Levitating Magnet: MIT's New Approach to Nuclear Fusion

University of Columbia researchers, along with their colleagues from MIT, have some results in reproducing the Earth’s magnetic fields for developing nuclear power plants based on nuclear fusion – the ultimate dream of scientists that uses the same principle that the Sun operates on, by using a levitating magnet.

Nuclear fusion does not emit carbon dioxide, uses far less radioactive materials and has a net output totally superior to any nuclear fission process existing today. The biggest disadvantage is that it can only be obtained at high temperatures – 10 million degrees, and the scientists need a container that can stand these enormous temperatures. The only solution found so far is containing the nuclear fusion plasma inside magnetic fields.

The experiment that led to the new discovery is called the Levitated Dipole Experiment (LDX), that uses a 500 kg, donut-shaped levitating magnet the size of a truck tire, made of superconducting wire coiled inside a stainless steel vessel. The huge donut magnet is suspended by a powerful electromagnetic field, and is used to control the motion of the extremely hot plasma contained in a 16-foot diameter outer chamber.

Contrary to what the scientists had believed, inside the magnetic chamber random turbulence causes the plasma to become more densely concentrated – an important step to make the atoms fuse, instead of spreading out, as it usually happens in a turbulence. This “turbulent pinching” of the plasma has been observed in the way plasmas in space interact with the Earth’s and Jupiter’s magnetic fields, but has never before been recreated in the laboratory.

Most experiments in fusion around the world use one of two methods: tokamaks, which use a collection of coiled magnets surrounding a donut-shaped chamber to confine the plasma, or inertial fusion, using high-powered lasers to blast a tiny pellet of fuel at the device’s center. But LDX’s levitating magnet technique takes a different approach. “It’s the first experiment of its kind,” says MIT senior scientist Jay Kesner, MIT’s physics research group leader for LDX, who co-directs the project with Michael E. Mauel, professor of applied physics at Columbia University’s Fu Foundation School of Engineering and Applied Science.

The results of the levitating experiment show that this approach “could produce an alternative path to fusion,” Kesner says, though more research will be needed to determine whether it would be practical. For example, though the researchers have measured the plasma’s high density, new equipment still needs to be installed to measure its temperature, and ultimately a much larger version would have to be built and tested.

Kesner cautions that the kind of fuel cycle planned for other types of fusion reactors such as tokamaks, which use a mixture of two forms of “heavy” hydrogen called deuterium and tritium, should be easier to achieve and will likely be the first to go into operation. The deuterium-deuterium fusion planned for devices based on the LDX levitating magnet design, if they ever become practical, would likely make this “a second-generation approach,” he says.

When operating, the huge LDX levitating magnet is supported by the magnetic field from an electromagnet overhead, which is controlled continuously by a computer based on precision monitoring of its position using eight laser beams and detectors. The position of the half-ton magnet, which carries a current of one million amperes (compared to a typical home’s total capacity of 200 amperes) can be maintained this way to within half a millimeter. A cone-shaped support with springs is positioned under the magnet to catch it safely if anything goes wrong with the control system.

The levitation of the magnet is crucial because the magnetic field used to confine the plasma would be disturbed by any objects in its way, such as any supports used to hold the magnet in place. In the experimental runs, they recreated the same conditions with and without the support system in place, and confirmed that the confinement of the plasma was dramatically increased in the levitated mode, with the supports removed. With the magnet levitated, the central peak of plasma density developed within a few hundredths of a second, and closely resembled those observed in planetary magnetospheres (such as the magnetic fields surrounding Earth and Jupiter).

Summarizing the difference between the two approaches, Kesner explains that in a tokamak, the hot plasma is confined inside a huge magnet, but in the LDX the levitating magnet is inside the plasma. The whole concept, he says, was inspired by observations of planetary magnetospheres made by interplanetary spacecraft. In turn, he says, for planetary research the experiments in LDX can yield “a lot more subtle detail than you can get by launching satellites, and more cheaply.”

The two universities’ experiment, if succeeded, will compete with the newly-opened science facilities that use ultra-dense deuterium to obtain nuclear fusion.

[source: ScienceDaily]

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