Scott N. Fochs

Interaction of a high-power laser beam with metal sheets

Experiments with a high-power laser beam directed onto thin aluminum sheets, with a large spot size, demonstrate that airflow produces a strong enhancement of the interaction. The enhancement is explained in terms of aerodynamic effects. As laser heating softens the material, the airflow-induced pressure difference between front and rear faces causes the metal to bulge into the beam. The resulting shear stresses rupture the material and remove it at temperatures well below the melting point. The material heating is shown to conform to an elementary model. We present an analytic model of elastic bulging. Scaling with respect to spot size, wind speed, and material parameters is determined.

Technical challenges for the future of high energy lasers

The Solid-State, Heat-Capacity Laser (SSHCL) program at Lawrence Livermore National Laboratory is a multi-generation laser development effort scalable to the megawatt power levels with current performance approaching 100 kilowatts. This program is one of many designed to harness the power of lasers for use as directed energy weapons. There are many hurdles common to all of these programs that must be overcome to make the technology viable. There will be a in-depth discussion of the general issues facing state-of-the-art high energy lasers and paths to their resolution. Despite the relative simplicity of the SSHCL design, many challenges have been uncovered in the implementation of this particular system. An overview of these and their resolution are discussed. The overall system design of the SSHCL, technological strengths and weaknesses, and most recent experimental results will be presented.

Intracavity adaptive correction of a 10-kW solid state heat-capacity laser

The Solid-State, Heat-Capacity Laser (SSHCL), under development at Lawrence Livermore National Laboratory (LLNL) is a large aperture (100 cm2), confocal, unstable resonator requiring near-diffraction-limited beam quality. There are two primary sources of the aberrations in the system: residual, static aberrations from the fabrication of the optical components and predictable, time-dependent, thermally-induced index gradients within the gain medium. A deformable mirror placed within the cavity is used to correct the aberrations that are sensed with a Shack-Hartmann wavefront sensor. Although it is more challenging than external correction, intracavity correction enables control of the mode growth within the resonator, resulting in the ability to correct a more aberrated system longer. The overall system design, measurement techniques and correction algorithms are discussed. Experimental results from initial correction of the static aberrations and dynamic correction of the time-dependent aberrations are presented.

The Use of Large Transparent Ceramics in a High Powered, Diode Pumped Solid-State Laser

The advent of large transparent ceramics is one of the key enabling technological advances that have shown that the development of very high average power compact solid-state lasers is achievable.

Plasma electrode pockels cell for the National Ignition Facility

The NIF, now under construction at LLNL, will be the largest laser fusion facility ever built. The NIF laser architecture is based on a multi-pass power amplifier to reduce cost and maximize performance. A key component in this laser design is an optical switch that closes to trap the optical pulse in the cavity for four gain passes and then opens to divert the optical pulse out of the amplifier cavity. The switch is comprised of a Pockels cell and a polarizer and is unique because it handles a beam that is 40 cm X 40 cm square and allows close horizontal and vertical beam spacing. Conventional Pockels cells do not scale to such large apertures or the square shape required for close packing. Our switch is based on a Plasma-Electrode Pockels Cell (PEPC). In a PEPC, low-pressure helium discharges are formed on both sides of a thin slab of electro-optic material. Typically, we use KH2PO4 crystals (KDP). The discharges form highly conductive, transparent sheets that allow uniform application of a high-voltage pulse across the crystal. A 37 cm X 37 cm PEPC has been in routine operation for two years on the 6 kJ Beamlet laser at LLNL. For the NIF, a module four apertures high by one wide is required. However, this 4 X 1 mechanical module will be comprised electrically of a pair of 2 X 1 sub-modules. Last year, we demonstrated full operation of a prototype 2 X 1 PEPC. In this PEPC, the plasma spans two KDP crystals. A major advance in the 2 X 1 PEPC over the Beamlet PEPC is the use of anodized aluminum construction that still provides sufficient insulation to allow formation of the planar plasmas. In this paper, we discuss full 4 X 1 NIF prototypes.

B-field interactions and electrode optimization in the plasma electrode Pockels cell

We use a 32 cm plasma electrode Pockels cell (PEPC) prototype at Lawrence Livermore National Laboratory to determine switching performance in the presence of external magnetic fields. The interaction with external magnetic fields is important because of the B-fields generated by the high current flow through amplifier flashlamp arrays, and their proximity to the PEPC. We have experimentally determined what is the maximum allowable magnetic induction for good PEPC operation, and then we calculate the magnetic induction generated by a flashlamp array to determine the minimum PEPC to amplifier spacing. We have also experimentally determined the effect of a tandem PEPC placement. We consider several cathode designs. We revisit the hollow cathode design and we investigate the tradeoffs between the hollow cathode and planar magnetron. The recent development of a metallic body for the future 1 X 2 PEPC has led us to do experiments with a biased boundary in the PEPC. Experimental results of various biasing potentials and dielectric coating materials for the PEPC body are discussed.

The design and operation of a 10 kW solid-state heat-capacity laser

Summary form only given. We have developed and demonstrated a new operational mode for solid-state laser systems in which the cooling of the gain medium is separated in time from the lasing cycle. In heat-capacity operation, no cooling takes place during lasing. The gain medium is pumped very uniformly and the waste heat from the excitation process is stored in the solid-state gain medium. By depositing the heat on time scales that are short compared to thermal diffusion across the optical aperture, very high average power operation is possible while maintaining low optical distortions. After a lasing cycle, aggressive cooling can then take place in the absence of lasing, limited only by the fracture limit of the solid-state medium. This mode of operation is ideally suited for applications that require 1-30 s engagements at very high average power. In order to demonstrate the operation of a high average power heat-capacity laser system, we have developed a flashlamp-pumped Nd:glass laser with output energies in the range of 500-1000 J/pulse in a 10/spl times/10 cm/sup 2/ beam.

Plasma Pockels cell based optical switch for the National Ignition Facility

Summary form only given. The National Ignition Facility (NIF), now under construction at Lawrence Livermore National Laboratory, is based on a multi-pass power amplifier. A key component in this laser design is an optical switch that closes to trap the optical pulse in the cavity for four gain passes and then opens to divert the optical pulse out of the amplifier cavity. The switch is comprised of a Pockels cell and a polarizer and is unique because it handles a beam that is 40 cm/spl times/40 cm square and allows close beam packing in four 4/spl times/12 clusters for a total of 192 beams. Conventional Pockels cells do not scale to such large apertures or the square shape required for close packing. Our switch is based on a plasma-electrode Pockels cell (PEPC). In this paper, we present here many interesting design details and experimental results which include observation of plasma and discharge effects which can degrade plasma uniformity including MHD plasma displacement from external return currents, current channel formation, and the effect of housing bias potential.

Plasma uniformity issues in a 2/spl times/1 plasma-electrode Pockels cell

Summary form only given. An optical switch based on large-aperture plasma-electrode Pockels cells (PEPC) is an important part of the National Ignition Facility (NIF) laser design. To achieve uniform electro-optic switching across the entire PEPC aperture, it is important that the plasma density be sufficiently high and sufficiently uniform. We have observed a number of plasma effects which can degrade plasma uniformity. These include magnetic displacement of the plasma by external return currents and nearby sources of magnetic interference, current channel formation due to plasma self-fields, anode and cathode electrode design, and the potential of the insulated metal housing that surrounds the plasma. We have studied these effects both analytically and experimentally in a 32 cm/spl times/32 cm plastic housing PEPC and more recently in an 80 cm/spl times/40 cm, two-aperture, aluminum-housing PEPC.