2011 Annual Report

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The fiscal year ending September 2011 (FY11) concluded the fourth year of the third five-year renewal of Cooperative Agreement DE-FC52-08NA28302 with the U.S. Department of Energy (DOE). This annual report summarizes progress in inertial fusion research at the Laboratory for Laser Energetics (LLE) during the past fiscal year including work on the National Ignition Campaign (NIC). It also reports on LLE’s progress on laboratory basic science research; laser, optical materials, and advanced technology development; operation of OMEGA and OMEGA EP for the NIC and high-energy-density (HED) campaigns, the National Laser Users’ Facility (NLUF), and other external users; and programs focusing on the education of high school, undergraduate, and graduate students during the year.

One of LLE’s principal missions is to conduct research in inertial confinement fusion (ICF) with particular emphasis on supporting the goal of achieving ignition on the National Ignition Facility (NIF).

Within the NIC, LLE plays a lead role in the validation of the performance of cryogenic target implosions, essential to all forms of ICF ignition. LLE is responsible for a number of critical elements within the Integrated Experimental Teams (IET’s) supporting the demonstration of indirect-drive ignition on the NIF and is the lead laboratory for the validation of the polar-drive approach to ignition on the NIF. LLE has also developed, tested, and constructed a number of diagnostics that are being used on the NIF for the NIC. During this past year, progress in the inertial fusion research program continued in three principal areas: NIC experiments; development of diagnostics for experiments on OMEGA, OMEGA EP, and the NIF; and theoretical analysis and design efforts aimed at improving direct-drive–ignition capsule designs and advanced ignition concepts such as fast ignition and shock ignition.

Experiments were conducted on OMEGA to measure the velocity and timing of multiple converging shock waves inside spherical targets filled with liquid (cryogenic) deuterium.

Pioneering experiments were conducted on OMEGA by a collaborative team led by scientists from MIT’s Plasma Science and Fusion Center (PSFC) and comprised of LLE, LLNL, and General Atomics (GA) scientists. The team obtained proton radiographic images of gas-filled hohlraum-driven implosions showing the dynamics of the interior of the hohlraums including inhibition of plasma-jet formation and the mitigation of plasma stagnation on the hohlraum axis.

The hydrodynamic-instability–induced mixing of ablator material into the hot spot of ignition-scale ICF implosions was measured with x-ray spectroscopy on NIF implosion experiments.

Measurements of hot-electron generation by the two-plasmon–decay instability (TPD) were carried out.

A joint team led by MIT-PSFC, with the participation of scientists from LLE and LLNL, measured for the first time the differential cross section for the elastic neutron–triton (n–³H) and neutron–deuteron (n–²H) scattering at 14.1 MeV using an inertial confinement facility.

Direct-drive, Rayleigh–Taylor (RT) growth experiments were conducted in planar cryogenic, liquid deuterium (D₂) targets on the OMEGA laser.

Scientists from LLE, Lodestar Research Corp., and the University of California, San Diego, analyzed the dynamics of hot-electron heating in direct-drive–implosion experiments caused by TPD instability.

A fully kinetic reduced particle-in-cell method, utilizing novel diagnostics, was applied to simulations of TPD instability in inhomogeneous plasma for parameters consistent with recent direct-drive experiments.

The annual report includes articles on advanced technology development at LLE, including:

• improvements to optimize the long-pulse, on-target energy of the OMEGA EP laser
• the use of photothermal heterodyne imaging (PHI) to evaluate the spatial distribution of absorbers in hafnia nonlayers
• the terahertz electro-optic (EO) response in cadmium manganese telluride (Cd,Mn)Te single crystals
• demonstration of a highly efficient pulsed-diode–pumped, room-temperature Yb:YAG ceramic laser with a slope efficiency of 78% and an optical-to-optical efficiency of 51%
• amplification of nanosecond, 1053-nm optical pulses from 15 pJ to 240 nJ by a Yb-doped all-fiber regenerative amplifier (AFRA) with an overall gain of 42 dB
• suppression of parasitic processes in noncollinear optical parametric amplifiers (NOPA) for walk-off and non-walk-off compensating configurations
• stress compensation in hafnia/silica optical coatings by the inclusion of alumina layers
• demonstration of a closed-loop, high-resolution beam-shaping system based on a liquid-crystal-on-silicon (LCOS) spatial-light modulator (SLM) in a multiterawatt laser system and in the OMEGA EP long-pulse front end
• development of new wavefront reconstruction algorithms for high-spatial-resolution applications
• results from femtosecond pump–probe spectroscopy studies of time-resolved optical reflectivity of all-oxide, YBa₂Cu₃O₇/La_0.7Sr_0.3MnO₃ superconductor/ferromagnet nano-bilayers

Under the facility governance plan that was implemented in FY08 to formalize the scheduling of the Omega Laser Facility as a National Nuclear Security Agency (NNSA) facility, Omega Facility shots are allocated by campaign. The majority (65.1%) of the FY11 target shots were allocated to the NIC conducted by integrated teams from the national laboratories and LLE and to HED campaigns conducted by teams led by scientists from the national laboratories.

