Photons & Fusion

March 2012

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

Interim Report Urges the Effective Use of NIF in IFE Planning

In an interim report published on March 7, the National Academies Committee on Prospects for Inertial Confinement Fusion Energy Systems has called for the establishment of a scientific-community-based roadmap for the development of inertial fusion energy (IFE) sources, with NIF as a major component.

In its only specific recommendation, the report said "planning should begin for making effective use of NIF as one of the major program elements in an assessment of the feasibility of inertial fusion energy." The committee suggested that many experiments could be performed on NIF to provide "experimental validation of predictive capabilities."

The report notes that scientific and technological progress in inertial confinement fusion (ICF) "has been substantial during the past decade, particularly in areas pertaining to the achievement and understanding of high-energy-density conditions in the compressed fuel, in numerical simulations of inertial confinement fusion processes, and in exploring several of the critical technologies required for inertial fusion energy applications (e.g., high-repetition-rate lasers and heavy-ion-beam systems, pulsed-power systems, and cryogenic target fabrication techniques)."

While acknowledging that "critical scientific and engineering challenges" remain to establishing the technical basis for an inertial fusion energy demonstration plant, the report said "the eventual achievement of ignition on the NIF, and particularly the achievement of moderate single-shot gain (10-20, say), would provide significant validation of key scientific underpinnings required for developing inertial fusion as a practical energy source."

Sponsored by the U.S. Department of Energy, the committee is tasked with assessing the prospects for generating power using IFE and advising DOE on how best to proceed with fusion power R&D with the goal of creating an IFE demonstration plant design. The committee's final report is expected to be published this summer. Click here to view the interim report.

Radiochemical Gas Analysis System Commissioned

The Radiochemical Analysis of Gaseous Samples (RAGS) system, which is designed to collect and analyze diagnostic gases produced by NIF ignition experiments, has been commissioned and has evaluated its first samples. Radiochemical diagnostic techniques, such as analysis of solid- and gas-phase capsule debris, will help characterize NIF ignition experiments.

Don Jedlovec adjusts the Radiochemical Analysis of Gaseous Samples (RAGS) system in Bldg. 581.

Developed for NIF by Sandia National Laboratories, RAGS is a cryogenic system that traps and removes samples of the Target Chamber gas following a shot. It consists of a two-part system. The first filters the gas to remove water vapor, particulates, and reactive gases, and the second collects and cryogenically fractionates (separates) the remaining noble gases on the basis of their volatility.

Glass microballoon filled with a deuterium-tritium mixture and doped with xenon (¹²⁴Xe) as a tracer for RAGS. The interaction of ¹²⁴Xe with neutrons produced by the DT fusion reaction created ¹²³Xe and ¹²⁵Xe activation products that were detected by the RAGS system.

As part of the final commissioning test, a small but known amount of a radioactive isotope of xenon (¹³⁵Xe) was injected into the Target Chamber. A sample of the Target Chamber gas was subsequently collected and transported to the Nuclear Counting Facility in Bldg. 151, where gamma-ray spectroscopy was used to count ¹³⁴Xe in the sample to determine the system's collection efficiency. The collection efficiency was determined to be 65 percent, exceeding the diagnostic's performance requirement.

On February 28, a diagnostics calibration experiment was conducted as part of the performance qualification and calibration of RAGS. The target was a 2.1-millimeter-diameter spherical glass shell filled with a 50/50 mixture of deuterium and tritium and a small amount of stable ¹²⁴Xe. RAGS successfully measured ¹²³Xe and ¹²⁵Xe – the activation products of ¹²⁴Xe – using the on-line scintillation detector. This test demonstrated RAGS' gas collection and counting capabilities.

Optics Progress Helps Enable High-Energy NIF Shots

A key enabling factor in this month's historic 1.8-megajoule (MJ) shot on NIF (see March 2012 Project Status) was the progress made by the NIF&PS optics team in improving the damage resistance and refurbishing time of critical optical components. The maximum output energy or power available from high-energy laser systems like NIF is typically limited by laser-induced damage to the optical system that transports the laser beam. Many NIF optics are made of fused silica (noncrystalline silicon dioxide); the finishing processes for fused silica optics can produce microscopic defects, or damage precursors, on the silica surface.

Improved finishing processes over the years, however, have reduced the precursors by orders of magnitude. To improve the damage resistance further, in 2010 the optics team developed the Advanced Mitigation Process (AMP), an optimized aqueous acid-based etching process used to largely remove these surface micro-fractures (usually scratches) and chemical impurities found on polished silica surfaces, increasing the surface damage threshold. NIF operators now can achieve more shots with each fused silica optic before it must be refurbished.

