Scientists detect characteristics of the birth of a major challenge to harvesting fusion energy on Earth

Date:
August 10, 2021
Source:
DOE/Princeton Plasma Physics Laboratory
Summary:
Novel camera detects the birth of high-energy runaway electrons,
which may lead to determining how to prevent damage caused by the
highly energetic particles.



FULL STORY ==========================================================================
A key challenge for scientists striving to produce on Earth the fusion
energy that powers the sun and stars is preventing what are called
runaway electrons, particles unleashed in disrupted fusion experiments
that can bore holes in tokamaks, the doughnut-shaped machines that house
the experiments. Scientists led by researchers at the U.S. Department
of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have used
a novel diagnostic with wide-ranging capabilities to detect the birth,
and the linear and exponential growth phases of high-energy runaway
electrons, which may allow researchers to determine how to prevent the electrons' damage.


========================================================================== Initial energy "We need to see these electrons at their initial energy
rather than when they are fully grown and moving at near the speed of
light," said PPPL physicist Luis Delgado-Aparicio, who led the experiment
that detected the early runaways on the Madison Symmetric Torus (MST)
at the University of Wisconsin-Madison.

"The next step is to optimize ways to stop them before the runaway
electron population can grow into an avalanche," said Delgado-Aparicio,
lead author of a first paper that details the findings in the Review of Scientific Instruments.

Fusion reactions produce vast amounts of energy by combining light
elements in the form of plasma -- the hot, charged state of matter
composed of free electrons and atomic nuclei that makes up 99 percent
of the visible universe.

Scientists the world over are seeking to produce and control fusion on
Earth for a virtually inexhaustible supply of safe and clean power for generating electricity.

PPPL collaborated with the University of Wisconsin to install the
multi-energy pinhole camera on MST, which served as a testbed for the
camera's capabilities.

The diagnostic upgrades and redesigns a camera that PPPL had previously installed on the now-shuttered Alcator C-Mod tokamak at the Massachusetts Institute of Technology (MIT), and is unique in its ability to record
not only the properties of the plasma in time and space but its energy distribution as well.

That prowess enables researchers to characterize both the evolution of
the superhot plasma as well as the birth of runaway electrons, which
begin at low energy. "If we understand the energy content I can tell
you what is the density and temperature of the background plasma as
well as the amount of runaway electrons," Delgado Aparicio said. "So
by adding this new energy variable we can find out several quantities
of the plasma and use it as a diagnostic." Novel camera Use of the
novel camera moves technology forward. "This certainly has been a great scientific collaboration," said physicist Carey Forest, a University
of Wisconsin professor who oversees the MST, which he describes as
"a very robust machine that can produce runaway electrons that don't
endanger its operation." As a result, Forest said, "Luis's ability to
diagnose not only the birth location and initial linear growth phase
of the electrons as they are accelerated, and then to follow how they
are transported from the outside in, is fascinating. Comparing his
diagnosis to modeling will be the next step and of course a better understanding may lead to new mitigation techniques in the future." Delgado-Aparicio is already looking ahead. "I want to take all the
expertise that we have developed on MST and apply it to a large tokamak,"
he said. Two post-doctoral researchers who Delgado-Aparicio oversees can
build upon the MST findings but at WEST, the Tungsten (W) Environment
in Steady-state Tokamak operated by the French Alternative Energies and
Atomic Energy Commission (CEA) in Cadarache, France.

"What I want to do with my post-docs is to use cameras for a lot
of different things including particle transport, confinement,
radio-frequency heating and also this new twist, the diagnosis
and study of runaway electrons," Delgado- Aparicio said. "We
basically would like to figure out how to give the electrons a
soft landing, and that could be a very safe way to deal with them." ========================================================================== Story Source: Materials provided by
DOE/Princeton_Plasma_Physics_Laboratory. Original written by John
Greenwald. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. L. F. Delgado-Aparicio, P. VanMeter, T. Barbui, O. Chellai,
J. Wallace,
H. Yamazaki, S. Kojima, A. F. Almagari, N. C. Hurst, B. E. Chapman,
K. J.

McCollam, D. J. Den Hartog, J. S. Sarff, L. M. Reusch, N. Pablant,
K.

Hill, M. Bitter, M. Ono, B. Stratton, Y. Takase, B. Luethi,
M. Rissi, T.

Donath, P. Hofer, N. Pilet. Multi-energy reconstructions, central
electron temperature measurements, and early detection of the
birth and growth of runaway electrons using a versatile soft x-ray
pinhole camera at MST. Review of Scientific Instruments, 2021; 92
(7): 073502 DOI: 10.1063/5.0043672 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210810161346.htm

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