Photons & Fusion is a monthly review of science and technology at the National Ignition Facility & Photon Science Directorate. For more information, submit a question.
Capsule implosions and fusion reactions during experiments on NIF last only a few picoseconds (trillionths of a second). Obtaining precise information about the physical processes occurring in the capsule, such as velocity, density, and internal energy (adiabat), has required the development of a new generation of ultrafast, ultrahigh-resolution diagnostic devices such as the Advanced Radiographic Capability (ARC), now moving rapidly along the path to completion and commissioning.
ARC, a petawatt-class laser with peak power exceeding a quadrillion (1015) watts, is designed to produce brighter, more penetrating, higher-energy x rays than can be obtained with conventional radiographic techniques. When complete, ARC will be the world’s highest-energy short-pulse laser, capable of creating picosecond-duration laser pulses to produce energetic x rays in the range of 50-100 keV for backlighting NIF experiments.
ARC uses up to four NIF beamlines, propagating two short-pulse beams for each NIF aperture in a split-beam configuration. Staggering the arrival of the eight ARC beamlets onto backlighter targets will produce an x-ray “movie” to diagnose the fuel compression phase leading up to ignition of a cryogenic deuterium-tritium target with tens-of-picoseconds resolution. ARC also will enable new experiments in frontier science and high-energy-density stewardship science.
Over the past year, the ARC utilities, including electrical cabling, vacuum and ventilation systems, platforms, and cleanrooms, were installed, and on Sept. 12, first light was propagated into ARC compressor vessel 1. A recent series of laser shots to the roving mirror diagnostic enclosure (RMDE) calorimeters at the exit of the NIF laser bay operationally tested a subset of the ARC systems, including the ARC injection laser system, Integrated Computer Control System (ICCS) automated shot software, and laser performance operations module (LPOM) shot setup and analysis software. An ARC shot on Feb. 18 fired 10.41 kilojoules (kJ) of light in four beams (eight beamlets or split beams), as recorded by the RMDE calorimeters in the image at left.
The laser shots were performed in parallel with the installation and alignment of the compressor and parabola vessel line replaceable units (LRUs) in the target bay and the ARC short-pulse diagnostics LRUs in the target bay and switchyard. Installation of the last optical LRU, the AM8 transport mirrors in the parabola vessel, was completed on May 9. The mirrors direct the converging ARC beams to the backlighters near target chamber center.
On May 30, the team removed all the covers from the optics (gratings and mirrors) in compressor vessel 1. As part of this activity the optic surfaces were inspected and any change in grating alignment caused by handling the covers was monitored. No change in grating alignment was needed and no new features (defects, scrapes, scratches or debris) were noted on the optic surfaces.
LLNL and its partners from the Institute Laue Langevin (ILL) in Grenoble, France, the Technical University of Darmstadt in Germany, and the European Synchrotron Radiation Facility (ESRF) in Grenoble have conducted a series of experiments to validate the performance of the LLNL-patented Dual Isotope Notch Observation (DINO) detector system.
In a successful test of this Laboratory Directed Research and Development (LDRD) program, ESRF’s high-energy beamline and the DINO concept were used to assay and map the location of lithium-7 (7Li) in several objects through isotope-specific material detection.
Invented by NIF&&PS Chief Technology Officer Chris Barty, DINO is based on gamma-ray excitation of nuclear resonance fluorescence (NRF). In the NRF process, an object is exposed to a continuous flux of gamma-ray photons whose energy has been tuned to the NRF absorption resonance in the isotope of interest. Excited by the gamma rays, the nucleus re-radiates high-energy, very-narrow-bandwidth photons. Because NRF energy levels depend on the exact nuclear structure of the emitter, NRF spectra are isotope specific.
