Fusion is the process that smashes together light elements in the form of plasma – the hot, charged state of matter composed of free electrons and atomic nuclei – generating massive amounts of energy. Confining and controlling the plasma is a key challenge in replicating fusion for a virtually inexhaustible supply of power to generate electricity.
Now, say the researchers, they have found unexpected currents arising in the plasma within doughnut-shaped fusion facilities known as tokamaks, which are used in nuclear-fusion research for magnetic confinement of plasma. The currents develop when a particular type of electromagnetic wave – such as those that radios and microwave ovens emit – forms spontaneously.
“It’s very important to understand which processes produce electrical currents in plasma and which phenomena could interfere with them,” says Ian Ochs, graduate student in Princeton University’s Program in Plasma Physics and lead author of a paper selected as a featured article in Physics of Plasmas. “They are the primary tool we use to control plasma in magnetic fusion research.”
The researchers found that when the frequency of the electromagnetic waves is high, the wave causes some electrons to move forward and others backward. The two motions cancel each other out and no current occurs.
However, when the frequency is low, the waves pushes forward on the electrons and backward on the atomic nuclei – or ions – creating a net electrical current. The researchers say they were surprised to find that they could create these currents when the low-frequency wave was a particular type – called an “ion acoustic wave” – that resembles sound waves in air.
The significance of this finding, say the researchers, extends from the relatively small scale of the laboratory to the vast scale of the cosmos.
“There are magnetic fields throughout the universe on different scales, including the size of galaxies, and we don’t really know how they got there,” says Ochs. “The mechanism we discovered could have helped seed cosmic magnetic fields, and any new mechanisms that can produce magnetic fields are interesting to the astrophysics community.”
The results from the pencil-and-paper calculations, say the researchers, consist of mathematical expressions that give scientists the ability to calculate how these currents, which occur without electrons directly interacting, develop and grow. The results deepen understanding of a basic physical phenomenon and appear to contradict the conventional notion that current drives require electron collisions.
Nathaniel Fisch, a coauthor of the paper, professor and associate chair of the Department of Astrophysical Sciences, and director of the Program in Plasma Physics, says, “The question of whether waves can drive any current in plasma is actually very deep and goes to the fundamental interactions of waves in plasma.”
These findings, say the researchers, lay the groundwork for future research. For more, see “Momentum-exchange current drive by electrostatic waves in an unmagnetized collisionless plasma.”
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