Scientists have found that electrical currents can form in ways not known before. The novel findings could give researchers greater ability to bring the fusion energy that drives the sun and stars to Earth.
From: DOE/Princeton Plasma Physics
Laboratory
September 4, 2020 -- Electric current is
everywhere, from powering homes to controlling the plasma that fuels fusion
reactions to possibly giving rise to vast cosmic magnetic fields. Now, scientists
at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory
(PPPL) have found that electrical currents can form in ways not known before.
The novel findings could give researchers greater ability to bring the fusion
energy that drives the sun and stars to Earth.
"It's very important to understand
which processes produce electrical currents in plasma and which phenomena could
interfere with them," said 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."
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. Scientists are seeking to replicate fusion for a virtually
inexhaustible supply of power to generate electricity.
The unexpected currents arise in the
plasma within doughnut-shaped fusion facilities known as tokamaks. The currents
develop when a particular type of electromagnetic wave, such as those that
radios and microwave ovens emit, forms spontaneously. These waves push some of
the already-moving electrons, "which ride the wave like surfers on a
surfboard," said Ochs.
But the frequencies of these waves
matter. When the frequency 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 after all. Ochs found that researchers
could surprisingly 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 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," Ochs said. "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 consist of mathematical expressions that give scientists the
ability to calculate how these currents, which occur without electrons directly
interacting, develop and grow. "The formulation of these expressions was
not straightforward," Ochs said. "We had to condense the findings so
they would be sufficiently clear and use simple expressions to capture the key
physics."
The results deepen understanding of a
basic physical phenomenon and were also unexpected. They appear to contradict
the conventional notion that current drives require electron collisions, Ochs
said.
"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," said 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. "What Ochs
derived in masterful, didactic fashion, with mathematical rigor, was not only
how these effects are sometimes balanced, but also how these effects sometimes
conspire to allow the formation of net electrical currents."
These findings lay the groundwork for
future research. "What especially excites me," Fisch said, "is
that the mathematical formalism that Ochs has built, together with the physical
intuitions and insights that he has acquired, now put him in a position either
to challenge or to put on a firm foundation even more curious behavior in the
interactions of waves with resonant particles in plasma."
Story Source:
Materials provided
by DOE/Princeton
Plasma Physics Laboratory. Original
written by Raphael Rosen. Note: Content may be edited for style and
length.
Journal Reference:
- Ian
E. Ochs, Nathaniel J. Fisch. Momentum-exchange current drive by
electrostatic waves in an unmagnetized collisionless plasma. Physics
of Plasmas, 2020; 27 (6): 062109 DOI: 10.1063/5.0011516
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