Around the Lab
Highlighting the History of the LLE
October, 2010This year, the University of Rochester’s Laboratory for Laser Energetics (LLE) marks its 40th anniversary as a unique national resource for investigating the interaction of intense radiation with matter. This important marker offers an opportunity to examine the highlights of the history of the LLE in order to prepare for the quest to harness nuclear fusion for the greater good. The most significant, long-term potential commercial application of fusion is the generation of electric power. Fusion does not generate nuclear waste nor does it enhance nuclear proliferation concerns in contrast to the nuclear fission reactors currently in use. The fuel for fusion is essentially inexhaustible. A demonstration of “ignition,” the beginning of a self-sustaining fusion reaction, in the laboratory is anticipated in the next few years on the National Ignition Facility (NIF). LLE is one of the institutional partners in the NIF and, as part of this National effort, stands on the threshold of the realization of fusion energy.
With the support of Robert Sproull, an eminent physicist and President of the University of Rochester, the LLE was founded in the fall of 1970 as an interdisciplinary entity within the College of Engineering and Applied Science. Moshe J. Lubin, a faculty member in the University’s Department of Mechanical and Aerospace Sciences (MAS), was the founding director of LLE. By 1970, Lubin and his early colleagues at Rochester recognized that lasers could generate sufficiently high energy density in matter to ignite thermonuclear reactions.
Moshe J. Lubin (1938–1993) was the founding director of LLE.
Between 1971 and 1975, the first four-beam LLE system, Delta, was built and operated. Delta was an ~1-kJ Nd:glass laser used to investigate the interaction of high-power laser radiation and plasma with particular emphasis on laser fusion.
A key year in the history of LLE was 1975. The vision of Lubin’s team—to build and operate a very large, 24-beam IR laser facility to be called OMEGA—would cost tens of millions of dollars. The invaluable annual research grants from the first sponsors—energy companies and the State of New York—amounted to tens of thousands of dollars during LLE’s formative years. The underwriting of OMEGA, however, was significantly beyond their ability. Up until 1975, Washington had not been targeted for seeking funds for the program. The Atomic Energy Commission (AEC) would have been the only realistic source of the needed funds, but it had just been disbanded and a new agency, the Energy Research and Development Administration (ERDA), would replace it. President Sproull knew the newly named administrator of ERDA, Dr. Robert Seamans, Jr., and he scheduled an appointment with him the second day of ERDA’s existence. Intrigued by the University’s interest in involving industrial and state sponsors, Seamans authorized the University to present its case to Congress so that LLE might be funded in ERDA’s pending budget. Lubin’s astute testimony, along with strong support from Rochester’s congressman, Frank Horton, who had just been appointed to the Joint House-Senate Committee on Atomic Energy, resulted in LLE receiving its requested funding.
The cornerstone-laying ceremony for the new LLE building to house the new laser facility took place on 2 April 1976. The new building, with architectural design work by United Engineers and engineering design efforts by Eastman Kodak, included 100,000 square feet of laboratory and office space. The part of the building dedicated to the laser was designed for clean operations in a well-controlled temperature and humidity environment. That part was decoupled from the rest of the building’s foundations to minimize vibration coupling to the laser equipment.
Photograph of the Delta laser from the 1972 LFFP brochure.
During the period from 1975 to 1980, the GDL (1-beam), ZETA (6-beam), and OMEGA (24-beam) laser systems were constructed and began operations. The GDL was a prototype beamline for OMEGA and demonstrated its 0.75-TW/beam performance in 1977. ZETA was a proof-of-design system incorporating the first six beams. It included a separate target chamber and operated for the first time in 1978. The dedication of the new laboratory took place on 17 October 1978 with OMEGA in its ZETA configuration. Some 200 scientists, politicians, industry representatives, and government officials attended the event, which included the first-ever public firing of a laser onto a DT-filled glass shell. Not only did the laser system perform flawlessly, but the target shot produced in excess of 300 million neutrons.
View of the 24-beam OMEGA laser.
Target fabrication was another important element of the LLE fusion experiments program during this period. LLE made significant contributions to the early state of the art of this technology including the development of drill, fill, and plug techniques; coating of smooth polymer layers; radiographic characterization of targets; target suspension techniques; and hemishell fabrication.
The early 1980s presented the LLE management with opportunities and challenges. Using shorter wavelengths appeared to be the most favorable approach for laser fusion. It was clear that embarking on such a project necessitated hard budget choices. It was also necessary to obtain the approval of the Department of Energy (DOE). The LLE management initiated a major effort to validate the physics of direct-drive inertial confinement fusion (ICF) on OMEGA with UV irradiation.
After a hard battle, approval for the conversion of the 24-beam OMEGA laser to the third harmonic was finally obtained from DOE. In return, DOE insisted on a phased implementation of the conversion over a period of three years. By the end of FY85, the full 24-beam UV conversion of OMEGA was completed–on time and on budget.
