Physics News Update, Number 421

Number 421, March 31, 1999 by Phillip F. Schewe and Ben Stein

CREATING ANTIMATTER WITH LASER LIGHT. Intense light from the Petawatt laser at Livermore, the world's most powerful laser, has been directed onto a thin gold film where it creates a plasma plume which acts as a sort of messy wakefield accelerator. In particular the laser electric fields rip electrons from the gold atoms and send the electrons shooting off with energies as high as 100 MeV. Some of these electrons radiate gamma rays which in turn can create electron-positron pairs (the first antimatter made in laser-solid interactions) and can also induce fission. Thus laser photons at the electron-volt level can, by teaming up, initiate the sort of million-electron-volt nuclear reactions that normally take place only at an accelerator. Moreover, the femtosecond laser pulses can be focused to a much smaller spot size than is possible with any conventional particle beam. Tom Cowan (925-422-9678, tcowan@llnl.gov) reported these results at last week's APS Centennial Meeting in Atlanta (see figure at Physics News Graphics).

TABLETOP THERMONUCLEAR FUSION. Yet another Livermore photonuclear breakthrough was reported at the APS meeting. Todd Ditmire described an experiment in which laser pulses (35 fsec long and intensities as high as 10¹⁷ W/cm²) were absorbed by a gas jet of deuterium molecules. These molecules actually resided in clusters (average size of 5 nm) which exploded under the laser bombardment. Some of the rocketing D's fused into helium-3 nuclei plus energetic neutrons. The neutrons, showing up with a characteristic energy of 2.45 MeV, were detected (about 10,000 per laser shot) via a time-of-flight technique. Ditmire said that this new approach to promoting fusion reactions (executed with a setup that fits on a 4'x11' table) could probably not be scaled up to provide commercial power, but that it might provide a cheap source of neutrons. The whole process is highly efficient: virtually all the laser energy was converted into ion kinetic energy.

MOLECULAR ASTROPHYSICS. To understand how molecules form in space, earthbound scientists are performing laboratory experiments that simulate the cold interstellar dust and gas clouds where molecules are manufactured. Some researchers study the formation of H2, the universe's simplest and most abundant molecule. Other researchers study the properties of polycyclic aromatic hydrocarbons (PAHs), flat rings of carbon and hydrogen which seem to exist in the interstellar clouds. At the APS meeting, Gianfranco Vidali of Syracuse (315-443-9115) presented studies on how two hydrogen atoms join together on an interstellar dust grain. Shooting H atoms onto a solid target (playing the role of an interstellar dust grain, with a temperature of 10 K) and observing how many of the atoms would react on the cold surface to form molecular hydrogen, he and his colleagues found that the rate of H₂ formation was higher on amorphous carbon than on olivine (a silicon-oxygen based material), suggesting that the former is a more likely candidate for interstellar dust, whose composition is still unknown. Louis Allamandola (650-604-6890) and his colleagues at NASA-Ames discussed recent experiments showing that shining UV light on PAHs can convert them to organic compounds that are present in henna, aloe, and St. John's wort. Combined with spectroscopic measurements that support the existence of PAHs in interstellar clouds, these experiments advance the notion that PAHs may be the precursors of biologically important molecules on our planet and possibly others.