What is the most common form of matter in the universe? It's not solid, liquid, or even gas. The answer is plasma--collections of electrically charged particles such as electrons and protons. Plasmas make up astrophysical objects such as stars and supernovas. On Earth, they exist naturally as lightning bolts and artificially as the charged particles inside fluorescent lightbulbs.
Physicists will discuss the latest discoveries in the universe of plasmas when the American Physical Society (APS) Division of Plasma Physics holds its annual meeting on November 15-19, 1999 at the Westin Hotel in Seattle, Washington. The APS, with over 40,000 members, is the largest professional organization in the world devoted to physics. DPP is one of its largest divisions.
IMAGES AND MEETING PROGRAM ON THE WEB
This press release with many associated pictures are available on the APS website: (http://www.aps.org/meet/DPP99/press/ ) The complete meeting program can be found on the web at http://www.aps.org/meet/DPP99/baps/
PLASMA RESEARCH IN 1999--ALREADY A REMARKABLE YEAR
In the spring, Livermore physicists (contact Todd Ditmire, 925-422-1349) announced that they had produced modest amounts of nuclear fusion on a tabletop--by shining a laser pulse on a small cluster of deuterium and tritium atoms. While tabletop nuclear fusion cannot generate enough power to be used as a practical energy source, the byproducts of the fusion reactions can provide neutrons useful for testing industrial materials and pursuing deep questions in basic physics.
Conducting basic research into the plasmas created by laser removal of material, University of Michigan researchers (contact Peter Pronko, 734-763-6008) accidentally discovered a tabletop method for separating chemical isotopes of the same element, providing a more compact alternative to the bulky techniques for extracting isotopes. Livermore researchers (contact Tom Cowan, 925-422-9678) created antimatter with laser light using the Petawatt, the world's most powerful laser. The Petawatt continues to produce new surprises in plasma physics, some of which will be reported at this meeting.
MEETING HIGHLIGHTS
Here are some highlights from among the many papers being given at the Seattle meeting:
---Solar Eruptions: A New Explanation. In efforts that may ultimately improve forecasting of space weather, Naval Research Laboratory scientists have come up with a new explanation for what triggers coronal mass ejections (CMEs), violent eruptions of plasma from the Sun. An important determinant of the environment between Earth and the Sun, CMEs can create geomagnetic storms which interfere with cell-phone communications on Earth. The prevailing theory for CMEs says that the energy responsible for these eruptions comes from the corona, the Sun's outermost atmosphere. But this theory often clashes with actual observations of CMEs. Examining a wealth of new data on CMEs from the SOHO spacecraft, James Chen (202-767-3134, chen@ppdchen.nrl.navy.mil ) and Jonathan Krall (202-404-7719; krall@ppdu.nrl.navy.mil ) of the Naval Research Laboratory argue that the magnetic energy responsible for these eruptions (about 10^15 grams of mass ejected at speeds of up to1000 km/s) is stored below the photosphere, the visible solar surface underneath the corona. Their explanation involves the concept of "solar flux ropes," giant magnetic field loops rooted below the photosphere. When sub-photospheric processes increase the amount of electrical current along a flux rope, the rope expands, taking plasma (mostly protons and electrons) with it and ejecting it into interplanetary space. The theory also addresses the properties of observed "magnetic clouds," interplanetary solar-wind structures believed to be associated with CMEs. Amazingly, particle signatures of magnetic clouds suggest that the flux ropes from the Sun can stretch out all the way to Earth while staying rooted in the photosphere. (Paper FO1.05)