From 1983 through 1987, significant work was carried out at LLE on characterizing the physics of UV laser-matter interaction; developing tools for the design of high-performance, direct-drive capsules; and developing high-density plasma diagnostics and direct-drive capsule fabrication and characterization capabilities.
Chirped-pulse amplification (CPA) was developed at LLE in 1985 and used to produce very high intensity ultrashort laser pulses.
LLE made investments in ultrafast science and technology in the early 1980s. This effort was supported: (a) to probe the extremes in energy and power density, (b) to provide advances in technology that might be important to the laser-fusion program, (c) to promote greater collaboration with the University of Rochester academic departments and colleges, and (d) to provide directions for growth for the Laboratory research program. The formation and development of the Ultrafast Sciences Group contributed many firsts to LLE’s innovations including high-power switching with picosecond precision; picosecond microwave pulse generation; picosecond electron diffraction; picosecond and subpicosecond electrical sampling; and femtosecond pulse generation. The development of the CPA technique (chirped-pulse amplification) made it possible to generate ultrashort laser pulses using conventional Nd:glass lasers. This technique enabled the development of petawatt lasers that are now of exceptional interest in the investigation of high-energy-density science.
In response to a request from the White House Office of Science and Technology Policy (OSTP), the National Research Council (NRC) of the National Academy of Sciences (NAS) completed a review of the LLE in 1986. The review recognized the important worked conducted by LLE in addressing ICF research and set a goal of compressing a cryogenic direct-drive target to a density of 100 to 200 times liquid DT density as a demonstration that would justify the upgrade of the OMEGA laser to 30 kJ. To meet this objective, the Laboratory installed a KMS Fusion cryogenic target system on OMEGA and then modified the system to meet the specifications of OMEGA experiments. To meet the direct-drive uniformity objectives, LLE developed and constructed distributed phase plates (DPP’s). The use of the DPP’s on OMEGA improved the overall uniformity by a factor of 6.
With the adapted cryogenic system and the newly developed beam smoothing, LLE demonstrated the goal of 100 to 200 × liquid DT density implosions on OMEGA as reported in Nature in 1988. This was the highest compressed fuel density recorded in ICF experiments (using either the direct- or indirect-drive approach) at that time and made a strong case for the direct-drive approach.
Between 1988 and 1990, several reviews of the ICF Program influenced the direction of the program and resulted in the Laboratory collectively focusing its attention to the OMEGA Upgrade. The OMEGA Upgrade laser was designed to be a 60-beam UV laser with an energy-on-target capability of 30 kJ and an eventual irradiation uniformity of 1% to 2% rms. To maximize its experimental utility, the system was designed to shoot at least one shot per hour. The total system cost of the laser was budgeted at ~$61 M (i.e., ~$2000 per UV joule). LLE surprised many people at DOE and the national laboratories by achieving the unachievable with respect to the new laser. Not only was the laser completed on time, its performance exceeded the specifications, and its cost was within budget.
From its first shots in 1995, the OMEGA Upgrade has set the standard for ICF research for the subsequent 15 years. In its first experimental campaign with DT-fueled targets, the OMEGA Upgrade produced fusion neutron yields in excess of 1.3 × 1014—or approximately 1% of the laser energy placed on target—exceeding by several times the yield obtained on the slightly higher-energy Nova facility at LLNL. This record fusion neutron yield stood for 15 years and is about to be surpassed in late 2010 in experiments conducted at the National Ignition Facility.
The team responsible for achieving the first LLE high-density cryogenic target milestone in 1988. Left to right: Hyo-gun Kim, Stephen Jacobs, John Soures, Stanley Skupsky, Robert McCrory, Frederick Marshall, Samuel Letzring, Terrance Kessler, James Knauer, and Robert Hutchison.
Target area (left) and laser bay (right) of the OMEGA 60-beam UV Laser System.
From 1995–1998, the OMEGA 60-beam UV laser became fully operational with pulse-shaping capability and broad-bandwidth, 2-D SSD. Beginning in FY96, OMEGA began to provide shots for indirect-drive and other high-energy-density physics experiments from the national laboratories. Two weeks of experiments were performed in June 1996 to demonstrate the utility of OMEGA for indirect drive. This campaign involved researchers from Los Alamos National Lab, Lawrence Livermore National Laboratory, and LLE. The main objective of these experiments was to validate the ability of the OMEGA system to perform hohlraum experiments, to reproduce results obtained with the Nova laser, and to demonstrate new capabilities not available on other lasers. All of these objectives were met.
LLE collaborations with the MIT Plasma Science and Fusion Center resulted in major accomplishments in charged-particle diagnostics during the 1990s. This collaboration led to the development of two magnet-based charged-particle spectrometers (CPS’s), a number of wedged-range-filter proton spectrometers, and most recently the development and implementation on OMEGA and on the NIF of the magnetic recoil spectrometer (MRS). For near-term OMEGA implosions, the MRS can measure the compressed areal density of DT fuel by measuring down-scattered neutrons below 14.1 MeV. Measurements of the yield and energy spectra of charged particle spectra have yielded valuable information about target conditions in highly compressed cores (especially for the cryogenic capsule implosions). This joint charged-particle–diagnostics effort has been significantly strengthened by collaboration with the SUNY Geneseo Physics Department.
