TEXT SIZE

Photons & Fusion

February 2011

Photons & Fusion is a monthly review of science and technology at the National Ignition Facility & Photon Science Directorate. For more information , submit a question.

NIF Reaches Ignition-Level Radiation Temperatures


The successful results of recent hohlraum radiation experiments on the National Ignition Facility were reported in two papers published in the Feb. 25 issue of Physical Review Letters. The papers also were highlighted in a synopsis, "Big Science in a Small Space," on the American Physical Society's "Physics" Website. "In experiments that simulate 'real' conditions more closely than any previous attempt," the synopsis noted, "the (NIF) team shows they are able to successfully generate the...levels of heat needed for laser-driven fusion."

Hohlraum SchematicSchematic of a hohlraum heated by 192 laser beams along with various optical and x-ray diagnostics. Gated x-ray images from implosions driven with megajoule laser energy show symmetric 9 keV capsule x-ray emission at peak compression, 1 to 1.5 nanoseconds after the end of the laser pulse. The hohlraum radiation temperature is measured through the laser entrance hole with the Dante detector, a broad-band, time-resolved x-ray spectrometer, while laser energy and power backscattered from the target is measured with temporally and spectrally resolved backscatter diagnostics using full-aperture backscatter (FABS) and near-backscatter imager (NBI) detectors on two quads of beams on the 30° and 50° cones.

The first paper, "Demonstration of Ignition Radiation Temperatures in Indirect-Drive Inertial Confinement Fusion Hohlraums" (DOI: 10.1103/PhysRevLett.106.085004), described experiments on NIF that achieved the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions.

In the experiments, cryogenically cooled gas-filled hohlraums containing cryogenic fuel capsules filled with helium or a mixture of helium and deuterium were irradiated with up to 1.2 megajoules of ultraviolet laser light delivered by NIF's 192 laser beams. Laser backscatter measurements showed that the hohlraums absorbed 87 to 91 percent of the incident laser power, resulting in peak radiation temperatures of 300 electron-volts (eV) and a symmetric implosion of the fuel capsule to a 100-micron-diameter hot core.

"The experimental data and modeling show a radiation drive of 300 eV in an ignition-scale hohlraum with a 2.2-mm-diameter capsule that will allow implosions with sufficient mass for ignition," the researchers reported. The paper's lead author, NIF & Photon Science plasma physicist Siegfried Glenzer, was joined by National Ignition Campaign (NIC) colleagues from LLNL, Los Alamos National Laboratory (LANL), UCLA, and General Atomics.

The second paper, "Observation of High Soft X-Ray Drive in Large-Scale Hohlraums at the National Ignition Facility" (DOI: 10.1103/PhysRevLett.106.085003), discussed the first soft X-ray radiation flux measurements from ignition-scale hohlraums using both 96- and 192-beam configurations at NIF. In the experiments, gold vacuum hohlraums, 6.4 mm long and 3.55 mm in diameter, were heated with laser energies between 150 and 635 kilojoules, reaching radiation temperatures of up to 340 eV. The experiments demonstrated x-ray radiation fluxes 20 to 30 percent higher than had been anticipated using conservative models benchmarked by smaller hohlraum targets on LLNL's Nova laser and the OMEGA laser at the University of Rochester.

The researchers found that simulations using less conservative atomic physics and electron heat transport models agree with the measurements and indicate a hohlraum x-ray conversion efficiency of about 85 to 90 percent. "The experimental results presented here show that vacuum hohlraums continue to 'work' at energy levels some 10–20 times greater than previously used (while keeping the energy density approximately fixed)," the researchers concluded. Lead author John Kline of LANL was joined on the paper by colleagues from LLNL, Sandia National Laboratories, the UK Atomic Weapons Establishment, and General Atomics.

Developing New Applications for Fiber Lasers

Fiber lasers, a class of lasers in which optical fibers doped with rare-earth minerals are used as the gain medium, are robust, easy to use, and reliable. Optical fibers are always in alignment, and they excel at generating high average power with excellent beam quality.

First developed about 25 years ago for use in the telecommunications industry, fiber lasers are now used widely in industry for applications such as welding and machining, and work by a team of eight scientists in the NIF & Photon Science Directorate is helping to extend their usefulness and credibility in research settings and in other applications requiring high-average-power pulsed lasers.

Diagram of Ribbon FiberCross section of ribbon-shaped fiber under development at LLNL.

For a Laboratory Directed Research and Development (LDRD) Strategic Initiative, the fiber laser group, led by Jay Dawson, is working to solve the challenge of average power scaling. Fiber laser R&D has led to gradual upgrades in system power, from the two-milliwatt output of the original fiber lasers to the ten kilowatts that single mode fiber lasers are now capable of producing. Conventional fibers, however, are approaching a fundamental limit in power. Nonlinear and thermal effects prevent further power scaling by simply increasing the diode pump power.

The LDRD project aims to scale fiber lasers by changing the shape of the waveguide from a circularly symmetric fiber to a rectangular, or ribbon-shaped, fiber (see graphic). Changing to a rectangular geometry significantly improves scalability, making it possible to propagate hundreds of kilowatts of average power without optical damage.Fiber Draw TowerThe fiber draw tower now under construction for the NIF & Photon Science directorate. The shape and size of the ribbon fiber was established through a combination of computer simulation and analytical modeling; the group is now working to demonstrate the physics of the ribbon fiber at various powers.

