



By contrast, the fast-ignitor concept adds two laser beams that are timed to strike the target at the moment of maximum compression. Because the Petawatt was conceived to use Nova, eight of Nova's ten beamlines would strike the target and form a plasma. Then a 1-TW, 100-picosecond channeling beam supplied by the Petawatt laser bores through the plasma and pushes the deuterium-tritium fuel in its path toward a higher density near the core of the target. At the optimum moment, a petawatt ignitor beam propagates through the channel formed by the channeling beam, striking the high-density, preimploded core. The petawatt pulse generates hot, high-energy electrons, which instantaneously raise a small region on the periphery of the core to over 100 million degrees Celsius. The fusion burn propagates from this small volume on the edge throughout the remaining fuel before hydrodynamic disassembly of the core.

The fast-ignitor technique offers, in principle, a method of reducing the energy and precision required to achieve ignition compared with conventional ICF. Perry cautions that, compared with the firm scientific foundation of conventional ICF, the fast-ignitor concept is still in its infancy because it resides in a region of untested physics. If, however, upcoming fast-ignition tests prove successful, a petawatt laser could be added to the NIF for fast-ignitor capability at a moderate additional cost.

The Petawatt's beam would be fired to inject energy into a small region of the deuterium-tritium target capsule to initiate ignition a few billionths of a second after NIF's beams are fired. A Petawatt-NIF combination might enable the achievement of a higher fusion energy gain than currently envisioned. A New Chapter in Physics

The ultrashort pulses and extremely high irradiance of the Petawatt laser will also enable researchers to advance their understanding of laser-matter interactions and, indeed, advance understanding of the fundamental nature of energy and matter. The enormous irradiance that will be generated by the Petawatt, some 1021 W/cm2, will make possible an irradiance unlike any produced in the laboratory to date. These unprecedented laboratory conditions will be characterized by electric fields about 100 times stronger than the field that binds electrons to atomic nuclei. Such fields have the potential to trap electrons and accelerate them to high energies within just a few centimeters, instead of many kilometers as in conventional particle accelerators.

The enormous electric fields created by the Petawatt will impart enormous oscillatory ("quiver") energy to the free electrons in the plasma. At 1021 W/cm2, the quiver energy of a free electron would be more than 10 million electron volts. The electrons would be moving at speeds approaching the speed of light and at densities never before seen in the laboratory.

These plasmas will be similar to those believed to exist in many astrophysical objects. Scientists could then study conditions predicted to exist in the center of stars and surrounding celestial bodies such as black holes and brown dwarves.

Additionally, high-energy photons (0.1 to 10 megaelectron volts) produced from the interaction of the petawatt pulse with high-atomic-number targets offer the potential for time-resolved radiography of dense objects. The short-pulse duration, potentially small source size, and simple production of multiple pulses separated in time make this an attractive source for multiple-exposure flash x-ray radiography. The plasmas themselves can provide important information to Lawrence Livermore scientists supporting DOE's Stockpile Stewardship and Management Program.

The Petawatt laser is currently undergoing a long series of tests as it is transformed into an operational facility for target experiments. Its development is expected to continue into the next decade as LLNL scientists continue to advance the state of the art in optics and the technology of short-pulse lasers. Perry notes that several years of hard work lie ahead in exploring the fast-ignitor concept with the Petawatt. The overall goal, as it was with the development of Livermore's first generation of lasers, is to speed the arrival of laser fusion as a source of virtually inexhaustible energy for society. Another goal, admittedly closer at hand, is to aid the nation's Stockpile Stewardship Program.