ESRF’s ID15A beamline was used to simulate, at 478 keV (thousand electron volts), the output of future million-eV-class mono-energetic gamma-ray (MEGa-ray) light sources based on laser-Compton scattering, such as those being developed at LLNL. The six-day experimental campaign demonstrated, among other things, that:
“This was an incredibly successful collaborative effort that puts LLNL and its collaborators once again in a leadership position with respect to defining the field of nuclear photonics,” Barty said. Added LLNL Responsible Individual Félicie Albert: “I was amazed at how quickly we found the 7Li NRF line. This happened within seconds, as opposed to several hours when we did the T-REX (Thomson-Radiated Extreme X-Ray) experiments at LLNL in 2008. None of this would have happened without the joint expertise of LLNL, ILL, Darmstadt University, and ESRF. This really shows the importance of further developing bright narrowband Compton scattering sources for nuclear detection and assay.”
The NIF & Photon Science Directorate Review Committee (DRC), chaired by Professor Robert Byer of Stanford University, met at the Laboratory May 5-8 to review the directorate’s current activities, plans, and challenges. Topics of discussion included NIF as a user facility; NIF and national security; NIF and directorate operations and capabilities, including current shot rate improvements; photon science activities including directed energy and advanced laser sources; and current experiments including the high-foot and fundamental science campaigns. There also were panel discussions on directorate strategy and future directions and a poster session on the evening of May 6.
A group of scientists and engineers from LLNL led by John Moody met on May 13 in Rochester, New York, with scientists from the Laboratory for Laser Energetics at the University of Rochester, the Naval Research Laboratory, General Atomics and Los Alamos National Laboratory to discuss plans for optical Thomson scattering experiments on NIF.
Thomson scattering is a powerful diagnostic technique in which light from a probe laser is scattered off the free electrons in a plasma. The spread in the wavelength of the scattered light can provide the density and temperature of the plasma localized to the overlap of the probe beam and collection aperture.
The group reviewed past experiments and their challenges and proposed a possible path to measure the density and temperature in a NIF hohlraum by using a modest probe laser beam of either the fourth or fifth harmonic of neodymium and the calculated properties of a NIF gas-filled hohlraum.
The HED Council, consisting of programmatic managers from LLNL and Sandia and Los Alamos national laboratories, met at the Laboratory May 15-16 to review proposals for FY2015 HEDSS (high energy density stewardship science) experimental campaigns on NIF. The Council reviewed 15 proposals in the areas of hydrodynamics, plasma and burn, nuclear physics, material properties, radiation transport, and code validation and will be submitting recommended prioritization to the programs and NIF for shot scheduling.
An international, multi-institution workshop to explore the results of recent research into hydrodynamic instabilities and mixing in NIF implosions was held at the Laboratory on May 28. Hosted by Dan Clark and Harry Robey, the workshop reviewed a variety of experiments designed to improve understanding of mixing, including hydrodynamic growth radiography (HGR) experiments measuring the growth of preformed ripples on NIF capsules due to the Rayleigh-Taylor instability.
The workshop’s goals were to review and assess what is known about mix in NIF implosions, how well it is known, what else needs to be measured, and how ignition pulse shapes can be improved to achieve both low mix and increased fuel compression and velocity as needed to achieve ignition. Researchers from a number of other national laboratories and institutions involved in inertial confinement fusion and high energy density research participated.
The first 2D radiography experiments of imploding NIF ignition-scale capsules at peak velocity (3.5- to 7-times compression), and the measurement and mitigation of a previously undetected low-mode asymmetry in the in-flight capsule shell, were reported by LLNL researchers and a collaborator in a Physical Review Letters paper published online on May 12.
The 2D radiography technique augments previous techniques that provided a partial record of the time-dependent drive symmetry but did not diagnose symmetry from the time of the final shock launch to that of stagnation. The new experimental platform provides shape information during this previously undiagnosed epoch of the capsule implosion, demonstrating that the shape of the hot-spot self-emission was sometimes misleading as to the symmetry of the surrounding cold shell.
The dependence of the in-flight shape on hohlraum length was measured, and the optimum length to minimize spherical harmonic Y40 was determined to be about 10 percent longer than the previous standard, chosen based on self-emission shape measurements. Residual temporal swings in the low modes remain.
