A legacy from the Livermore lasers program of the 1970s is helping Laboratory researchers achieve the world's brightest laser, thereby making possible a new world of plasma physics experiments. In a project funded under the Laboratory Directed Research and Development Program, the Janus laser, a milestone in the Laboratory's development of glass lasers, has been incorporated in JanUSP (for Janus-pumped, ultrashort-pulse laser). This new instrument recently produced the highest irradiance (power per unit area) ever recorded: two sextillion (2 X 1021) watts per square centimeter.
Achieving this long-sought level of brightness is a requirement for exploring plasmas present only in the interiors of stars and detonating nuclear devices. The characteristics of such extreme plasmas include electric fields 100 times stronger than those binding electrons to atomic nuclei, magnetic fields like those found on the surface of white dwarf stars, electron oscillatory ("quiver") energies similar to gamma-ray bursts, and pressures one trillion times that of Earth's atmosphere at sea level.
JanUSP is a significant upgrade to Lawrence Livermore's longstanding ultrashort-pulse laser facility. Research on ultrashort-pulse lasers (with pulse lengths lasting from a billionth to a trillionth of a second) has been the focus of intense activity at Livermore since the mid-1980s. The work arrived at a major milestone in the late 1990s when the Laboratory's Petawatt laser achieved record-breaking levels of power (more than a quadrillion, or 1015, watts) and irradiance (approaching a sextillion, or 1021, watts per square centimeter) at full energy of about 680 joules before it was shut down (see S&TR, March 2000, The Amazing Power of the Petawatt)
At 200 terawatts (2 X 1014 watts) and 15 joules, JanUSP has a fraction of the power and energy, respectively, of the Petawatt. However, with its shorter pulse length (85 femtoseconds, less than a tenth of a trillionth of a second) and smaller spot size (2 micrometers), it can access much different regimes of matter.
The machine's front end is a commercial oscillator that produces 75- to 80-femtosecond pulses of 800-nanometer light. The low-energy laser pulses are passed through diffraction gratings, made by Livermore's Diffractive Optics Group. The gratings drastically stretch pulses out in time so that they do not distort and eventually damage the laser optics.

Laser Beam with Push
The stretched pulses are energized by a series of amplifiers using increasingly larger titanium-doped sapphire crystals. The final amplification stage features a 10-centimeter-diameter, 5-centimeter-thick, titanium-doped sapphire crystal, the largest in the world and one that required three years to be produced commercially. Energizing this crystal is 130-joule green light from the Janus laser. The fully amplified light is recompressed to its original duration and focused onto a target inside a 2-meter-diameter chamber.
"We want to use JanUSP to explore the uncharted regime of matter subjected to irradiance above 1021 watts per square centimeter," says physicist Paul Springer. To meet that goal requires heating a relatively thick (several micrometers) sample of metal extremely fast, and researchers are tapping the enormous pressure of JanUSP's laser light to do so. Physicist Jacques Denavit first proposed the technique of using light to literally push ions from an aluminum or gold foil some 500 nanometers thick onto a target of uranium or other dense metal. "We don't typically think of light as having a noticeable pressure, but JanUSP's laser beam is so intense that it can generate hundreds of petapascals of pressure at the surface of a target," observes physicist Scott Wilks, who has been using the two-dimensional Zohar computer code (written by physicist A. Bruce Langdon) to model JanUSP-plasma interactions.
If all goes according to plan, later this summer a laser pulse from JanUSP will literally push electrons forward and out of the foil in less than a trillionth of a second. The negatively charged electrons, in turn, will drag positively charged protons and uncharged neutrons with them. The heavy ions, traveling at 8,000 kilometers per second (about 3 percent of the speed of light), will be deposited onto a target made of a heavy metal such as uranium.
In less than a billionth of a second, the impinging ions will travel 5 to 10 micrometers into the target, rapidly creating a superhot plasma of 1,000 volts (about 10 million kelvins). The ions, with about 1,800 times the mass of electrons, are much slower and therefore travel only one-thousandth the distance. As a result, the plasma they create will be confined to a thickness of several micrometers. The plasma will be in thermal equilibrium, meaning that all ions and electrons will radiate at the same temperature. "It's the same kind of plasma that is found at the center of a star or an exploding nuclear device," Springer says. In those cases, thermonuclear energy is transferred through ions, not electrons.

