RAMPANT, occasionally rancorous, competition among scientists, institutions, and schools of thought mark much of scientific research today. Much less is heard about the genuine cooperation that abounds in the research community, particularly that between men and women from different research centers working toward a common goal.
A telling illustration of close scientific collaboration is the long-standing relationship of laser experts at the Laboratory for Laser Energetics (LLE) of the University of Rochester and at Lawrence Livermore National Laboratory. Their common goal has been to harness the potential of the laser as a future energy source and as a tool for revealing the secrets of matter at extreme temperatures and pressures.
Involved in inertial confinement fusion (ICF) research since the late 1960s, LLE today operates the only fusion research program jointly supported by the federal government, state government, industry, utilities, and a university. The U.S. Department of Energy has designated LLE as the National Laser Users' Facility to enable academic institutions, industrial research establishments, and government laboratories to have access to its facilities.

Showcasing the Omega Laser
LLE's showcase facility is its 60-beam Omega laser, which can deliver more than 40 kilojoules of energy on a target less than 1 millimeter in diameter. (By comparison, the 192-beam National Ignition Facility [NIF], now under construction at Lawrence Livermore, will produce 1.8 megajoules of energy.)
Completed in 1995, Omega is the nation's principal direct-drive laser fusion research facility. With direct drive, laser light strikes a minuscule capsule directly to compress it. In the other approach to inertial fusion-indirect drive-laser light first strikes the inner wall of a metal cylinder called a hohlraum, causing the production of x rays that symmetrically implode a capsule located inside.
Omega is the latest achievement of LLE's laser program, which has paralleled Lawrence Livermore's for nearly four decades. During that time, researchers at each institution have readily adopted the breakthrough technologies developed by the other, often collaborating to improve them or modifying them to suit unique experimental goals. "Such shared technologies translate to a `national win,'" says LLE director Bob McCrory.

Sharing Improves Technologies and Reduces Cost
When Omega was upgraded from 24 to 60 beams, it married technologies pioneered by both Livermore and LLE. Livermore scientists advised their LLE colleagues about disk amplifier technology they had developed, recalls Howard Powell, Livermore physicist and program leader for Laser Science and Technology. "We told them everything we knew about how to use flashlamps to pump disk amplifiers and how to cool the amplifiers," he says.
Although NIF will use nitrogen gas as a flashlamp coolant, Omega scientists decided to use water. This new technology has paid off-the Omega laser beams have only a 45-minute turnaround time. "The fact that flashlamp cooling works so well for them means we're very confident about using flashlamp cooling techniques for NIF," says Powell.
Livermore laser scientists point to two key developments by their LLE colleagues. The first, achieved in 1980, uses crystals of KDP (potassium dihydrogen phosphate) to efficiently convert a laser's infrared wavelengths to ultraviolet to better couple the laser energy to the target. Early generations of Livermore's neodymium-doped glass lasers-Janus, Cyclops, Argus, and Shiva-produced successively higher peak power and output energy at 1,050-nanometer wavelengths. This wavelength was not short enough to produce effective implosions. Livermore researchers took advantage of the LLE breakthrough in 1985 to convert the laser light on their 10-beam Nova laser to the 351-nanometer wavelength.
McCrory points out that until the Omega upgrade began operation, Nova was the world's most powerful laser. Because of technology advances, Omega was built for roughly one-third the cost of Nova. Further advances make NIF's cost per unit of output energy even lower. NIF has about 60 times the output energy of Omega at roughly 20 times the cost, continuing the trend of advancing technology from Nova to Omega to NIF.
The second major LLE breakthrough was smoothing by spectral dispersion (SSD). This technology shimmers the beam on the target to get rid of speckling and intensity variation, thereby avoiding destructive hot spots. Although it was originally developed for direct-drive experiments, Livermore researchers discovered it was also useful for indirect drive. As a result, SSD was modified and implemented on Nova; it will also be used on NIF.
"The real contest," says McCrory, "is the quality of the laser beam." In that respect, he says, SSD is comparable in importance to Livermore's development of spatial filters in the late 1970s. These filters prevent damage to laser glass by smoothing the shape of and eliminating the high-frequency noise in the beam. At the time, says McCrory, spatial filters were the "salvation" of solid-state lasers.