Advanced Mitigation Process Etching Station

The etching station at the Laboratory's optical processing facility is used to treat fused silica optics with the Advanced Mitigation Process. The white-framed, wedged-shaped final focusing lens (WFL) is visible through the glass.

In a thank-you note following a March 13 tour of the NIF optics facilities, LLNL Director Parney Albright told the team, "You are pushing the cutting edge of optical science, developing the required engineering and technology tools, and laying out an effective systems approach to deal with practical aspects of optical life-cycle considerations. NIF simply could not work without these crucial developments and contributions. I look forward to seeing your team continuing to stretch the boundaries of optical science and systems performance."

Controlling Hot-spot Mix in NIF Ignition-scale Implosions

Inertial confinement fusion (ICF) depends on the formation of a central "hot spot" with sufficient temperature and areal density for ignition. Shock-timing, implosion velocity, and implosion symmetry experiments are now under way on NIF with ignition-scale targets to optimize hot-spot formation. Hydrodynamic instabilities are predicted to mix ablator material into the hot spot as it forms, greatly enhancing hot-spot radiative losses.

In an invited Physics of Plasmas paper published online on March 30 (doi.org/10.1063/1.3694057), researchers presented "conclusive experimental evidence" of hot-spot mix occurring in ignition-scale implosions. The paper also quantified the amount of hot-spot mix mass.

Simulations indicate that for ignition to occur, the hot-spot mix mass must be less than 75 nanograms (ng). In the experiments, the amount of hot-spot mix mass was found to be comparable to the 75-ng allowance. Strategies to control hot-spot mix include reducing capsule surface perturbations, adding small amounts of other materials ("dopants") to the ablator, changing the laser pulse shape, and changing to another ablator material, such as copper-doped beryllium.

Lead author Sean Regan of the Laboratory for Laser Energetics at the University of Rochester was joined by collaborators from LLNL; Prism Computational Sciences, of Madison, Wisconsin; General Atomics of San Diego; Los Alamos and Sandia national laboratories; and the University of Nevada.

NIF Petawatt Laser Test Bed Is Operational

The alignment tools and strategy for NIF's Advanced Radiographic Capability (ARC) petawatt (quadrillion-watt) laser are being tested in a newly completed test bed – the ARC full aperture compression test bed (AFACT) – housed in a cleanroom in Bldg. 381.

When complete, ARC will use four beamlines of the NIF laser system to create picosecond-duration laser pulses to produce x-rays for backlighting NIF experiments. AFACT can be used to develop and verify the alignment procedures for the four meter-scale, high-efficiency, multilayer dielectric diffraction gratings used in the ARC compressor. ARC's high peak power is enabled by a process called chirped pulse amplification, in which a short, broadband (multi-wavelength) pulse is first stretched in time to reduce its peak intensity, then amplified at intensities below the damage threshold in the laser amplifiers, and finally compressed to a short pulse and highest peak power in the large compressor.

John Halpin, lead technician for the ARC full aperture compression test bed (AFACT), uses an "angular chirp" diagnostic instrument to determine the parallelism of the AFACT diffraction gratings – the large rectangular devices in the photo that disperse multi-wavelength light into its color components, similar to a prism.

By verifying the grating alignment procedures at full-scale using the identical configuration that will be used in the ARC compressor vessels in the NIF target bay, AFACT will reduce the impact on NIF operations and the experimental program from ARC installation and commissioning.

Using Three Laser Wavelengths to Optimize NIF Hohlraum Energy

NIF experiments have validated simulations showing that using three tunable wavelengths on NIF laser pulses provides a way to control implosion symmetry while at the same time significantly reducing energy-sapping instabilities in NIF shots (see November 2010 Photons & Fusion Newsletter). In a Nature Physics article published online on February 26 (doi:10.1038/nphys2239), LLNL researchers and their collaborators reported that energy transfer between beams using "three-color" laser tuning improves laser-target coupling and provides direct control over capsule compression symmetry.

Cross-beam transfer can occur in a plasma when two or more high-power lasers traveling in different directions overlap, redirecting the incoming laser light from one beam to another. Recent experiments in NIF hohlraum targets have demonstrated symmetric capsule implosion by carefully controlling the balance between different cones of laser beams through two and three-color tuning of the cross-beam power transfer.

Readout and Target Diagram Showing Three-Color Scheme

(Left) Readout in the NIF Master Oscillator Room shows the three infrared wavelengths used in a three-color NIF experiment. (Right) Diagram of a NIF target shows the path of the inner and outer laser beams through the hohlraum laser entrance hole.

This technique for high-power laser redirection can be used to direct laser light around poor coupling regions and obstacles. In addition, it could be used on NIF to combine many of the beams into a single pump beam that could then be used to create laser intensity in the range of about 10²² to 10²³ W/cm² and provide the capability to explore physics at laser intensity frontiers.