In October 2001, a major enhancement to the OMEGA Laser System (now called OMEGA EP—for Enhanced Performance) was proposed to include four new high-energy beamlines, a versatile high-intensity capability, and a new auxiliary target chamber. The high-intensity beams are generated using the CPA technique originally developed and demonstrated at LLE 20 years ago. OMEGA EP uses a chirped-pulsed–amplification (CPA) technique developed at LLE in the late 1980s. With the CPA method, the laser pulse is first stretched thousands of times, amplified, and recompressed into a very short and very intense pulse. The OMEGA EP laser intensity on target is expected to eventually reach 1021 W/cm2, inducing an electric field so large that the electrons of the target material will be accelerated to a velocity close to the speed of light. One of the advanced ignition techniques to be explored at the Omega Laser Facility will be “fast ignition.” If successful, fast ignition could lead to the highest energy densities and inertial confinement fusion conditions ever achieved in a laboratory.
The LLE–MIT team responsible for the design and construction of the NIF MRS system shown above in its assembled configuration ready for shipment to LLNL for installation on the NIF. Kneeling in front from right to left are Daniel Casey (MIT), Michelle Burke (LLE), Tim Clark (LLE), Brian Rice (LLE); standing (left to right) Mark Romanofsky (LLE), Robert Till (LLE), Oscar Lopez-Raffo (LLE), Chad Abott (LLE), Tom Lewis (LLE), Jason Magoon (LLE), Johan Frenje (MIT), and Milt Shoup (LLE). Photos of John Szczepanski (LLE) and Nick Fillion (LLE) are inserted upper right.
Schematic of the OMEGA EP facility. The four new beamlines on the right of the figure are NIF-scale beams, with the power-amplification stage following the NIF architecture very closely with minor modification. It is possible to inject chirped pulses into two of the beamlines for subsequent compression to short, high-intensity pulses with widths ranging from 1 to 100 ps. A switchyard in the central portion of the figure enables two of the beams to be delivered to the OMEGA target chamber, temporally compressed to high intensities using large-aperture gratings within a cylindrical compression vessel. In addition, it is possible to route all four beams to the new target chamber, with up to two temporally compressed.
DOE began funding the OMEGA EP in FY03. The project began on 1 April 2003 with $13 million in FY03 funding. The University of Rochester authorized funding for an 82,000-sq-ft addition to LLE to house the new facility, located adjacent to the existing OMEGA laser. Building construction began in August 2003 and was completed in January 2005. The OMEGA EP project was completed on time and on budget and dedicated in April 2008. From its initial operations, OMEGA EP has produced exciting new results in high-density physics including record yields of positrons produced when a high-intensity short-pulse beam interacts with a high-Z target, short-pulse Compton radiographs of imploding high-density targets, and copious high-energy x-ray production for short-pulse radiography applications.
In 2008, LLE passed a Department of Energy milestone: Direct-drive cryogenic targets with design characteristics similar to those eventually used to demonstrate ignition on the NIF were successfully compressed to an areal density (i.e., density × radius) of 200 mg per square centimeter. These targets achieved a compressed fuel density of over 500× that of liquid deuterium (approximately 4x the density achieved in the first LLE cryogenic experiments in 1989). This was a major step toward demonstrating the validity of direct-drive–ignition NIF targets.
In 2010, the Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) of France and LLE celebrated the tenth anniversary of collaborative experiments on the Omega Laser Facility and jointly published a volume dedicated to that collaboration. CEA has developed, produced, and delivered more than 500 targets to the Omega facility. Many of the laser experiments have allowed for the development of expertise in target component fabrication, assembly, and implementation. These technologies and the expertise developed for more than ten years at CEA, particularly for the experimental studies carried out on OMEGA, have paved the way for future targets for the LMJ program.
With the advent of the NIF and the expectation of an ignition demonstration on the NIF laser within the next few years, it is appropriate to consider what may be in store for LLE in the future. There is no doubt that the viability and productivity of the LLE facility with its world-class laser facilities and scientific personnel will continue its focus on a broad range of high-energy density-physics research topics and on a broad-based education mission. The Laboratory, however, has never had a propensity to sit on its laurels. On the contrary, LLE’s defining characteristic is to literally reach for the stars. Clearly, the most important mission that LLE could undertake in the wake of an ignition demonstration on the NIF is to work toward the development of inertial fusion energy as an alternative, safe, and inexhaustible energy source. The University of Rochester Laboratory for Energetics can be expected to be a key player in the development of this important technology.
The radiography of massive objects with high-resolution MeV-photon imaging was demonstrated by a collaborative team led by CEA in 2010 OMEGA EP experiments. On the left are the test objects named “flower” (bottom) and its radiographic image (top); on the right are shown a step-wedge object named the “tower of Hanoi” (bottom) and the resulting radiography image (top). These images were produced using a single OMEGA EP beam operating at ~1 kJ in <10 ps.