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January 2003 (Volume 12, Number 1)
Astrophysical Jets in the Lab.
Many astronomical objects, from galactic nuclei to black holes surrounded by accretion disks, emit very long plumes of plasma, called astrophysical jets. In a new laboratory plasma experiment, Caltech researchers have shown how magnetic forces can create these jets. Magnetic forces squeeze the plasma into a narrow plume and eject this plume along the axis, forming a jet- like structure. These results should help to shed light on the long-standing problem of how jets are formed. In the experiment, up to 150 kilo-Amperes of electric current are run through a hydrogen plasma inside a cylindrical metal chamber the size of a large closet. Some of the jet-like plumes show a spiral structure similar to what is occasionally observed in space, enabling researchers to improve their understanding of real astrophysical jets.
Lab-Based Recreations of Extreme Astrophysical Phenomena.
Using a new technique, researchers from Imperial College, London, and the Rutherford Appleton lab in the UK have created super-strong magnetic fields that are hundreds of times more intense than any previous magnetic field created in an Earth laboratory and up to a billion times stronger than our planet's natural magnetic field. Such intense magnetic fields may soon enable researchers to recreate, extreme astrophysical conditions, such as the atmospheres of neutron stars and white dwarfs (in their very own laboratories.)
At the Appleton Lab, researchers at the VULCAN facility aimed intense laser pulses, lasting only picoseconds, at a dense plasma. The resulting magnetic fields in the plasma were on the order of 400 Megagauss. Due to technological advances, peak laser intensities are likely to increase still further and consequently even higher magnetic fields may soon be possible, making it possible to put models of extreme astrophysical conditions to the test by using X-rays to generate nuclear fusion.
Working toward the vision of generating clean energy from nuclear fusion, researchers have successfully imploded fuel capsules by bombarding them with intense x-rays. The results show that the process generates significant fusion and that the implosion method looks capable of generating large-scale energy production.
In one set of experiments, a high degree of symmetry has been achieved in the implosion process. In another set of experiments, researchers observed significant production of neutrons, a sign of nuclear fusion. These successful experiments are an important step toward ignition, the level at which the fusion reaction becomes self-sustaining and excess energy can be drawn from the process for other applications.
First 3D Magnetic Reconnection Measurements.
In work that promises new insights into the cosmos and fusion-energy production alike, physicists have reported they have made the first 3D laboratory measurements of magnetic recon-nection, the main process by which magnetic fields release energy in the universe. This process is thought to heat the solar corona, as well as to accelerate particles to high energies, possibly even to the very high energies of cosmic rays. Magnetic recon- nection is also an important process in some experimental fusion energy reactors that use magnetic fields to confine the plasma.
Until recently, this process has been studied only in two dimensions. Now, 3D experimental measurements of magnetic reconnection have been made at the Swarthmore Spheromak Experiment (SSX) at Swarthmore College. At SSX, physicists merge rings of magnetized plasma called spheromaks. Measurements reveal a swept and sheared magnetic structure in the reconnection region. Researchers hope to elucidate fundamental plasma physics processes on the sun and understand new plasma structures in magnetic confinement fusion machines.
Hollow Plasma Doughnut Currents.
Doughnuts of plasma can be coaxed into configurations with hollow current rings, providing practical advantages over conventional "filled doughnut" shapes. Simulations suggest they will allow faster turn-on and greater efficiency of future nuclear fusion power plants. Plasma doughnuts normally carry large electric currents throughout their volume but researchers expected the direction of the current could be changed back and forth.
However, in recent experiments at the Joint European Torus (JET) and JT-60U tokamaks in England and Japan, researchers tried to reverse the current and found that the current doughnut became hollow. Now computer simulations conducted by researchers at the DOE's Princeton Plasma Physics Laboratory (PPPL) using super-computers at the National Energy Research Supercomputer Center have explained this phenomenon. Instead of the electric current reversing direction, the plasma experiences magnetic reconnection and the core becomes stabilized with zero current. As soon as a current tries to reverse in the center, it is pulled into the outer ring. This new understanding should allow a more practical design of compact next-generation fusion experiments.
Turbulence Restrains Itself.
Magnetically confined plasmas in tokamaks and related fusion devices exhibit a high degree of turbulence, which can generally destroy the optimal conditions for producing fusion energy. Now, scientists have experimentally confirmed that turbulence can actually limit its own ability to wreak havoc. Researchers at the DIII-D tokamak at General Atomics have discovered that turbulence generates its own flows that act as a self-regulating mechanism. These flows create a "shearing" action that destroys turbulent eddies.
These turbulent flows have been clearly observed in recent experiments at DIII-D by using a special imaging system, which is helping to advance researchers' understanding of this complex and crucial phenomena taking place in high temperature fusion plasmas.
The roiling turbulence inside tokamaks represents some of the most complex physics on the planet. Using the full power of the world's largest supercomputers, scientists have now been able to fully simulate the movement of tokamak particles and heat due to turbulence. Implementing new algorithms to incorporate very complex physics, they included the effects of super-fast electrons and the recent practice of rotating the plasma, for higher-pressure tokamak operation and higher-energy output. These simulations may also help greatly in making reliable predictions for larger tokamaks and future commercial-scale fusion reactors.
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