Fusion Pellets, Most Powerful Laser Highlight DPP Meeting

Highlights from the Fall 1998 DPP meeting.

Reaching Out for Plasma Science In addition to the usual technical program, a number of science education and outreach activities were featured during the DPP meeting in New Orleans. For example, the DPP Concerns of Young Scientists Committee sponsored a workshop on career opportunities on Tuesday, and two invited talks on science education were featured at a special Wednesday afternoon session. Also featured was the 11th annual Science Teachers' Day, including workshops with area educators, followed by the poster session of undergraduate research and education outreach projects all day Tuesday. This year, the program focused on the science and technology of plasmas and plasma applications, with a special emphasis on fusion energy, and was organized by MIT with support from various universities, laboratories and industry. The 5th annual Plasma Science Expo was held all day Thursday, intended as a means for the DPP scientific community to share some of the exciting challenges of plasma science with the local community. A variety of industrial exhibitors, laboratories and universities sponsored displays for the event, many of which were interactive hands-on exhibits. On display throughout the DPP meeting was a large, poster display - created for the upcoming APS Centennial Meeting in March 1999 - to acquaint APS unit members and the general public with the importance and applicability of plasma physics.

The latest discoveries in the universe of plasmas were presented during the annual meeting of the APS Division of Plasma Physics (DPP), held November 16-20, 1998 in New Orleans, Louisiana. Over 1,500 attended and presented approximately 1500 papers were presented at the meeting, which is one of the largest physics meetings in the world held each year.

Nuclear Fusion Research
A major pursuit of many plasma physicists is to develop nuclear fusion into an abundant source of energy for the world in the 21st century and beyond. In the last year, scientists at the Joint European Torus (JET) in England produced 21 Megawatts of power -a new world record for nuclear fusion. Like all fusion demonstrations to date, the recent JET experiment did not generate as much power as had been poured into the reactor to start the fusion process. Still, the ratio of output power to input power was a record 65%, more than double previous records. There were also dramatic advances in a promising approach for achieving fusion known as "Z-pinch."

Physics with the Petawatt
The Petawatt is currently the world's most powerful laser, located at Lawrence Livermore National Laboratory. 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 U.S. LLNL's Stephen P. Hatchett described how the laser can produce highly improved, sub-millimeter resolution images of objects through 145 mm of lead. 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. When these electrons were made to decelerate rapidly and release high-energy photons as a result, the researchers observed the photons to induce nuclear fission of uranium-238. 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.

Photon-Induced Nuclear Fission and Positron Emission
Some of the high-energy electrons created at LLNL's Petawatt laser pass straight through the solid material in which they are created; as they penetrate, they are significantly slowed down, producing high-energy photons. Researchers have observed these photons to induce nuclear fission of uranium-238 and create positron-electron pairs. According to LLNL's Thomas E. Cowan, 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.

Less Expensive Road to Ultrashort, Powerful Lasers?
Producing ultrashort, ultrapowerful laser pulses typically requires equipment with prohibitive costs. In new theoretical work, researchers 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] 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 to begin this month at the Max-Planck Institute for Quantum Optics in Garching, Germany.

New Experimental Information on ICF
In inertial confinement fusion (ICF), 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.

Energy Transfer Between Crossed Beams in Flowing Plasmas
In ICF 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.

But 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. Several researchers at the meeting proposed possible solutions to the problem.

3-D Laboratory Simulations of Solar Eruptions: Laboratory simulation of solar prominences. (Image courtesy of Paul Bellan, Caltech)

3-D Laboratory Simulations of Solar Eruptions
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.

Reducing Energy and Particle Loss in Magnetic Fusion Reactors
In magnetic fusion, magnetic fields trap a hot plasma and allow it to reach the conditions necessary for nuclear fusion. The most widely used device for achieving these fusion conditions is 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.

It produces shear forces that reduce small-scale turbulence and the loss of particles and heat.