Scientists propose a novel method for controlling fusion reactions


Date:
August 4, 2020
Source:
DOE/Princeton Plasma Physics Laboratory
Summary:
Researchers have developed a pulsed method for stabilizing magnetic
islands that can cause disruptions in fusion plasmas.



FULL STORY ========================================================================== Scientists have found a novel way to prevent pesky magnetic bubbles in
plasma from interfering with fusion reactions -- delivering a potential
way to improve the performance of fusion energy devices. And it comes from managing radio frequency (RF) waves to stabilize the magnetic bubbles,
which can expand and create disruptions that can limit the performance
of ITER, the international facility under construction in France to
demonstrate the feasibility of fusion power.


========================================================================== Magnetic islands Researchers at the U.S. Department of Energy's (DOE)
Princeton Plasma Physics Laboratory (PPPL) have developed the new
model for controlling these magnetic bubbles, or islands. The novel
method modifies the standard technique of steadily depositing radio
(RF) rays into the plasma to stabilize the islands - - a technique that
proves inefficient when the width of an island is small compared with
the characteristic size of the region over which the RF ray deposits
its power.

This region denotes the "damping length," the area over which the RF power would typically be deposited in the absence of any nonlinear feedback. The effectiveness of the RF power can be greatly reduced when the size of
the region is greater than the width of the island -- a condition called
"low- damping" -- as much of the power then leaks from the island.

Tokamaks, doughnut-shaped fusion facilities that can experience such
problems, are the most widely used devices by scientists around the world
who seek to produce and control fusion reactions to provide a virtually inexhaustible supply of safe and clean power to generate electricity. Such reactions combine light elements in the form of plasma -- the state of
matter composed of free electrons and atomic nuclei that makes up 99
percent of the visible universe - - to generate the massive amounts of
energy that drives the sun and stars.

Overcoming the problem The new model predicts that depositing the rays in pulses rather than steady state streams can overcome the leakage problem,
said Suying Jin, a graduate student in the Princeton Program in Plasma
Physics based at PPPL and lead author of a paper (link is external) that describes the method in Physics of Plasmas. "Pulsing also can achieve
increased stabilization in high-damping cases for the same average power,"
she said.



==========================================================================
For this process to work, "the pulsing must be done at a rate that is
neither too fast nor too slow," she said. "This sweet spot should be
consistent with the rate that heat dissipates from the island through diffusion." The new model draws upon past work by Jin's co-authors
and advisors Allan Reiman, a Distinguished Research Fellow at PPPL,
and Professor Nat Fisch, director of the Program in Plasma Physics at
Princeton University and associate director for academic affairs at
PPPL. Their research provides the nonlinear framework for the study of
RF power deposition to stabilize magnetic islands.

"The significance of Suying's work," Reiman said, "is that it expands considerably the tools that can be brought to bear on what is now
recognized as perhaps the key problem confronting economical fusion using
the tokamak approach. Tokamaks are plagued by these naturally arising and unstable islands, which lead to disastrous and sudden loss of the plasma." Added Fisch: "Suying's work not only suggests new control methodologies;
her identification of these newly predicted effects may force us to
re-evaluate past experimental findings in which these effects might have
played an unappreciated role. Her work now motivates specific experiments
that could clarify the mechanisms at play and point to exactly how best
to control these disastrous instabilities." Original model The original
model of RF deposition showed that it raises the temperature and drives
current in the center of an island to keep it from growing. Nonlinear
feedback then kicks in between the power deposition and changes in the temperature of the island that allows for greatly improved stabilization.

Governing these temperature changes is the diffusion of heat from the
plasma at the edge of the island.



========================================================================== However, in high-damping regimes, where the damping length is smaller
than the size of the island, this same nonlinear effect can create a
problem called "shadowing" during steady state deposition that causes
the RF ray to run out of power before it reaches the center of the island.

"We first looked into pulsed RF schemes to solve the shadowing problem,"
Jin said. "However, it turned out that in high-damping regimes nonlinear feedback actually causes pulsing to exacerbate shadowing, and the ray runs
out of power even sooner. So we flipped the problem around and found that
the nonlinear effect can then cause pulsing to reduce the power leaking
out of the island in low-damping scenarios." These predicted trends
lend themselves naturally to experimental verification, Jin said. "Such experiments," she noted, "would aim to show that pulsing increases the temperature of an island until optimum plasma stabilization is reached."

========================================================================== 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. S. Jin, N. J. Fisch, A. H. Reiman. Pulsed RF schemes for tearing
mode
stabilization. Physics of Plasmas, 2020; 27 (6): 062508 DOI:
10.1063/ 5.0007861 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200804165116.htm

--- up 2 weeks, 6 days, 1 hour, 55 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)