Novel Plasma Applications Highlight 1995 DPP Meeting

More than 1,200 plasma scientists gathered in Louisville, Kentucky to hear about the latest in plasma applications, as well as plasma transport, laser plasmas, fusion and the magnetohydrodynamic (MHD) process, during the 37th annual meeting of the APS Division of Plasma Physics (DPP), held 6-10 November. Over 1,450 papers were presented, including five review and prize addresses and 58 invited talks. There were three special evening symposia featured a panel discussion on challenges to physics graduate education, future directions in plasma physics and fusion research, and short-pulse lasers and wakefields.

In addition, the DPP program included sessions on: career/employment issues, including one focusing on mid-career changes; science education; public outreach; government science policy; women in plasma physics; and human rights, among others.

Plasma Applications. Researchers at the Princeton Plasma Physics Laboratory, in conjunction with the Charged Injection Corporation (CIC), have been working on electrostatic atomization and its applications to fuel injection, paint spray, and agriculture, among other areas. The team analyzed charged droplet sprays generated by a simple capillary source - using a quadrupole mass spectrometer and a charge detector - and found that the droplet distribution shows complex mono- and multimodal distributions for a given charge-to-mass ratio. In addition to the atomization experiment, an extensive numerical modeling effort was made to help develop electrostatic sprays at unlimited flow rates with arbitrary droplet sizes.

Improvements in plasma processing - in which a partially ionized gas is used for semiconductor etching reactions - continue to increase the technique's applicability to the industry. Scientists at the University of Wisconsin, Madison carried out a series of etching experiments in three types of electrodeless high-density, low-pressure etch tools plasma sources: electron cyclotron resonance, inductively coupled and helicon etching tools.

Although the physical processes resulting in ionization and electron heating in the three sources are quite different, the results showed that the etch rate process is identical for the three tools when viewed from the wafer sheath boundary. "In a real sense, the etch rate does not depend on the plasma source when high density-low pressure sources are employed," said group leader Noah Hershkowitz. "Major differences in tool etch characteristics are more likely determined by tool wall material, chemistry, and geometry."

Francis Chen of the University of California, Los Angeles, receipient of the 1995 APS Maxwell Prize, reported that new high-density RF plasma sources are needed for the fabrication of the next generation of computer chips, which will be faster and smaller, requiring more exacting processing techniques. The newest of these is the helicon source, based on low-frequency whistler waves.

The pulsed laser ablation technique for deposition of thin films has proven extremely successful at growing high-quality films of very complex and novel materials, such as high-temperature superconducting compounds and diamond-like carbon. Modeling this can be difficult because of the complex physics involved, including laser-solid interactions at the target, plasma formation off the target, vapor/plasma plume transport towards the deposition substrate, and plume-solid interactions at the substrate. Scientists at Oak Ridge National Laboratory (ORNL) have developed a global physics and computational approach to the laser ablation process.

While the team concentrated on silicon to experimentally confirm their models, ORNL's Jean-Noel Leboeuf said, "the application of our physics results go beyond silicon, given the universality of many experimental observations, such as plume splitting, for a wide variety of laser-ablated materials, including carbon, copper, or yttrium."

Transport and Self-Organization. Chaotic radial transport plays a central role in the formation and evolution of energetic particle populations trapped in planetary magnetospheres. A recent experiment at Columbia University used electron-cyclotron resonance heating to create a localized population of magnetically trapped, energetic electrons which periodically became unstable. The observed instabilities drove electrostatic fluctuations, which in turn resonated with the precessional drift motion of energetic electronics. They found that increases in the flux of energized electrons to the detector occurred only when fluctuations which met the conditions for global chaos were present, according to Columbia's Harry Warren. Furthermore, transport simulations indicated that the particle motion is strongly chaotic. Quasilinear models do not reproduce several important features of the experimental measurements.

Scientists at Oak Ridge National Laboratory have developed a model for transport based on the concept of self-organized criticality (SOC). The model seeks to describe the dynamics of the transport without relying on the underlying local fluctuation mechanisms, according to ORNL's D.E. Newman. The dominant scales are system sizes rather than the underlying local fluctuation scales.

Reverse Shear Discharges. A promising operating scenario for the next generation of high-performance non-inductively driven tokamaks is the use of plasma discharges with reverse central magnetic shear. This allows access to the second stability regime, which has produced high values of beta in previous experiments. Recently, Princeton Plasma Physics Laboratory (TFTR) and General Atomic (DIII-D) tokamak researchers performed experimental investigations in this regime in an attempt to improve tokamak plasma confinement and stability at high beta. The effects are dramatic. The particle confinement is enhanced, the thermal losses are reduced by a factor of over 40, and the central density increases by a factor of 3 in the shear-reversed region in TFTR. Future emphasis, say fusion scientists, will be to extend the volume of the reversed-shear regime.

Inertial Fusion. Understanding drive symmetry in gas-filled hohlraums is currently of interest because the baseline design of the indirect drive ignition target for the planned National Ignition Facility (NIF) uses such a device. Scientists at the Nova laser facility at Lawrence Livermore National Laboratory conducted a series of symmetry measurement experiments using thin wall gold hohlraums filled with methane or propane gas, in which the gas serves to tamp the motion of the gold ablating from the hohlraum walls, reducing spot motion and swings in drive symmetry. The results showed that the gas is effective in impeding the motion of the wall blowoff material, and that the resulting implosion performance of the capsule is not significantly degraded from vacuum result. The LLNL team also obtained data on neutron yield, implosion time, and spectroscopy of argon emission from the imploded core.

Laser Plasma. Researchers at the University of Maryland have successfully demonstrated the channeling of intense laser pulses over distances much greater than a Rayleigh length. They used a two-pulse technique in which the first pulse prepares a shock driven, axially-extended radial electron density profile which guides the second pulse, injected after an adjustable delay, according to group leader H.M. Milchberg.

Livermore's L.B. Da Silva has found that the reliability and characteristics of collisionally pumped soft x-ray lasers make them ideal for a wide variety of plasma diagnostics. His team has used x-ray lasers to probe high-density, laser-produced plasmas by taking advantage of recently developed multilayer beamsplitters to construct an interferometer operating at 255 angstroms. They have also combined x-ray lasers with a multilayer imaging system to study hydrodynamic imprinting of laser speckle pattern on directly driven thin foils, and used x-ray laser moire deflectometry to measure the electron density profile in ICF hohlraums.

Fluctuations and Transport in Toroidal Systems. Herbert L. Berk of the University of Texas at Austin's Institute for Fusion Studies has developed a basic nonlinear theory for a kinetic system driven by a source of energetic particles, which he believes is directly applicable to such problems as the bump-on-tail instability and fishbone oscillations, as well as the alpha particle interaction with Alfven waves in a fusion reactor.

Recent measurements of magnetic fluctuation-induced electron thermal transport confirm key aspects of the theory that accounts for clumping of electrons that stream along the magnetic field, according to the University of Wisconsin, Madison's P.W. Terry.

Explosive Instabilities and Detonation in MHD. According to UCLA's Steven Cowley, many plasma systems exhibit large scale explosive events, such as solar flares, magnetic substorms, and tokamak disruptions, which almost always require nonlinear destabilization to achieve their fast time scales. He has developed a new mechanism for explosive behavior using a nonlinear MHD model of the instability, in which the system crosses the instability threshold in a small region of space. The model demonstrates that the nonlinearity causes the linear instability to broaden into the linearly stable region, forming shocks.