New stellerator a step forward in plasma research
Date: Saturday, March 10, 2007 @ 17:31:32 MST
A picture of the helically symmetric experiment. Credit: HSX/University of Wisconsin-Madison
A project by University of Wisconsin-Madison researchers has come one step closer to making fusion energy possible.
The research team, headed by electrical and computer engineering
Professor David Anderson and research assistant John Canik, recently
proved that the Helically Symmetric eXperiment (HSX), an odd-looking
magnetic plasma chamber called a stellarator, can overcome a major
barrier in plasma research, in which stellarators lose too much energy
to reach the high temperatures needed for fusion.
Published in a recent issue of Physical Review Letters,
the new results show that the unique design of the HSX in fact loses
less energy, meaning that fusion in this type of stellarator could be
Plasma is very hot, ionized gas that can conduct electricity -
essentially, it's what stars are made of. If heated to the point of
ignition, hydrogen ions could fuse into helium, the same reaction that
powers the sun. This fusion could be a clean, sustainable and limitless
Current plasma research builds on two types of magnetic plasma
confinement devices, tokamaks and stellarators. The HSX aims to merge
the best properties of both by giving a more stable stellarator the
confinement of a more energetically efficient tokamak. "The slower
energy comes out, the less power you have to put in, and the more
economical the reactor is," says Canik.
Tokamaks, the current leader in the fusion race, are powered by
plasma currents, which provide part of the magnetic field that confines
the plasma. However, they are prone to "disruptions."
"The problem is you need very large plasma currents and it's not
clear whether we'll be able to drive that large of a current in a
reactor-sized machine, or control it. It may blow itself apart," says
Stellarators do not have currents, and therefore no disruptions,
but they tend to lose energy at a high rate, known as transport. The
external magnetic coils used to generate the plasma-confining field are
partially responsible for the high transport rates in conventional
stellarators. The coils add some ripple to the magnetic field, and the
plasma can get trapped in the ripple and lost.
The HSX is the first stellarator to use a quasi-symmetric magnetic
field. The reactor itself looks futuristic: Twisted magnetic coils wrap
around the warped doughnut-shaped chamber, with instruments and sensors
protruding at odd angles. But the semi-helical coils that give the HSX
its unique shape also direct the strength of the magnetic field,
confining the plasma in a way that helps it retain energy.
The team designed and built the HSX with the prediction that
quasisymmetry would reduce transport. As the team's latest research
shows, that's exactly what it does. "This is the first demonstration
that quasisymmetry works, and you can actually measure the reduction in
transport that you get," says Canik.
These results excited and relieved the researchers who have spent
years working on the project. "We all thought the machine would do what
it's turning out to do, but there are a million reasons why it might
not: the theory might be wrong, (or) we might have built it badly,"
says Anderson. "But everything is panning out and supporting the fact
that the ideas on which it was based were correct, and really points
the way of the future for the stellarator."
The next step for the project is to establish how much symmetry in
the coils is necessary to achieve low transport rates. They hope to
make the coils easier to engineer, with the mindset that the principles
used in the HSX could someday be incorporated into fusion generators,
the reason that Anderson and his team began designing the HSX 17 years
"It's an exciting field. It's something where one can contribute
positively to mankind with an energy source that's completely
sustainable, doesn't involve nuclear proliferation or radioactive
waste, with a limitless fuel supply," says Anderson. "Plus, the
machines look cool."
Link: Helically Symmetric eXperiment (HSX) -- http://www.hsx.wisc.edu/
Source: University of Wisconsin-Madison