---Advances in Plasmatrons for Cars and Trucks. Dan Cohn of MIT (617-253-5524) will report advances in a fuel-preparation device known as a plasmatron. A wine-bottle-sized device, the plasmatron can greatly reduce pollution emissions in vehicles while being completely compatible with conventional automobile technology. Head of the MIT Plasma Technology Division, Cohn believes that the plasmatron can be a "game changer" in the automobile field. Specifically, Cohn believes that the plasmatron can provide a reasonable alternative to much publicized fuel-cells--considerably sooner and at much lower cost--if implemented in hybrid electric-gasoline vehicles which offer high fuel efficiency. How does this device work? When connected to a fuel tank, the plasmatron converts some of the fuel into a hydrogen-rich gas. The hydrogen then travels to the engine along with untreated fuel. Because of its favorable combustion properties, the hydrogen enables the engine to run with a greater proportion of air--bringing about a lower engine temperature (greatly reducing nitrogen oxide pollutants) and more efficient operation (because of the properties of air molecules). Cohn now estimates that the plasmatron can reduce hydrocarbon emissions by up to 90% at engine startup, the time at which most automotive emissions occur. Along with co-plasmatron inventors Leslie Bromberg and Alexander Rabinovich at MIT, Cohn has done work indicating that employing the plasmatron in diesel engines (widely used in trucks) might significantly reduce pollution in those vehicles. With the success of their laboratory tests, Cohn and his colleagues have proposed to demonstrate the plasmatron within a year in a bus that runs on natural gas, with the aim of significantly reducing the smog that results from these vehicles (Paper SI2.04).
---Neutrino Mechanism May Resolve Supernova Mystery. Few things in nature are as powerful as a type II supernova--a star which blows up after collapsing under its own weight. Surprisingly, current physics theories cannot explain the origin of all the energy required to fuel these powerful explosions. In fact, present-day computer simulations--reflecting conventional understanding of supernova physics--predict that the supernova's explosion should stall after a few moments, preventing the supernovae from occurring. What is providing the missing energy required to sustain these explosions? Scientists have pointed to neutrinos, produced abundantly in all these processes. But there's a problem--neutrinos rarely interact with matter. Even if one adds up all the interactions between individual neutrinos and single particles of ordinary matter in these environments, neutrinos just can't transfer enough energy to explain this process. Now, physicists have identified a new neutrino mechanism which may resolve the problem. Physicists have shown that intense fluxes of neutrinos--present in supernova environments--can drive collective motions in groups of electrons or other particles. For example, neutrinos moving through a neutral plasma (consisting of electrons and ions) can kick some electrons from their original position, but keep the heavier ions in place, setting up an electric field which creates an collective motion of electrons bouncing back and forth. Exactly analogous to what happens when photons and electrons pass through neutral plasmas, these collective interactions often result in a greater energy transfer from neutrinos to matter than is possible with a single-particle mechanism. Presenting the first rigorous framework for these interactions, Luis Silva of UCLA (310-825-4683; silva@physics.ucla.edu ) will show that sufficient neutrino energy can be deposited into a supernova shock wave through these interactions--providing an important contribution to prevent the shock from stalling and keeping the explosion going. In addition, collective neutrino interactions might be important in explaining the birth kicks of neutron stars, and in explaining the "energy deficit" in certain explanations of gamma-ray bursts by collective conversion of neutrino fluxes into gamma rays. (RP1.107, QI1.06)
---Powerful Protons from a Tiny Spot. In a development that may provide benefits to electronics manufacturing and medical radiation therapy, Livermore researchers have devised a way to generate intense beams of powerful ions from a tiny spot. Using a single pulse of light from Livermore's Petawatt laser, the most powerful in the world, the researchers have generated 30 trillion protons with energies of up to 50 MeV, from a tiny spot approximately 400 microns in size. Although no other laser is as powerful as the Petawatt, the researchers nonetheless believe that their technique can be widely applied to provide more compact sources of high-velocity ions than previously possible. In their demonstration, a single laser pulse strikes a thin slab of plastic or gold, ejecting electrons which form a cloud of negative charge around the back of the target. The cloud pulls positively charged ions from the back of this target which are rapidly accelerated to high energies. The ions are accelerated to extremely high energies over a short distance (almost 1 MeV/micron for protons)--orders of magnitude higher than conventional ion accelerators. In principle, any type of high-velocity ion can be generated simply by depositing atoms of the desired species onto the back of the target. The researchers envision the possibility of creating an "ion lens"--by shaping a concave section from a target, one can imagine that the ejected ions focus toward a point, further enhancing the brightness of the ion beam. (Talks FI2.04, O1.11, QO1.12; contact Scott Wilks, 925-422-2974, Steve Hatchett, 925-422-5916, Richard Snavely, 925-423-8597, rasnavely@llnl.gov )
