Compact, Efficient Laser Systems Required for Laser Inertial Fusion Energy

Effect of deuterium content on the structural, optical, and thermal properties of DKDP crystals: a systematic analysis

Deuterated potassium dihydrogen phosphate (DKDP) crystals with different deuterium contents have a wide range of applications, such as frequency conversion in high power lasers, electro-optic modulation, and Q-switching crystals for Pockels cells. However, there is a lack of systematic research on the effect of deuterium content on the fundamental structure and properties of these DKDP crystals. To this end, in this study, a series of DKDP crystals with different deuterium contents have been grown using the "point-seed" rapid growth method, and the structure and properties of the crystals have been characterized. The results indicate that as the deuterium content increases, the cell parameter along the a(b)-axis direction gradually increases, and the transmittance gradually increases in the infrared range. A small amount of doping (low H or D ratio) reduces the structural integrity of the crystal, and the crystals at intermediate deuterium concentrations have better crystallinity. The thermal properties of the crystals do no change significantly with the variation in the deuterium content. Overall, these findings can serve as a useful reference for boosting the application of DKDP crystals with various deuterium contents.

Entirely reflective slit spatial filter for high-energy laser systems

An entirely reflective slit spatial filter is proposed to provide spatial filtering, gain isolation, and ASE mitigation for high-energy laser systems. The traditional circular pinhole is replaced by two orthogonal slits, which lowers the intensity at the spatial filter plane by up to two orders of magnitude, and by using reflective optics we reduce spatial dispersion and eliminate B-integral effects. A ray trace model of the spatial filter shows excellent transmitted wavefront, but also indicates aberrations at the foci from using cylindrical optics at 45°. It is expected that the use of off-axis parabolic mirrors would mitigate this issue but comes at the cost of more complicated, expensive optics and more complex alignment. We created a numerical model based on Fourier optics to explain this effect and guide design requirements to mitigate it. High-quality imaging and filtering capabilities are demonstrated experimentally.

Bidirectional ring amplifier with twin pulses for high-power lasers

A novel bidirectional ring amplifier with twin pulses for high-power lasers is proposed, and the performances on output energy capability and extraction efficiency are comprehended with detailed simulation. The results show that an extraction efficiency of 62.3% and the output energy of 13.4 kJ per pulse at the B integral limit can be obtained at low average fluence of 10.3 J/cm² and the low injection energy of 3.9 mJ in the bidirectional ring amplifier. Compared with the multi-pass amplifier, the bidirectional ring amplifier is more compact and the extraction efficiency is much higher at low injection energy and low laser fluence operation, which is beneficial to simplify the preamplifier system and reduce the effects of nonlinear phase shift.

1-J operation of monolithic composite ceramics with Yb:YAG thin layers: multi-TRAM at 10-Hz repetition rate and prospects for 100-Hz operation

Experimental amplification of 10-ns pulses to energy of 1 J at repetition rate of 10-100 Hz in cryogenic multipass total-reflection active-mirror (TRAM) amplifier is reported for the first time. By using a monolithic multi-TRAM, which is a YAG ceramic composite with three thin Yb:YAG active layers, efficient energy extraction was achieved without parasitic lasing. A detailed measurement of output characteristics of the laser amplifier is presented; results are discussed and compared with numerical calculations.

Gigashot optical degradation in silica optics at 351 nm

As applications of lasers demand higher average powers, higher repetition rates, and longer operation times, optics will need to perform well under unprecedented conditions. We investigate the optical degradation of fused silica surfaces at 351 nm for up to 10(9) pulses with pulse fluences up to 12 J/cm(2). The central result is that the transmission loss from defect generation is a function of the pulse intensity, I(p), and total integrated fluence, φ(T), and is influenced by oxygen partial pressure. In 10(-6) Torr vacuum, at low I(p), a transmission loss is observed that increases monotonically as a function of number of pulses. As the pulse intensity increases above 13 MW/cm(2), the observed transmission losses decrease, and are not measureable for 130 MW/cm(2). A physical model which supports the experimental data is presented to describe the suppression of transmission loss at high pulse intensity. Similar phenomena are observed in anti-reflective sol-gel coated optics. Absorption, not scattering, is the primary mechanism leading to transmission loss. In 2.5 Torr air, no transmission loss was detected under any pulse intensity used. We find that the absorption layer that leads to transmission loss is less than 1 nm in thickness, and results from a laser-activated chemical process involving photo-reduction of silica within a few monolayers of the surface. The competition between photo-reduction and photo-oxidation explains the measured data: transmission loss is reduced when either the light intensity or the O(2) concentration is high. We expect processes similar to these to occur in other optical materials for high average power applications.

Design and optimization of an adaptive optics system for a high-average-power multi-slab laser (HiLASE)

We report numerical and experimental results obtained with an optical setup that simulates the heating and cooling processes expected in a multi-slab high-average-power laser head. We have tested the performance of an adaptive optics system consisting of a photo-controlled deformable mirror (PCDM) and a Shack-Hartmann wavefront sensor for the effective correction of the generated wavefront aberrations. The performance of the adaptive optics system is characterized for different layouts of the actuator array and for different configurations of the heating mechanisms. The numerical results are benchmarked using a PCDM, which allowed us to experimentally compare the performances of different deformable mirrors.

LIFE: a sustainable solution for developing safe, clean fusion power

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) in California is currently in operation with the goal to demonstrate fusion energy gain for the first time in the laboratory-also referred to as "ignition." Based on these demonstration experiments, the Laser Inertial Fusion Energy (LIFE) power plant is being designed at LLNL in partnership with other institutions with the goal to deliver baseload electricity from safe, secure, sustainable fusion power in a time scale that is consistent with the energy market needs. For this purpose, the LIFE design takes advantage of recent advances in diode-pumped, solid-state laser technology and adopts the paradigm of Line Replaceable Units used on the NIF to provide high levels of availability and maintainability and mitigate the need for advanced materials development. The LIFE market entry plant will demonstrate the feasibility of a closed fusion fuel cycle, including tritium breeding, extraction, processing, refueling, accountability, and safety, in a steady-state power-producing device. While many fusion plant designs require large quantities of tritium for startup and operations, a range of design choices made for the LIFE fuel cycle act to reduce the in-process tritium inventory. This paper presents an overview of the delivery plan and the preconceptual design of the LIFE facility with emphasis on the key safety design principles being adopted. In order to illustrate the favorable safety characteristics of the LIFE design, some initial accident analysis results are presented that indicate potential for a more attractive licensing regime than that of current fission reactors.