Latest Discoveries On 4th State Of Matter

The Petawatt is the name for what is currently the world's most powerful laser, located at Lawrence Livermore National Laboratory in California. It can produce pulses of 1.3 quadrillion (peta) watts for half a trillionth of a second, more than 1300 times the entire electrical generating capacity of the US, if only for a short time. It can do some interesting physics, too. Stephen P. Hatchett of Livermore will describe how the laser can produce highly improved, sub-millimeter resolution images of objects through 145 mm of lead (almost 6 inches). (Stephen P. Hatchett and Barbara F. Lasinski, LLNL, 925-422-5916, Papers B1S.06 and B1I2.03.) Shining the laser on a gold target, other researchers have ejected electrons with as much as 100 MeV energy, a new record for electrons coming from a solid. (Previous experiments with solids have only observed electrons with several MeV.) In addition to shedding insights on the fundamental interplay between light and solids, studying such electrons may help physicists develop models for understanding the generation of gamma ray bursts and other phenomena in high-energy astrophysics. (Thomas E. Cowan, Michael D. Perry, Thomas C. Sangster and Scott C. Wilks, Lawrence Livermore National Laboratory, 925-422-9678, Papers K6F.02 and K6F.06).

Some of the high-energy electrons created at Livermore's Petawatt laser (see item above) pass straight through the solid material in which they are created; as they penetrate the material they are significantly slowed down. In their abrupt deceleration, they produce high-energy photons. Researchers have observed these photons to induce nuclear fission of uranium-238 and create positron-electron pairs. This result is striking because the process of nuclear fission is usually initiated by a massive particle such as a neutron. Although photon-induced nuclear fission and positron production have been seen before, the advantages of the Petawatt laser light may allow researchers to obtain newly detailed information on thermonuclear processes. (Thomas E. Cowan and coworkers, LLNL, Univ. Alabama-Huntsville, NASA-Marshall Space Flight Center, Harvard Univ., and University Space Research Assoc., 925-422-9678, Postdeadline Paper at the meeting).

Producing ultrashort, ultrapowerful laser pulses typically requires equipment with prohibitive costs. In new theoretical work, researchers have now proposed a tabletop scheme for producing such pulses by colliding a short laser pulse with a long laser pulse inside a plasma. Theoretical simulations of this process show that the short pulse would remain short while being amplified by orders of magnitude. This method of ultra-short [less than 10 femtoseconds (fs), where 1 fs=10^15 s] pulse amplification may provide an alternative to the widely used chirped-pulse amplification technique, which requires large and expensive gratings. First experiments on this technique are slated for January 1999 at the Max-Planck Institute for Quantum Optics (Gennady Shvets and N. Fisch, Princeton, 609-243-2609, A. Pukhov, MPQ, Garching, J5Q23).

In inertial confinement fusion, a laser or other energy source implodes a capsule containing nuclear fuel, and heats its contents to the high temperatures and densities necessary for nuclear fusion to occur. For the first time, researchers have made experimental measurements of the energy spectrum of the charged particles produced in capsule implosions. Obtained at the Omega laser facility at the University of Rochester, these measurements provide new insights into the physical conditions in the imploding capsules. Interestingly, the researchers have discovered that imploding capsules with the same mixtures of nuclear fuel inside the capsule can produce a wide range of outcomes, apparently because of the complexities of the implosion process. (Richard D. Petrasso, MIT, 617-253-8458, Q7I2.06).

In inertial confinement fusion experiments planned at the National Ignition Facility (NIF) being built at Livermore, a set of 192 powerful laser beams will converge not on a fuel pellet directly, but rather inside a gold cylinder called a "holhraum" (a German word meaning "cavity"). In this scheme, the laser light turns gas inside the hohlraum into a hot plasma (which then will tend to flow out of the laser entrance holes in the hohlraum), and the hohlraum's hot gold walls generate x-rays which symmetrically heat and compress a fusion fuel pellet at the center. Researchers have discovered a new phenomenon which could compromise this process: When two laser beams with the same color cross paths in a plasma, energy can be transferred from one beam to the other when the velocity at which the plasma flows equals the speed of sound in the plasma. This phenomenon may cause unwanted energy transfer between the NIF beams, preventing a target from being heated uniformly. This "resonant energy transfer" has been observed and measured at the Nova laser at Livermore and at the laser facility at LULI in France. Researchers at the meeting will propose solutions to the problem. (Kenneth B. Wharton, UCLA and LLNL, 925-422-2081, B1I2.06; C. Labaune, LULI, Ecole Polytechnique, B1I2.04).

In a burgeoning plasma physics field called "laboratory astrophysics," researchers are creating plasmas which simulate astrophysical phenomena such as exploding stars and galaxy formation. In the past year, Caltech researchers have produced improved laboratory versions of solar prominences, huge luminous arches extending outwards from the surface of the sun. Using a newly designed plasma gun and a two-camera system, they have obtained 3-D photographs that provide useful insights into how prominences evolve. In particular, the 3D photographs show helix-shaped writhes, and the development of twisted filaments that thread each other. These experiments also show that the simulated prominence depends strongly on the voltage distribution over the plane corresponding to the surface of the sun. This voltage has been ignored in the past, but the experiments suggest it affects both the formation and shape of prominences because it drives electric current along the bundles of magnetic field lines in the prominence, producing light and twisting the plasma structure. (Freddy Hansen and Paul M. Bellan, 626-395-4827, Caltech, F3S.35; image at www.aip.org/physnews/graphics/html/prom.htm ).

In the magnetic fusion approach, magnetic fields trap a hot plasma and allow it to reach the condtions necessary for nuclear fusion. The most widely used device for achieving these fusion conditions is a doughnut-shaped chamber called a tokamak, which produces multilayered magnetic fields to trap the plasma and allow it to reach high temperatures and densities. However, large-scale and small-scale turbulence in the plasma hinders the process somewhat by causing particles and heat to leak out of the tokamak. Researchers have recently found ways to reduce this leakage by causing the plasma to flow around the system parallel to the walls of the chamber, but with different speeds at different points in the multilayered field structure. This flow is produced by making an electric field in the plasma which changes with position. As Keith H. Burrell of General Atomics points out, the flow produced by the combined effects the magnetic fields and the electric fields on the particles in the plasma produces shear forces that reduce small-scale turbulence and the loss of particles and heat, even when energy is added to produce the so-called "E x B flow." This fortuitous situation not only improves fusion conditions in tokamaks, it provides some fascinating basic physics: introducing energy into a turbulent system usually increases turbulence and heat loss, rather than decreasing these quantities (Keith H. Burrell, General Atomics, 619-455-2278, C2E.03). Zhihong Lin (zlin@pppl.gov) of the Princeton Plasma Physics Lab will present three-dimensional, massively parallel computer code simulations of microturbulence in a tokamak plasma. These simulations, which employ 400 million plasma particles, agree well with the experimental results and provide new insights into the physics of the system (Zhihong Lin et al., Princeton University, F3Q.01).

* Modified per author's instructions, 11/13/98 1:00pm ET US, from "21 Megawatts of power"

For more information, please contact
Ben Stein, American Institute of Physics,
301-209-3091, bstein@aip.acp.org,
(Reachable from 11/13 onward)
Bruce Remington, APS Division of Plasma Physics,
925-423-2712, remington2@llnl.gov
or Saralyn Stewart, APS Division of Plasma Physics,
504-529-7111 at the meeting (11/12-11/20), 512-471-4378 at other times stewart@peaches.ph.utexas.edu

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