Unexpected electrical current that could stabilize fusion reactions


Date:
September 4, 2020
Source:
DOE/Princeton Plasma Physics Laboratory
Summary:
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.



FULL STORY ========================================================================== 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:
1. 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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200904125106.htm

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