ICF, Plasma Astrophysics and Education Outreach Highlight 1996 DPP Meeting
Plasma scientists from around the world gathered in Denver, Colorado, to hear about the latest research in inertial confinement fusion (ICF), ITER physics, laser plasmas, astrophysics and plasma applications during the 1996 annual meeting of the APS Division of Plasma Physics (DPP), held 11-15 November. Over 1,400 papers were presented, including four review papers, one prize recipient address and 55 invited talks.
Several special symposia organized throughout the week featured such topics as fusion energy sciences restructuring, science education, plasma science career opportunities, and magnetic reconnection in the laboratory and space. The keynote speaker at Wednesday evening's banquet was James Randi, a conjuror, lecturer, author and amateur astronomer who received the APS Forum Award in 1989 for his success in exposing scientific fraud.
Inertial Confinement Fusion
Magnetically-driven z-pinch implosions have been used for more than 40 years to evaluate the effects of x-ray radiation on materials and electronics. Keith Matzen of Sandia National Laboratories reported on a new use for these implosions in a Monday morning review session, of special significance to the national stockpile stewardship program which requires that the safety and reliability of nuclear weapons be determined without underground tests. In a z-pinch implosion, electrical energy is coupled into the kinetic energy of a magnetically-driven plasma, formed from a gas puff, an annular array of wires, or a metal foil. On the Saturn and PBFA Z accelerators, a dramatic increase in x-ray energy and power from fast z-pinch implosions was obtained using an annular array of 100 to 200 thin wires in the shape of a cylindrical shell. When the confined plasma, produced by ionization of the wire material, "pinches" on axis, a large fraction of the energy is emitted as soft x rays. The improvement in x-ray power represents a new world laboratory record (by a factor of four).
In ICF, laser light irradiating a plasma at very high irradiances can induce nonlinear phenomena, such as parametric instabilities, and decay into various combinations of waves of different frequencies. Recent experiments at Los Alamos National Laboratory indicate that ion-driven parametric instabilities, which affect the propagation of the laser through plasmas, are prevalent in many ICF plasmas.
However, most particle-in-cell (PIC) algorithms are either incapable of simulating the actual physics behind the phenomenon, or computationally inefficient. So an LANL team of scientists have developed a 3-D hybrid PIC code written for the massively parallel CRAY-T3D platform. "We believe HERCULES is the first PIC computational tool capable of simulating low-frequency ion- driven parametric instabilities in a large, 3-D plasma volume, and offers a unique opportunity for examining issues that are potentially vital to ICF," said LANL's H.X. Vu of the new code.
ICF Laser Imprinting
In direct-drive ICF, in which laser light impinges directly on the ICF pellet containing the fusionable material, nonuniformities in laser illumination seed ripples at the ablation front in a process known as "imprinting." These nonuniformities grow during the capsule implosion and can penetrate the capsule shell, impede ignition, or degrade burn. Scientists at Lawrence Livermore National Laboratory have developed a novel technique for studying the imprint of a direct-drive laser beam on a thin foil, using an x-ray laser as a backlighter. This technique allows the LLNL team to measure small variations in the foil thickness, and in turn to measure modulations due to imprint.
Researchers at the Imperial College in London, England, have developed a novel direct-drive smoothing scheme called foam buffered direct drive which substantially reduces initial non-uniform imprinting. The foam plasma helps to smooth out any laser drive structure.
The International Thermonuclear Experimental Reactor/Engineering Design Activity (ITER/EDA) is a joint project of the European Union, Japan, the Russian Federation and the U.S. to carry out the engineering design of a reactor-scale tokamak capable of producing 1 to 1.5 GW of fusion power. According to John Wesley of the ITER Joint Central Team, it is expected to be the principal facility for fusion research for the period 2010-2030. It is being designed to be capable of conducting comprehensive physics studies of reactor-regime plasmas, and to reliably produce the fusion power level and burn duration needed for testing of reactor components at appreciable neutron fluence.