Heating Matter with Ions
Springer notes that although lasers have been used for years to create plasmas from solid targets, the interactions have been with targets' electrons, not their neutrons and protons. "Lasers with prepulse-the less intense, first part of a pulse-couple their energy to hot electrons, which heat up materials to lower temperatures than what we want," says Springer. "Heating with ions is a better way to create the high-energy-density plasmas we're looking for."
Typical laser pulses have what's known as a pedestal, or a slow rise time before the pulse achieves its full intensity. As a result, the prepulse boils electrons off the target's surface. This initial impact creates relatively low-density plasma that interacts with the main pulse, which arrives an instant later. The key to creating plasmas with ions is making the pulse rise time extremely fast. "You don't want the target to know the laser is there until the main pulse arrives," explains Wilks. "The main pulse can then couple its energy directly to the ions and not into the electrons."
The Petawatt laser achieved its record power level with a much longer pulse length (440 femtoseconds) and a much larger aperture (57 centimeters). As a result, its prepulse was a million times larger than the one Livermore scientists hope to see with JanUSP. Indeed, the goal for the so-called pulse contrast (ratio between the main pulse and prepulse) is an unprecedented 10 billion.
Alternatives for creating extreme states of plasma with ions don't exist. Gas guns are too slow by a factor of 1,000. Much more energetic lasers like the National Ignition Facility (now under construction at Lawrence Livermore) heat vastly more mass, but they were not designed to achieve the temperatures of JanUSP. And typical ultrashort-pulse lasers don't have the light intensity or pulse contrast necessary to couple energy to ions.
Springer also notes that the rapid time scales involved in JanUSP experiments are driving improvements in diagnostics for measuring the fleeting plasmas. The JanUSP team has improved upon the x-ray streak camera that was successfully used on Lawrence Livermore's Nova laser, which was decommissioned last year. The new camera achieves 50 times greater resolution than its predecessor. The techniques used to improve Nova's diagnostics can also be applied to those being built for the National Ignition Facility, Springer says.

New Plasma Regimes to Help Stockpile Stewards
JanUSP's high-temperature, high-density plasmas will shed new light on a wide range of astrophysical studies. Such plasmas, especially those caused by strong shocks in solid materials, are also important to the Department of Energy's Stockpile Stewardship Program to ensure the reliability and safety of the nation's nuclear arsenal. A major goal of the Stockpile Stewardship Program is gaining a better understanding of materials under extreme pressures and temperatures.
Another possible application for JanUSP is testing the concept of an ion "lens." The lens involves curving the foil target to focus ejected ions into a 0.3-micrometer-wide, intense beam at the back of the target. Wilks, who's done computer simulations on the concept, says the ion lens could be useful for radiation therapy and for integrated circuit manufacturing (for doping materials onto silicon substrates). In both cases, he says, "you're focusing substantially more ions on a tiny spot than is possible with conventional methods."
The facility is currently attracting researchers from within Lawrence Livermore (including those who worked on the Petawatt), other national laboratories, and other nations for investigating new concepts. In February, a team from Germany's Max Planck Institute conducted experiments designed to accelerate electrons to 1,000 megaelectronvolts, an energy never before achieved outside a particle accelerator. At that energy, it is possible to create subatomic particles called pi-mesons, which are responsible for nuclear interactions.
Physicist Dwight Price notes that several research institutions worldwide are developing facilities similar to JanUSP because of its unique plasma-generating capabilities. "Other research centers are rushing to catch up to us," he says. For at least the next few years, Springer expects JanUSP to provide a wealth of new data to confirm-or contradict-models of extreme states of matter that until recently could not be tested in the laboratory.
—Arnie Heller

Key Words: ion lens, Janus, JanUSP, Nova, Petawatt, stockpile stewardship.

For further information contact Paul Springer (925) 423-9221 (springer6@llnl.gov).

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