E. Utterback

The mercury project : A high average power, gas-cooled laser for inertial fusion energy development

Abstract Hundred-joule, kilowatt-class lasers based on diode-pumped solid-state technologies, are being developed worldwide for laser-plasma interactions and as prototypes for fusion energy drivers. The goal of the Mercury Laser Project is to develop key technologies within an architectural framework that demonstrates basic building blocks for scaling to larger multi-kilojoule systems for inertial fusion energy (IFE) applications. Mercury has requirements that include: scalability to IFE beamlines, 10 Hz repetition rate, high efficiency, and 109 shot reliability. The Mercury laser has operated continuously for several hours at 55 J and 10 Hz with fourteen 4 × 6 cm2 ytterbium doped strontium fluoroapatite amplifier slabs pumped by eight 100 kW diode arrays. A portion of the output 1047 nm was converted to 523 nm at 160 W average power with 73 % conversion efficiency using yttrium calcium oxy-borate (YCOB).

Silicon monolithic microchannel-cooled laser diode array

A monolithic microchannel-cooled laser diode array is demonstrated that allows multiple diode-bar mounting with negligible thermal cross talk. The heat sink comprises two main components: a wet-etched Si layer that is anodically bonded to a machined glass block. The continuous wave (cw) thermal resistance of the 10 bar diode array is 0.032 °C/W, which matches the performance of discrete microchannel-cooled arrays. Up to 1.5 kW/cm2 is achieved cw at an emission wavelength of ∼808 nm. Collimation of a diode array using a monolithic lens frame produced a 7.5 mrad divergence angle by a single active alignment. This diode array offers high average power/brightness in a simple, rugged, scalable architecture that is suitable for large two-dimensional areas.

Full System Operations of Mercury: A Diode Pumped Solid-State Laser

Abstract Operation of the Mercury laser with two amplifiers has yielded 30 Joules at 1 Hz and 12 Joules at 10 Hz with over 8x104 shots on the system. Static distortions in the Yb:S-FAP amplifiers were corrected by a magneto-rheological finishing technique.

Initial operation of the mercury laser: a gas-cooled 10-Hz Yb:S-FAP system

We report initial operation of the Mercury laser with seven 4 x 6 cm S-FAP amplifier slabs pumped by four 80 kW diode arrays. The system produced up to 33.5 J single shot, 23.5 J at 5 Hz, and 10 J at 10 Hz for 20 minute runs at 1047 nm. During the initial campaign, more than 2.8 x 104 shots were accumulated on the system. The beam quality of the system was measured to be 2.8 x 6.3 times diffraction limited at 110 W of output, with 96% of the energy in a 5X diffraction limited spot. Static wavefront glass plates were used to correct for the low order distortions in the slabs due to fabrication and thermal loading. Scaling of crystal grown has begun with the first full size slab produced from large diameter growth. Using an energetics optimization code we find the beam aperture is scalable up to 20 x 30 cm and 4.2 kJ.

Full System Operations of Mercury: A Diode-Pumped Solid-State Laser

Laser operations with two amplifiers activated produced 35 Joules at 1 Hz, 12 Joules at 10 Hz, and 8x104 total system shots. Static distortions in the Yb:S-FAP amplifiers were corrected by magneto rheological finishing technique.

Modeling the effect of heatsink performance on high-peak-power laser-diode-bar pump sources for solid state lasers

We derive approximate expressions for transient output power and wavelength chirp of high-peak-power laser-diode bars assuming one-dimensional heat flow and linear temperature dependences for chirp and efficiency. The model is derived for pulse durations, 10 less than (tau) less than 1000 microseconds, typically used for diode-pumped solid-state lasers and is in good agreement with experimental data for Si heatsink mounted 940 nm laser-diode bars operating at 100 W/cm. The analytic expressions are more flexible and easily used than the results of operating point dependent numerical modeling. In addition, the analytic expressions used here can be integrated to describe the energy per unit wavelength for a given pulse duration, initial emission bandwidth and heatsink material. We find that the figure-of-merit for a heatsink material in this application is ((rho) CpK)1/2 where (rho) Cp is the volumetric heat capacity and K is the thermal conductivity. As an example of the utility of the derived expressions, we determine an effective absorption coefficient as a function of pump pulse duration for a diode-pumped solid-state laser utilizing Yb:Sr5(PO4)3F (Yb:S-FAP) as the gain medium.

High energy hybrid fiber - Yb:SFAP laser with dynamic spectral and temporal pulse shaping

We have commissioned a turnkey 500 mJ, 10 Hz front end laser. The system delivers temporally and spectrally tailored pulses to correct signal distortions within itself or subsequent amplifiers from single longitudinal mode to 250 GHz RF bandwidth.

The Mercury Project:A kW Scale Yb:S-FAP Laser for Inertial Fusion Energyand Target Experiments

The laser is nearing completion with demonstration of 73% frequency-conversion efficiency, deformable mirror operation that generated a 4-times diffraction limited spot, and commissioning of an advanced front end to be installed on the main laser.

Activation of a Spatial, Temporal, and Spectrally Sculpted Front End for the Mercury Laser

We have produced over 500 mJ using a hybrid fiber-based master-oscillator system coupled with a Yb:S-FAP power amplifier. This system is designed with spatial, temporal, and spectral sculpting enabling broadband amplification correctable for gain narrowing.

System operations of mercury; a diode-pumped solid-state laser

The Mercury laser project is part of a national inertial fusion energy program in which four driver technologies are being considered including solid-state lasers, krypton fluoride gas lasers, Z-Pinch and heavy ions. Mercurys operational goals of 100 J, 10 Hz, 10% efficiency in a 5 times diffraction limited spot will demonstrate the critical technologies required for scaling the system to the multi-kilojoule level. Five one hour runs were conducted to assess system stability and reliability; energy fluctuations during the 55 J operations showed a 0.6% rms deviation. Current beam quality during average power operation is approximately 10 times diffraction limited. In the future, active wavefront control, and corrector plates for steady state thermal distortions will be implemented to achieve the 5 times diffraction limited spot.