Nuclear fusion could prove an abundant source of clean energy. But the process can be difficult to control, and scientists have yet to demonstrate a fusion plant that produces more energy than it consumes. Now physicists at MIT have addressed one of the many technological challenges involved in harnessing nuclear fusion as a viable energy source. They’ve demonstrated that pulses of radio frequency waves can be used to propel and heat plasma inside a reactor.
MIT’s doughnut-shaped fusion reactor, the Alcator C-Mod, uses magnets to confine hydrogen in a turbulent, electrically charged state of matter called a plasma. By infusing large amounts of energy into the plasma, physicists can kick off fusion reactions that, in turn, release large amounts of energy. The MIT reactor is too small to generate practical fusion reactions that generate enough energy to keep themselves going–what’s called a burning plasma. But the researchers have been working on ways to achieve this state in larger reactors, such as the planned International Thermonuclear Experimental Reactor (ITER).
The challenge is keeping the plasma confined in a stable rotation, with just the right amount of turbulence and the ideal temperature gradients so that it keeps burning. Traditionally, physicists control plasmas by injecting high-power beams of inert atoms. Controlling turbulence and temperature is critical: the better confined the plasma, the smaller the reactor needs to be and the less power required.
When directed well, inert beams in today’s reactors “have substantial momentum and drag the plasma with them,” says Earl Marmar, head of MIT’s Alcator project. They also “heat” the plasma, supplying energy to kick-start fusion reactions. Marmar anticipates that in the future, the beam technique simply won’t work: it will be able to impart enough energy, but not enough momentum.
MIT researchers led by John Rice and Yijun Lin have experimentally demonstrated that radio waves–which will be able to penetrate large plasmas like ITER’s–can give plasma both energy and momentum. The MIT group placed powerful antennas at the edge of the reactor to launch two frequencies of radio waves into the plasma. One group of waves is attuned to protons. When these waves collide with protons, they heat up; the protons, in turn, collide with the hydrogen isotope fuel. The second group of waves is attuned to lightweight helium isotopes that the MIT group adds to the mix. These waves collide with the helium, imparting their momentum to the isotopes, which push the rest of the plasma. These experiments were described last week in the journal Physical Review Letters.
“People have been thinking about doing this for a long time, but the results were always inconclusive,” says Marmar. He says that the key to the MIT group’s success with radio waves was its development of more-effective methods for monitoring the plasma. “Most of the time, [physicists] do the measurements using the same neutral beams used to drive flow,” says Marmar. The MIT group tracks the flow of its plasma by introducing impurities that it can monitor using x-ray spectroscopy.
Wayne Houlberg, a scientist in the Fusion Science and Technology Department at ITER, believes that the MIT group’s work is interesting but still in its early stages. “Its applicability to the plasma conditions we expect in ITER will take time to evaluate,” he says.
MIT’s reactor is currently down for maintenance; running these experiments is so complex and expensive that reactors like MIT’s typically run for only three to four months a year. When experiments start up again, says Rice, he and his colleagues will work on fine-tuning the use of radio waves for controlling the plasma. “Ultimately, you’d like to control the shape of the rotation,” he says. For example, it might be better for the plasma in the center than for the plasma at the edges to rotate more quickly, or vice versa.
Practical fusion power plants are still decades away. In addition to confronting technological and scientific hurdles, fusion researchers have seen their funding stagnate. This year, the United States was to have significantly increased its financial support of ITER; the measure didn’t pass Congress. “We never know for sure what our budget will be,” says Marmar. “ITER is our best hope, but funding is caught in limbo.”
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