In FY11 29% of the facility shots were allocated to basic science experiments. Nearly half of these were conducted for university basic science under the NLUF Program and the remaining shots were allotted to the Laboratory Basic Science (LBS) Program comprising peer-reviewed basic science experiments conducted by the national laboratories and LLE/Fusion Science Center (FSC).

The Omega Laser Facility is also being used for several campaigns by teams from the Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) of France and Atomic Weapons Establishment (AWE) of the United Kingdom. These programs are conducted on the facility on the basis of special agreements put in place by DOE/NNSA and participating institutions.

During FY11, the Omega Laser Facility conducted 1348 target shots on OMEGA and 457 target shots on OMEGA EP for a total of 1805 target shots (see Tables 128.V and 128.VI) OMEGA averaged 10.3 target shots per operating day with availability and experimental effectiveness averages for FY11 of 93.3% and 96.1%, respectively. OMEGA EP was operated extensively in FY11 for a variety of internal and external users. Of the 457 target shots, 401 were shot in the OMEGA EP target chamber and 56 were joint shots in the OMEGA target chamber. OMEGA EP averaged 5.5 target shots per operating day with availability and experimental effectiveness averages for FY11 of 85.6% and 95.2%, respectively. Some of the highlights of the facility during FY12 include:

• The OMEGA EP short-pulse (IR) and long-pulse (UV) energy on target was increased.
• At the request of users, the OMEGA EP shortest UV pulse durations were extended from the previous limit of 1 ns to 100 ps.
• Using a static wavefront corrector, wavefront correction was developed for OMEGA EP to correct high-order residual wavefront that is beyond the spatial resolution of the existing adaptive optics.
• The daily operation of the OMEGA EP laser was improved with enhancements to the OMEGA EP infrared alignment table (IRAT).
• Target experiments have been shown to slowly degrade the UV transmission, primarily of the final debris shield, causing a decrease in on-target energy relative to the diagnostic prediction. During FY11, the study of UV transmission resulted in a better understanding of the loss mechanisms.
• Diagnostic capabilities continue to evolve with the commissioning of 24 new diagnostic instruments on OMEGA and 9 new diagnostic instruments on OMEGA EP.
• Several safety improvements were implemented in the experimental area.

As the only major university participant in the National ICF Program, education continues to be an important mission for the Laboratory. Laboratory education programs span the range of high school to graduate education.

During the summer of 2011, 16 students from Rochester-area high schools participated in the Laboratory for Laser Energetics’ Summer High School Research Program. The goal of this program is to excite a group of high school students about careers in the areas of science and technology by exposing them to research in a state-of-the-art environment.

The students spent most of their time working on their individual research projects with members of LLE’s technical staff. The projects were related to current research activities at LLE and covered a broad range of areas of interest including experimental systems and diagnostic development, computational modeling of implosion physics, chemistry, materials science, laser system development and diagnostics, and database development.

Two hundred and eighty-one high school students have now participated in the program since it began in 1989. Thirty of the participating students have gone on to gain semi-finalist status in the Intel Science Talent Search national competition and four of the students have gained finalist status at this competition.

Approximately 41 undergraduate students participated in work or research projects at LLE this past year. Student projects include operational maintenance of the OMEGA Laser Facility; work in laser development, materials, and optical-thin-film-coating laboratories; computer programming; image processing; and diagnostics development. This is a unique opportunity for students, many of whom will go on to pursue a higher degree in the area in which they gained experience at the Laboratory.

Graduate students are using the OMEGA Facility as well as other LLE facilities for fusion and HED physics research and technology development activities. These students are making significant contributions to LLE’s research program. Twenty-five faculty from the five University academic departments collaborate with LLE scientists and engineers. Presently, 77 graduate students are involved in research projects at LLE, and LLE directly sponsors 38 students pursuing Ph.D. degrees via the NNSA-supported Frank Horton Fellowship Program in Laser Energetics. Their research includes theoretical and experimental plasma physics, HED physics, x-ray and atomic physics, nuclear fusion, ultrafast optoelectronics, high-power-laser development and applications, nonlinear optics, optical materials and optical fabrication technology, and target fabrication.

In addition, LLE directly funds research programs within the MIT Plasma Science and Fusion Center, the State University of New York (SUNY) at Geneseo, and the University of Wisconsin. These programs involve a total of approximately 6 graduate students, 25 to 30 undergraduate students, and 10 faculty members.