---Shock Waves in Dusty Plasmas. We usually think of plasma as a gaslike substance, with particles flying by one another with little deflection. But under the right conditions, physicists can make plasmas act like a liquid or solid, in which particles sit almost stationary, right next to their nearest neighbors and interacting almost exclusively with their nearest neighbors. This is especially true when plasmas are mixed with dust, as is the case in interstellar space. In laboratory experiments at the University of Iowa (John Goree, 319-335-1843, john-goree@uiowa.edu ), the "dusty plasmas" are micron-sized spheres loaded up with approximately 10,000 electrons apiece. When illuminated by an intense sheet of light, the researchers can see the movements of these particles in a way that is not possible with real atomic matter. For this reason, they act as a model system for gaining insights into the properties of solid and liquid matter at the atomic scale. By firing a particle at the dusty plasma at supersonic speeds, the researchers produced a Mach cone, similar to the V-shaped shock wave produced by a supersonic airplane. Mach cones are well known in gases (airplanes for example), but almost unknown in solids. One of the only other known examples is in seismology: a sound wave traveling down the surface of a liquid-filled borehole moves faster than the sound speed in the surrounding rock, causing a Mach cone to be produced in the rock. (Paper H12.02)
---Recreating Supernova Remnants. In the laboratory, scientists now have the ability to simulate exploding stars, or supernovae, by shining lasers (which provide an energy source for the explosion) on a target made of light elements (which simulate starstuff). Previous experiments concentrated on the supernova explosion itself. Now, Paul Drake of the University of Michigan (734-763-4072, rpdrake@umich.edu ) and colleagues have been simulating an event currently being observed in the aftermath of Supernova 1987A, an exploding star first seen 12 years ago. They are studying the structures formed as the supernova's shock wave approaches a dense ring of gas. By observing the structures formed as their laboratory shock wave approaches a dense plastic plate, they are gaining important insights and testing computer models of the boundary between the exploding star and the gas ring. These experiments are helping the scientists to understand the structures produced within a supernova remnant after an explosion. (QI1.03)
---New Insights into Vortex Crystals. It's difficult to anticipate all of the patterns that can arise in nature, but some patterns seem to defy concepts of what is physically possible. For several years, UC-San Diego researchers have been observing surprising patterns in turbulent plasmas of electrons. In their experiments, they trap billions of electrons in magnetic fields to make them act like fluid particles flowing turbulently on a flat surface. Many important turbulent flows in nature are principally two dimensional, such as the Great Red Spot of Jupiter and large-scale ocean currents. The researchers have noticed that the electrons can spontaneously form a "vortex crystal," consisting of 2-20 tightly spinning whirlpools frozen in place amidst an utterly turbulent background. Physicists have lacked a comprehensive theory of what enables these structures to arise. Presenting the first quantitative theory of vortex crystals, Dezhe Jin of UC-San Diego (858-534-6883; djin@physics.ucsd.edu ) says that the large whirlpools or vortices must shuffle around other particles in a flow to optimize how randomly these background particles are distributed--thereby maximizing the amount of disorder, and creating the most stable state for the system--before they have the opportunity to merge with one another and form a single larger whirlpool. Says Jin, "We should not be surprised if some day we observe orderly sets of large scale hurricanes or storms in large scale fluid systems of some planets--even the Earth or on Jupiter."(UI2.04)