Many key issues concerning ITER's design still require work. Understanding the scaling and effect of plasma turbulence on ITER's performance is being vigorously pursued. Physicists must also find a method for producing plasma conditions with acceptably low peak heat loads.
Laser Plasma Astrophysics
Recent radio and x-ray observations of supernova SN1987A provide evidence for the shock interaction with an ionized region created in the dense plasma wind from a previous evolutionary phase, according to Roger Chevalier of the University of Virginia. The supernova's proximity "gives us an unprecedented opportunity to observe the development of the supernova shock wave as it interacts with mass lost prior to the explosion," he said, adding that the ionizing radiation from the progenitor star probably played an important role in shaping the supernova environment. In the case of another nearby supernova, SN1993J, dense gas was present close to the explosion, giving rise to a cooling shock wave and radiative phenomena at an early phase. Experiments simulating astrophysical shock conditions are being performed in high intensity laser plasma experiments at NRL and LLNL.
The MRX magnetic reconnection experiment at Princeton Plasma Physics Laboratory was constructed to investigate the fundamental physics of magnetic reconnection. PPPL's Masaaki Yamada reported on some of the results on Thursday evening. The initial experiment measured a two-dimensional profile of the neutral current sheet layer in which reconnection occurs and the relationship between the reconnection rate and plasma conditions, such as the merging angle and plasma conductivity.
Recent successes in confining antimatter in the form of positrons and antiprotons have created new scientific and technological opportunities, according to C.M. Surko of the University of California, San Diego, who described work by recent groups on trapping antimatter plasmas. Surko's group is studying the physics of electron-positron plasmas, considered relevant to astrophysical processes, as well as the interaction of an electron beam with a trapped positron plasma, and the interaction of cold positrons with atoms and molecules. In addition, scientists at CERN have succeeded in accumulating and cooling large numbers of antiprotons.
According to Surko, the ability to produce and trap cold antihydrogen atoms will enable precise comparisons of the properties of matter and antimatter, including tests of CPT invariance and the measurement of gravitational masses. Other scientific and technological uses of cold antiparticles include the transportation of antimatter plasmas in portable traps; the possible reflection of positronium or antihydrogen atoms from material surfaces at low temperatures; and potential medical uses of antiprotons.
Inductively coupled plasmas (ICPs) have been re-discovered by the multi-billion-dollar semiconductor industry as an important class of high-density, low-pressure plasma sources suitable for the manufacture of next-generation integrated circuits. However, according to M. Tuszewski of Los Alamos National Laboratory, the approach is still prohibitively expensive for upcoming 300-mm diameter wafers. There is an urgent need for basic ICP plasma research, such as experimental characterization and predictive numerical modeling. "Inductive radio frequency (rf) power absorption is fundamental to the ICP electron heating and resulting plasma transport, but remains poorly understood," said Tuszewski by way of example.
Photocathode-driven free electron lasers (FELs) are proving extremely attractive for material processing applications, according to Alan Todd of Northrop Grumman. These include broad- band tunability across the infrared and ultraviolet spectra; high peak and average radiated power for economic processing in quantity; and high brightness. The most promising areas for application are in polymer, metal and electronic material processing, micromachining and defense applications. Unfortunately, although the usefulness of the process has been proven, the power levels and costs of lamps and lasers do not yet scale to production margins, Todd reported.
Education and Outreach
The third annual Science Museum Open House was also a highlight of the DPP meeting, intended as a means of reaching out to the community to share the exciting challenges of plasma science and fusion. In addition to Thursday evening lectures on plasma science, several industrial exhibitors, laboratories and universities set up displays for the general public, primarily hands-on and interactive. In addition, the conference featured a special Science Teacher's Day focusing on the science and technology of plasmas and plasma applications, with an emphasis on fusion energy, co-sponsored by the DPP, APS, and General Atomics.