Lead author John Moody was joined on the paper by colleagues from LLNL, Los Alamos National Laboratory, and General Atomics in San Diego.

New Adjustable-Depth 'Gray Blockers' Enhance NIF Optics Conditioning

One technique used to protect NIF optics from high-energy laser pulse damage is to block the laser light before it reaches flaws on the optics, known as damage initiation sites, which have the potential to grow in size in response to subsequent high-fluence (energy per unit area) laser shots.

In 2010, NIF installed programmable spatial shapers (PSS) in all 48 preamplifier modules that can temporarily shadow these isolated sites from high-fluence laser pulses. The "blockers" that produce the shadows are introduced in the laser's low-fluence region, upstream of the main amplifier chain.

This month, new Integrated Computer Control System software has added a new variable transmission capability called "gray blockers." Among other upgrades, the software control enhancements allow the blocker light transmission levels to be adjusted, affecting the amount of energy transmitted to the final optics (an opaque blocker allows no light to reach the optics).

Preparing New Database for Gray Blockers

Database Team members Donna Maloy (left) and Suzy Townsend configure the database during the ICCS 9.1.2 software release. The software enhancements allow the depth, or amount of light that can pass through the variable transmission "gray blockers" to be adjusted.

After calibration, the system has proved capable of achieving transmissions within the required 10 percent tolerance of the requested amount. This variable transmission capability allows NIF to condition the laser optics in the vicinity of pre-existing blockers in conjunction with executing high-energy physics experiments. By eliminating the need for separate, time-consuming conditioning shots, this significantly reduces the cost of operating the facility.

Measuring Energetic Electrons in Imploding ICF Capsules

Energetic (high-energy) electrons generated during inertial confinement fusion (ICF) experiments can penetrate the plastic target capsule (the ablator) and "preheat" the hydrogen fuel, making it more difficult to compress. The first direct measurements of the energy deposited into a plastic ablator by energetic electrons in a NIF ICF hohlraum were reported by LLNL researchers and their colleagues in a Physical Review Letters article published on March 30 (DOI:10.1103/PhysRevLett.108.135006).

By imaging hard x-ray emission from energetic electrons slowing in the ablator shell under ignition drive conditions, the researchers were able to estimate the degree of preheating of the deuterium-tritium (DT) fuel from high-energy electrons generated by laser-plasma interaction. This enabled them to infer an upper bound on the DT fuel preheat in NIC experiments, and the amount of preheating was determined to be acceptable. The results are consistent with detailed radiative hydrodynamics simulations, they said.

Tilo Döppner of NIF & Photon Science was the paper's lead author; he was joined by collaborators from LLNL, Los Alamos National Laboratory, and General Atomics.

NIF Visitors

Henri Poincaré Institute Director Tours NIF

French Mathematician Tours NIF Noted French mathematician Cédric Villani (right), director of Institut Henri Poincaré in Paris, toured NIF on March 2. Ed Moses and Plasma Physics Group Leader Siegfried Glenzer conducted the tour. Villani received the Fields Medal, the highest honor in mathematics, in 2010 for his work on Landau damping and the Boltzmann equation. Discussions on future collaborations between our Laboratory and his institute were very encouraging.

Ambassador Mitchell Reiss Visits

Ambassador Mitchell Reiss, President of Washington College, visited NIF on March 8, accompanied by his Laboratory host, Neil Joeck of the Center for Global Security Research (right). NIF Director Ed Moses (left) led the tour. Ambassador Reiss, the former director of the State Department's Policy Planning Office, negotiated the Northern Ireland peace agreement, and as the director of the Korea Energy Development Organization, he negotiated the implementation of the Framework Agreement with North Korea. He was at the Laboratory to give a talk titled, "Should We Talk to Terrorists?"

California State Legislators Tour

Gina Bonanno (right) leads California Assembly Member Joan Buchanan (second from right) and State Senator Mark DeSaulnier on a March 9 tour of NIF. Members of their Capitol and district office staffs joined them. The visit also included tours of the TeraScale Facility and High Performance Computing Innovation Center.

Photo of the Month: The NIF Target Chamber

This dramatic image of NIF beamlines entering the lower hemisphere of the NIF Target Chamber, as seen from the ground floor of the Target Bay, was taken by NIF photographer Damien Jemison. Five exposures were taken to capture the range of light in the dimly lit Target Bay. Jemison used the high dynamic range (HDR) Efex Pro program to process the five images into a single photo of one of the most spectacular views in the facility. He converted the image to monotone to simplify the chaos while enhancing the drama, then highlighted the barely visible Target Chamber by adding its blue hue back into the image. "The end result is my artistic view of how I feel when standing face-to-face with the highest-energy laser in the world," Jemison said.