In the quest for fusion energy on earth, researchers use magnetic fields to insulate hot plasma from the walls of the chamber to maintain the reaction and prevent damage to interior surfaces.
The tiled walls inside the Alcator C-Mod tokamak might say they've been scarred by sudden disruptions in the hydrogen fuel that periodically impacts them. MIT researchers are discovering ways to spread out the focused energy from these disruptions so that the vessel walls are not damaged.
Credit: M. Garrett
In the tokamak, a leading contender to achieve a sustained fusion burn, electrical currents flowing in the plasma inside the doughnut-shaped vacuum chamber can become unstable if the plasma current or pressure gets too high or the control system breaks, leading to a sudden termination of the discharge.
This sudden termination, called a disruption, can produce concentrated heating and mechanical forces on a section of the interior surface, forcing the plant to shut down for repairs.
Researchers at MIT's Plasma Science and Fusion Center (PSFC), General Atomics, Oak Ridge National Laboratory, University of Washington, and the University of California, San Diego, believe that if the intense energy of these disruptions could be uniformly spread out around the interior of the vessel, the plasma could be prevented from melting the wall—a necessity for the next-step fusion device, ITER, under construction in Cadarache, France. Several groundbreaking experiments at the Alcator CMod tokamak at MIT and the DIII-D tokamak in San Diego are guiding the way towards better protection for the vessel walls during disruptions.
Scientists at Alcator C-Mod and DIII-D investigating plasma disruptions have discovered that injecting gases heavier than the background hydrogen fuel (such as argon or neon) just before an impending disruption will spread the resulting energy around the vessel.
However, the Alcator C-Mod team found that the argon or neon does not uniformly spread out quite enough to prevent damage. Sometimes the heat load is still asymmetric, concentrated in one sector of the device. Even using multiple injection sites around the vessel does not necessarily improve the asymmetry, and sometimes heightens it (Olynyk, 2012 APS DPP). To explain this unexpected result, computer models (Izzo, 2012 APS DPP) indicated that internal instabilities within the plasma should determine the radiation asymmetry rather than the distribution of gas injectors.
The DIII-D team has for the first time tested the theory that internal plasma instabilities determine the radiation asymmetry. The team used 3D magnetic fields to "lock" the plasma instability in one direction or another. They found that by varying the direction in which the instability locked, they could reproducibly change the amount of energy deposited at a given location within the vessel, as expected from the computer. Moreover, no indication of the expected localized heating around the gas injector itself was found. The DIII-D results show that simply increasing the number of gas injectors does not alleviate radiation asymmetry during disruption mitigation. The results do, however, suggest that rotating the instability could spread the heat more evenly.
Using rotation to lower the heat load to the walls is exactly what was discovered at the Alcator C-Mod tokamak. The Alcator C-Mod team has discovered that the plasma can spontaneously rotate rapidly during a portion of the disruption known as the "quench." The rotation appears to be driven by smaller-scale instabilities, and the rotation ends up moving the radiating regions around the vessel quickly and thus lowering the average heat load. Future research will determine if we can control or encourage this spontaneous rotation, and thus distribute the heat more uniformly to the wall.
Robert Granetz, MIT, (617)-253-8634, email@example.com
N.W. Eidietis, General Atomics, (858)-455-2459, firstname.lastname@example.org
N. Commaux, Oak Ridge National Laboratory, (858)-455-2073
V.A Izzo, University of California, San Diego, (858)-455-4144, email@example.com
Abstracts:CO4.00009 Effects of Magnetic Shear on Toroidal Rotation in C-Mod Plasmas with LHCD
James Riordon | EurekAlert!
Significantly more productivity in USP lasers
06.12.2016 | Fraunhofer-Institut für Lasertechnik ILT
Shape matters when light meets atom
05.12.2016 | Centre for Quantum Technologies at the National University of Singapore
In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.
Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
07.12.2016 | Health and Medicine
07.12.2016 | Life Sciences
07.12.2016 | Health and Medicine