Construction has also begun on a fiber draw tower, creating an in-house capability at Lawrence Livermore Lab to produce photonic crystal fibers. The 8.2-meter tower will be capable of drawing fibers from 80 to 500 micrometers in diameter starting this year. In conjunction with this work, the fiber laser group and the U.S. Air Force Academy developed a code that can model photonic crystal fiber structures on LLNL supercomputers. Depending on the complexity of the design, the tower and associated modeling capability will allow scientists to make new waveguide designs within just a week or two and will create opportunities for innovation.

Another thrust of fiber laser work in NIF & PS is an 18-month, $3.2-million-dollar laser beam combining project for the U.S. Department of Defense's Defense Advanced Research Projects Agency (DARPA). The goal is to create a high-energy, high-average-power laser through beam combination, while meeting DARPA's stringent efficiency, size, and weight requirements.

A Raman crystal efficiently combines multiple modest quality, high-energy lasers into a single high-fidelity, high-energy pulse. The fiber group has developed a set of Raman amplifier modeling tools, designed an optical assembly to combine pump and signal lasers in the Raman crystals, and narrowed the list of candidate materials for the Raman crystal. Ultimately, the system will need to fit in a 10-kilogram, roughly 50-square-centimeter box so that it can be installed on an unmanned aerial vehicle; modeling indicates that this is achievable with the proposed design.




Japanese Symposium Examines HED Research on NIF

NIF & Photon Science researchers and their colleagues described NIF's potential for conducting high energy density (HED) experiments at a symposium on February 14 and 15 in Tokyo sponsored by the Science Council of Japan.

Science Council of Japan Logo

Titled "High Energy Density Science with Large-Scale High-Power Lasers," the symposium featured talks by NIF Director Ed Moses, LLNL Associate Director of Physics and Advanced Technologies Bill Goldstein, NIF User Office Director Chris Keane, materials science researcher Gilbert (Rip) Collins, and NIF collaborator Ray Jeanloz of the University of California, Berkeley. Acting Principal Associate Director for Science and Technology Tomás Díaz de la Rubia, David Crandall of the Office of the Under Secretary for Science at the U.S. Department of Energy, Prof. Robert Rosner of the University of Chicago, and a number of Japanese researchers and government officials also participated.

Improving Laser Damage Resistance in NIF Optics

Growing laser damage sites on multilayer high-reflector coatings can limit the performance and lifetimes of mirrors in high-peak-power laser systems such as NIF. One strategy to improve laser damage resistance is to replace growing damage sites with predesigned benign mitigation structures. These structures can be created by various techniques including femtosecond laser machining, single-crystal high-speed diamond machining, and magnetorheological finishing. By mitigating the weakest site on the optic, large-aperture mirrors will have a laser resistance comparable to the intrinsic value of their multilayer coating. An optimal mitigation structure can routinely yield a laser damage threshold that is higher than the operational fluence.

Schematic of 3D SimulationSchematics of the 3D simulation domain showing a multilayer coating with a conical pit with a 15° cone angle on a BK7 glass substrate. Hafnia layers are represented by the green color, while the blue colors represent silica layers, the glass substrate, and the cap layer.

LLNL researchers reported on their efforts to examine conical pits as potential mitigation geometries in the February 7 issue of Applied Optics (doi:10.1364/AO.50.00C373). The impact of polarization on multilayer laser damage resistance was also investigated. Based on both simulations (see graphic) and experiments, the researchers found that the field intensification induced by the mitigation pit is strongly dependent on the polarization and the angle of incidence (AOI) of the incoming wave. Their findings suggest a global mitigation strategy over the multilayer coating by fabricating conical pits with the cone angle matching with the AOI of the incoming wave. A more application-specific approach to determine the optimal conical angle could also be used.

Contributing to the paper were LLNL's S. Roger Qiu, Justin E. Wolfe, Michael D. Feit, and Christopher J. Stolz. They were joined by colleagues from the University of California, Berkeley, and Panoramic Technology, Inc.

Diamond X-ray Detector Deployed on NIF

A new instrument for measuring the x-ray bang-time in NIF inertial confinement fusion capsules was described in an article by NIF & PS researchers in the February 15 issue of the Journal of Instrumentation (JINST 6 P02009; doi: 10.1088/1748-0221/6/02/P02009). The five-channel instrument now being deployed at NIF consists of chemical vapor deposited polycrystalline diamond photoconductive x-ray detectors with highly ordered pyrolytic graphite x-ray monochromator crystals at the input. Capsule bang-time can be measured in the presence of relatively high thermal and hard x-ray background components due to the selective bandpass of the crystals combined with direct and indirect x-ray shielding of the detector elements.

Characteristics of the instrument have been measured, and they demonstrate that x-ray bang-time can be measured with ±30 picosecond precision, characterizing the soft x-ray drive to +/- 1 electron volts or 1.5 percent.

Top of Page
Privacy & Legal Notice LLNL-WEB-474613