February 15, 2012
Researchers Break Down Science Behind Plasmas
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Researchers have discovered clues to help give scientists a better understanding of how plasma jets from the sun occur.The team from the Division of Engineering and Applied Science at the California Institute of Technology (Caltech) used high-speed cameras to look at jets of plasma in the lab.
During the experiments, they fired jets of hydrogen, nitrogen, and argon plasmas at speeds of about 6 to 30 miles per second across a distance of 8 inches in a vacuum.
The experiment required 200 million watts of power to produce jets that are 20,000 degrees Kelvin and carry a current of 100,000 amps.
The researchers used cameras that can take a snapshot in less than a microsecond, or one-millionth of a second.
They found the corkscrew shape that developed in the jets grew exponentially and extremely fast. The jets formed 8-inch-long coils in just 20 to 25 microseconds.
They also noticed that tiny ripples began appearing on the inner edge of the coil just before the jet broke, which is the moment when there was a magnetic reconnection.
After a few months of additional experiments the researchers determined that the kink instability spawns a completely different kind of phenomenon known as Rayleigh-Taylor instability.
This occurs when a heavy fluid that sits on top of a light fluid tries to trade places with the light fluid. During this process, ripples form and grow at the interface between the two, allowing the fluid to swap places.
The team realized that kink instability creates conditions that give rise to a Rayleigh-Taylor instability. As the coiled plasma expands because of the kink instability, it accelerates outward.
The plasma tries to swap places with the trailing vacuum by forming ripples that expand. The Rayleigh-Taylor instability is revealed by the ripples on the trailing side of the accelerating plasma, and it grows in about a microsecond.
Paul Bellan, professor of applied physics at Caltech, said that although the Rayleigh-Taylor instability has been studied for over 100 years, no one had considered the possibility that it could be caused by a kink instability.
He said the two types of instabilities are so different that to see them so closely coupled was a shock.
The researchers say one of the key advances in this study is being able to relate the phenoma at large scales, such as the kink instability, to those at small scales, like the Rayleigh-Taylor instability.
They said that although kink and Rayleigh-Taylor instabilities may not drive magnetic reconnection in all cases, this mechanism is a plausible explanation for at least some scenarios in nature and the lab.
The research was published in the February 16 issue of the journal Nature.
Image Caption: An argon plasma jet forms a rapidly growing corkscrew, known as a kink instability. This instability causes an even faster-developing behavior called a Rayleigh-Taylor instability, in which ripples grow and tear the jet apart. This phenomenon, the Caltech researchers say, has never been seen before and could be important in understanding solar flares and in developing nuclear fusion as a future energy source. Credit: A. L. Moser and P. M. Bellan, Caltech
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