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Demonstrating a technique that may lead to advances in certain forms of radiation therapy and electronics manufacturing, Livermore's Petawatt, the world's most powerful laser, impinges upon a target to generate 30 trillion protons from a tiny spot only 400 microns in size. Two other research groups, in Michigan and the United Kingdom, have demonstrated this technique with smaller-scale lasers. Figure courtesy of Scott C. Wilks of Lawrence Livermore National Laboratory
Physicists discussed the latest discoveries in the universe of plasmas when the APS Division of Plasma Physics (DPP) -- one of the Society's largest units -- held its annual meeting November 15-19, 1999, in Seattle, Washington, capitalizing on what has proven to be a banner year for plasma research. Last spring, Livermore physicists 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 (see APS News, June 1999). Conducting basic research into the plasmas created by laser removal of material, University of Michigan researchers accidentally discovered a tabletop method for separating chemical isotopes of the same element. Other Livermore researchers created antimatter with laser light using the Petawatt, the world's most powerful laser.
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 and Jonathan Krall of the Naval Research Laboratory argue that the magnetic energy responsible for these eruptions (about 1015 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 with it and ejecting it into interplanetary space.
Advances in Plasmatrons
Dan Cohn of MIT reported on 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, he 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.
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 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.
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). 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.
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 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."
-Philip F. Schewe and Ben P. Stein, AIP Public Information
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