Putting a new spin on tokamak disruptions

Nov 13, 2013
Putting a new spin on tokamak disruptions
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 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. 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 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 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.

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More information: Abstracts:

CO4.00009 Effects of Magnetic Shear on Toroidal Rotation in C-Mod Plasmas with LHCD
Session CO4: C-Mod Tokamak
2:00 PM–5:00 PM, Monday, November 11, 2013
Room: Plaza D

GO4.00002 Overview of DIII-D Disruption Mitigation Experimental Results
Session GO4: DIII-D Tokamak
9:30 AM–12:30 PM, Tuesday, November 12, 2013
Room: Plaza D

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User comments : 15

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antialias_physorg
1 / 5 (1) Nov 13, 2013
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

If disruptions can't be completely ruled out then maybe the interior should be tiled with some (non-magnetic, ceramic(?)) tiles that can easily be replaced.
Would certainly be less expensive (and time consuming) than replacing a torus segment.
Doug_Huffman
1 / 5 (10) Nov 13, 2013
Ceramic specific heat is too low.
WillieWard
1 / 5 (10) Nov 14, 2013
Tokamak is a flawed concept. Why they don't try new approaches such as aneutronic fusion reactor?
antialias_physorg
not rated yet Nov 14, 2013
Tokamak is a flawed concept

And the reason for this statement is ... ?

Tokamaks have been shown to work as a geometry that can create fusion since the 1950s.

Why they don't try new approaches such as aneutronic fusion reactor?

Because the conditions in a tokamak are already difficult enough to manage. Aneutronic fusion is a lot harder.

It's more sensible to first get a technology up and running where you are reasonably certain you're going to succeed (especially given the cost which you harp on about incessantly) than to dump that money into something where the chances are good that it won't work at all (by which time you'd doubtlessly start whining again about the cost/waste, too).
WillieWard
1 / 5 (10) Nov 14, 2013
Tokamaks have been shown to work as a geometry that can create fusion since the 1950s.
...the conditions in a tokamak are already difficult enough to manage.

Copy the nature sometimes they say. But after at least 50 expensive Tokamaks experiments around the world, the mainstream scientists have not learned yet that do not exist doughnut-shaped star in the universe.
Aneutronic fusion can be pretty easy and cheap with correct technology. I believe it is time to try aneutronic star on Earth. http://www.flickr...0507711/
antialias_physorg
not rated yet Nov 14, 2013
not learned yet that do not exist doughnut-shaped star in the universe.

Hint: Stars cause fusion to happen via constriction from gravity. Unless you have some secret patent on creating that kind of gravity on Earth your argument makes no sense.

Different forces necessitate different geometries - especially if you want to have sustained fusion reactions (which are sort of the point of a powerplant) and not just singular explosions.
cantdrive85
1.4 / 5 (11) Nov 14, 2013
Tokamak is a flawed concept. Why they don't try new approaches such as aneutronic fusion reactor?

Some are, the folks at the Focus Fusion Society are trying just that...
http://focusfusio...gory/C63

It involves the Dense Plasma Focus and would create electrical energy directly, no steam turbines necessary.
cantdrive85
1.3 / 5 (14) Nov 14, 2013
Hint: Stars cause fusion to happen via constriction from gravity. Unless you have some secret patent on creating that kind of gravity on Earth your argument makes no sense.

Electrical and magnetic forces contain fusion everywhere, not the incomparably weak gravity.
antialias_physorg
5 / 5 (2) Nov 15, 2013
Electrical and magnetic forces contain fusion everywhere, not the incomparably weak gravity.

Funny how temperature of stars is very well corelated to size (heard of the '
main sequence', have you?) - and not to how fast they (or their cores) rotate.
If electrical forces had anything to do with fusion and not gravity then we'd see a different corelation.

Observation trumps fantasy - every time.
WillieWard
1 / 5 (10) Nov 15, 2013
...If electrical forces had anything to do with fusion and not gravity then we'd see a different corelation...
Electrostatic acceleration has been used since early days of nuclear science and is far more powerful than gravity. Electrostatic fusion machines have better design than tokamaks. http://www.flickr...0508387/

TheGhostofOtto1923
1 / 5 (1) Nov 15, 2013
f disruptions can't be completely ruled out then maybe the interior should be tiled with some (non-magnetic, ceramic(?)) tiles that can easily be replaced
ITER design already has such a beryllium tiled lining.
https://www.iter..../blanket

-And the effects of disruptions on this blanket are already being studied.

"The prediction of electromagnetic loads on blanket module 1 of the ITER device during a plasma disruption event is considered. This analysis is performed for a number of design variations (of the blanket module) and different disruption events.

"The key features of the analysis procedure will be presented including the geometric description of the blanket module composed of a first wall, shield block, and vacuum vessel. The modeling of the plasma current will also be described."
http://www.ans.or.../a_18069
TheGhostofOtto1923
1 / 5 (2) Nov 15, 2013
Because the socialistic government of EU is full of lobbyists groups of scientists and provider companies
"Fridman considers most of the major applications of plasma chemistry, from electronics to thermal coatings, from treatment of polymers to fuel conversion and hydrogen production and from plasma metallurgy to plasma medicine. It is helpful to engineers, scientists and students interested in plasma physics, plasma chemistry, plasma engineering and combustion, as well as chemical physics, lasers, energy systems and environmental control."

-Industrial-scale manufacturing using materials in plasma form will be central to future technologies, including LENR. It will enable the use of such things as antimatter in the creation of all sorts of exotic compounds.

The only known way of containing, manipulating, and transporting bulk quantities of plasma is in closed bottles such as the tokamak. Which is why massive amounts of time, effort, and money are being expended at this time.
cantdrive85
1 / 5 (10) Nov 15, 2013
how temperature of stars is very well corelated to size (heard of the '
main sequence', have you?) - and not to how fast they (or their cores) rotate.
If electrical forces had anything to do with fusion and not gravity then we'd see a different corelation.

Observation trumps fantasy - every time[\q]
Correlation doesn't necessarily mean causation. There is much confusion if the properties of plasma, electrical and magnetic forces dominate by a ridiculously large margin.
antialias_physorg
not rated yet Nov 17, 2013
Correlation doesn't necessarily mean causation.

When we're dealing with ridiculously low alpha values it's a pretty good bet that stuff is related causally.

There is much confusion if the properties of plasma, electrical and magnetic forces dominate by a ridiculously large margin.

That's why they're local. The whole point why we get such an intricate mixture of electrically negative and electrically positive stuff (read: atoms) that it cancels out any far reaching effects IS PRECISELY because electrical forces are so much stronger than gravity.

'Stronger' always results in 'shorter range' in this universe - because anything that it affects has had enough time to get on top of one another faster than for weaker forces.
cantdrive85
1 / 5 (8) Nov 17, 2013
When we're dealing with ridiculously low alpha values it's a pretty good bet that stuff is related causally.

A pretty good bet is all it is, a bet made upon faulty models of the plasma that stars are made.

That's why they're local. The whole point why we get such an intricate mixture of electrically negative and electrically positive stuff (read: atoms) that it cancels out any far reaching effects IS PRECISELY because electrical forces are so much stronger than gravity.

'Stronger' always results in 'shorter range' in this universe - because anything that it affects has had enough time to get on top of one another faster than for weaker forces.


This assumption that "an intricate mixture of electrically negative and electrically positive stuff (read: atoms)" cancels itself out ignores decades of laboratory and in situ plasma research, such a notion is based on theoretical MHD guesses which fail to model many aspects (DL's, currents, etc.) of the highly complex plasma.