concepts for Reducing Costs and Improving Efficiency of Solid-State Laser Drivers for Inertial Fusion Energy
Author: Alvin C. Erlandson
Requested Type: Consider for Invited
Submitted: 2006-12-18 20:32:27
Co-authors: A.C. Erlandson, E. Ault, C.P.J. Barty, A. Bayramian, R. Beach, J. Caird, R. Campbell, R. Cross, C. Ebbers, T. Ladran, Z. Liao, J. R. Murray, R. Page, K. Schaffers, T. Soules, S. Sutton, and S. Telford
Contact Info:
Lawrence Livermore National Laboratory
PO Box 808
Livermore, CA 94550
USA
Abstract Text:
The development of low-cost, high-efficiency drivers is a key challenge in the development of inertial fusion energy (IFE). At this workshop, we describe new concepts for reducing costs and increasing efficiency of solid-state laser drivers for IFE, which are based on two guiding principles: 1) capital costs can be decreased by increasing duty cycles on laser components, thus making better use of laser hardware, and 2) laser performance can be improved by separating two important functions of the laser – energy-storage and ns-pulse-production – so that each function can be optimized independently. Both principles are embodied in our proposed laser-pumped-laser architecture, in which a “pump” laser that is pumped directly by diodes pumps a second “driver” laser that produces ~ 5-20-ns pulses appropriate for driving IFE targets.
In the laser-pumped-laser architecture, we reduce diode costs by using (in the pump laser) a gain medium that has a long storage lifetime. (Peak power and cost of the diodes vary in inverse proportion to the storage lifetime of the gain medium that the diodes pump.) The pump laser has high stored-energy density and is relatively compact, since gain media with long storage lifetime tend to have high saturation fluence. Stored energy is extracted efficiently by multi-passing the gain medium with a high fluence beam having sufficiently long pulselength (100-200 ns) for damage risk to be at an acceptable level. A harmonic converter inside the multipass cavity efficiently converts output from the pump laser to a wavelength that is absorbed by the driver laser. The driver laser produces ns-long pulses efficiently by using a gain medium that has high gain and low saturation fluence. Perhaps most significantly, we reduce costs by using pulse stacking in which each laser beamline produces a train of pulses, with several tens of ns between pulses, that follow different paths but arrive at the target simultaneously. We will present model predictions for a laser system that uses this new architecture, a harmonically-doubled Yb3+ laser pumping a Ti-doped sapphire laser.
We will also report recent progress made in the development and testing of diode-pumped-solid-state-laser technology on the Mercury Laser at LLNL. A diode-pumped Yb:S-FAP, gas-cooled-slab laser, Mercury has shown that diode-pumped solid-state lasers of appropriate design are inherently scalable to sizes needed for IFE. Recently, Mercury produced an average power of 617 W (61.7 J / 10 Hz) while maintaining five-times-diffraction-limited beam quality. Additionally, laser output was frequency doubled with 73% energy efficiency. A novel high-average-power Pockels-cell switch was successfully tested.
This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
Characterization: B2
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