Omega Stands in for Nova
When Nova was decommissioned in May 1999, Omega became the only facility in the nation doing laser fusion implosion experiments. Although it was designed to do direct-drive experiments, it is working well as a facility for Livermore's indirect-drive experiments.
The decision to close Nova and transfer experiments to Omega until NIF begins operation in 2002 was not made lightly. Livermore physicist Ted Perry notes that because Omega was designed as a direct-drive facility, it can use only about 40 of its 60 beams for the indirect-drive targets used on Nova and NIF.
"Omega had to demonstrate that it could do the experiments. It passed all the tests," says Perry, who credits the ingenuity of Livermore and Rochester scientists working together to optimize the facility for indirect drive.

Omega Contributes to Stockpile Stewardship
Most of Livermore's planned shots on Omega for 1999 are earmarked as part of DOE's science-based Stockpile Stewardship Program to ensure that the nation's nuclear weapon stockpile remains safe and reliable. Small Livermore teams travel to Rochester with their laser targets and stay for about a week-long "campaign" of 20 to 30 shots. (Omega averages 10.5 shots per day.)
Most experiments are unclassified, especially fundamental hydrodynamics experiments that can be applied as much to astrophysics as to understanding nuclear weapons. Diagnostic instrumentation originally built for Nova works well on Omega, thanks to what McCrory calls "shared modularity." In turn, instruments built for Omega can be readily adapted to work on NIF.
Livermore physicists have been impressed with the precision of Omega's 60 beams. "Omega is more precise than Nova because it has more modern technology," says Powell. "Precision is everything in laser fusion."
Livermore physicist Kim Budil points out that Omega's 60 beams give experimenters more flexibility to design experiments than Nova did. What's more, she says, working with the complicated geometry of the beams is good preparation for NIF's 192 beams.
For ICF, it is important that the spherical target stays round as it is squashed by the x rays in the hohlraum. It is relatively easy to detect sausage- and pancake-shaped deviations from spherical implosions, but more complicated deviations from roundness, such as a cross, are harder to measure. A team led by Livermore physicist Nino Landen recently concluded experiments on Omega that demonstrate the detection capability for these subtler deviations, so-called high-order asymmetries, which were difficult, if not impossible, to isolate on Nova. The control of these high-order asymmetries is important to achieving highly spherical implosions and eventually ignition on NIF.
In addition to supporting stockpile stewardship experiments by Livermore and Los Alamos personnel, LLE scientists are preparing for direct-drive experiments. McCrory says that LLE is facing the same kinds of technical challenges to make direct-drive work that Nova experimenters faced in the mid-1980s proving indirect drive.
Direct drive is an attractive option to indirect drive because of the potential for higher energy gain, says Charles Verdon, head of the Livermore group that designs laser targets and LLE deputy director from 1994 to 1997. "As Rochester solves its technical issues, such as handling cryogenic targets, the results will help Livermore scientists prepare for NIF," Verdon says. In addition, he says that as a multiuser facility, Omega is serving as a model for how best to operate NIF as a stockpile stewardship facility for all three weapons laboratories.






Omega after NIF
Verdon says that Omega will continue to be an important facility to Livermore even after NIF begins operation. Lawrence Livermore will use Omega to scope out scientific ideas more easily and cheaply. High-power or high-energy experiments, however, will require NIF.
The strong LLE connection to NIF is evident in other areas. LLE optics experts are applying essential multilayer coatings to several NIF optical components, such as the polarizers that form part of the giant laser's optical switches and the deformable mirrors used to control beam quality.
Looking beyond NIF, Livermore and LLE researchers are collaborating on a proposal to develop a DOE "virtual laboratory" to design a diode-pumped solid-state laser for inertial fusion energy. The laser would fire some 10 times per second with 10 percent efficiency. A similar virtual laboratory for a heavy-ion laser facility was formed last year as a collaboration between Livermore and Lawrence Berkeley National Laboratory.
"The ICF program has worked synergistically. There is always pride in ownership, but there haven't been a lot of `not invented here' roadblocks," says Verdon.
"We compete," adds Powell, "but it's a healthy competition."
-Arnie Heller




Key Words: diode-pumped solid-state laser, direct drive, flashlamp cooling, indirect drive, inertial confinement fusion (ICF), KDP (potassium dihydrogen phosphate) crystals, Laboratory for Laser Energetics (LLE), Lawrence Berkeley National Laboratory, National Ignition Facility (NIF), Nova laser, Omega laser, smoothing by spectral dispersion (SSD), spatial filters.

For further information contact Robert McCrory (716) 275-4973 (rmcc@lle.rochester.edu) or Charles Verdon (925) 423-4449 (verdon1@llnl.gov.


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