Results from the new platform are being used to improve several models used in inertial confinement fusion (ICF) simulation and design codes and to determine the implosion velocity, center-of-mass velocity, areal density, and low-mode shape of the imploding capsule shell for new NIF ICF configurations. Lead author Ryan Rygg was joined by LLNL colleagues and by John Kline of Los Alamos National Laboratory.
Controlling the impact of laser–plasma interactions (LPI) is one of the most difficult and uncertain challenges facing laser-driven ICF, for both the indirect-drive and direct-drive approaches. Laser-driven parametric instabilities in indirect-drive ignition experiments on NIF, for example, can dramatically affect capsule implosion symmetry.
A Physics of Plasmas article by LLNL researchers and colleagues from the University of Rochester, published online on May 27, highlights some of the discoveries and recent advances in understanding these instabilities, “the most important of which is the realization that the collective interaction of multiple beams is ubiquitous throughout laser fusion,” the researchers said.
The instabilities described in the article are cross-beam energy transfer (in both indirectly driven targets on NIF and in direct-drive targets), multiple-beam stimulated Raman scattering (for indirect drive), and multiple-beam two-plasmon decay instability (in direct drive). The review discusses the general principles by which multiple-beam instabilities can be either avoided or mitigated.
“The final implication,” the researchers said, “is that laser-plasma interactions in ICF should be viewed from a description based on multiple-beam rather than single-beam concepts.” Lead author Jason Myatt of the Laboratory for Laser Energetics was joined by University of Rochester colleagues and by LLNL’s Denise Hinkel, Pierre Michel, and John Moody.
In high energy density science seminars on May 5 and May 13, Professor Steven Rose of the Blackett Laboratory at Imperial College, London, and Carl Brune of the University of Ohio discussed ways in which experiments on high-power laser systems such as NIF can enhance understanding of nuclear physics and provide insights into a variety of astrophysical phenomena.
On May 5, Professor Rose discussed how the study of plasmas created in inertial confinement fusion (ICF) experiments on NIF, the Laser Megajoule in France, and other large laser systems could aid in measuring opacity in the center of the sun, a subject of interest for understanding stellar structure and evolution. He also described other astrophysically relevant phenomena susceptible to laboratory astrophysics studies, such as the Breit–Wheeler process for electron-positron pair production from two photons, and two-photon emission by an electron scattered from a high-intensity laser pulse; known as double Compton scattering, this process is important in radiation-dominated astrophysical plasmas.
Brune said that compared to accelerator-based studies, the ICF plasma environment provides opportunities to study reactions involving tritium (3H), the low-mass implosion environment, and the possibility of high neutron fluxes. He noted that the first two of these factors has led to a determination of the T(t,2n)α neutron spectrum at NIF with unprecedented precision. “The absolute cross section of the reaction is seriously being pursued in ICF experiments,” he said.
The high neutron flux generated by ICF shots also provides the opportunity to study neutron-induced reactions, and in many cases, such as for the T(t,2n)α reaction, the improved nuclear physics inputs lead to an improved understanding of nuclear diagnostics. Future ICF studies of the 10B(p,α)7Be reaction, via the detection of 7Be radioactivity, are expected to provide important insights into the distribution of velocities of the reacting nuclei.
The role of HED science in astrophysics research also was highlighted by LLNL researchers at the 10th International Conference on High Energy Density Laboratory Astrophysics, held May 12-16.
Invited speaker Daniel Casey said the emerging field of plasma nuclear science, performed at facilities such as NIF and the OMEGA laser at the University of Rochester, is “opening exciting new opportunities to access conditions much like the interior of stars.” He noted that ICF implosions produce extremely dense and hot plasmas that may provide paths for studying plasma electron screening and other plasma-nuclear effects present in stellar cores. ICF implosions also have very short and intense fusion burns producing extraordinarily high neutron fluxes that are opportune for studying reactions on short-lived and excited states relevant to supernovas, he said.
Other LLNL researchers presenting at HEDLA 2014 included: