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A Step Forward for Fusion
18 September 2012 11:45 am
In the high-stakes race to realize fusion energy, a smaller lab may be putting the squeeze on the big boys. Worldwide efforts to harness fusion—the power source of the sun and stars—for energy on Earth currently focus on two multibillion dollar facilities: the ITER fusion reactor in France and the National Ignition Facility (NIF) in California. But other, cheaper approaches exist—and one of them may have a chance to be the first to reach "break-even," a key milestone in which a process produces more energy than needed to trigger the fusion reaction. Researchers at the Sandia National Laboratory in Albuquerque, New Mexico, will announce in a Physical Review Letters (PRL) paper accepted for publication that their process, known as magnetized liner inertial fusion (MagLIF) and first proposed 2 years ago, has passed the first of three tests, putting it on track for an attempt at the coveted break-even. Tests of the remaining components of the process will continue next year, and the team expects to take its first shot at fusion before the end of 2013.
Fusion reactors heat and squeeze a plasma—an ionized gas—composed of the hydrogen isotopes deuterium and tritium, compressing the isotopes until their nuclei overcome their mutual repulsion and fuse together. Out of this pressure-cooker emerge helium nuclei, neutrons, and a lot of energy. The temperature required for fusion is more than 100 million°C—so you have to put a lot of energy in before you start to get anything out. ITER and NIF are planning to attack this problem in different ways. ITER, which will be finished in 2019 or 2020, will attempt fusion by containing a plasma with enormous magnetic fields and heating it with particle beams and radio waves. NIF, in contrast, takes a tiny capsule filled with hydrogen fuel and crushes it with a powerful laser pulse. NIF has been operating for a few years but has yet to achieve break-even.
Sandia's MagLIF technique is similar to NIF's in that it rapidly crushes its fuel—a process known as inertial confinement fusion. But to do it, MagLIF uses a magnetic pulse rather than lasers. The target in MagLIF is a tiny cylinder about 7 millimeters in diameter; it's made of beryllium and filled with deuterium and tritium. The cylinder, known as a liner, is connected to Sandia's vast electrical pulse generator (called the Z machine), which can deliver 26 million amps in a pulse lasting milliseconds or less. That much current passing down the walls of the cylinder creates a magnetic field that exerts an inward force on the liner's walls, instantly crushing it—and compressing and heating the fusion fuel.
Researchers have known about this technique of crushing a liner to heat the fusion fuel for some time. But the MagLIF-Z machine setup on its own didn't produce quite enough heat; something extra was needed to make the process capable of reaching break-even. Sandia researcher Steve Slutz led a team that investigated various enhancements through computer simulations of the process. In a paper published in Physics of Plasmas in 2010, the team predicted that break-even could be reached with three enhancements.
First, they needed to apply the current pulse much more quickly, in just 100 nanoseconds, to increase the implosion velocity. They would also preheat the hydrogen fuel inside the liner with a laser pulse just before the Z machine kicks in. And finally, they would position two electrical coils around the liner, one at each end. These coils produce a magnetic field that links the two coils, wrapping the liner in a magnetic blanket. The magnetic blanket prevents charged particles, such as electrons and helium nuclei, from escaping and cooling the plasma—so the temperature stays hot.
Sandia plasma physicist Ryan McBride is leading the effort to see if the simulations are correct. The first item on the list is testing the rapid compression of the liner. One critical parameter is the thickness of the liner wall: The thinner the wall, the faster it will be accelerated by the magnetic pulse. But the wall material also starts to evaporate away during the pulse, and if it breaks up too early, it will spoil the compression. On the other hand, if the wall is too thick, it won't reach a high enough velocity. "There's a sweet spot in the middle where it stays intact and you still get a pretty good implosion velocity," McBride says.
To test the predicted sweet spot, McBride and his team set up an elaborate imaging system that involved blasting a sample of manganese with a high-powered laser (actually a NIF prototype moved to Sandia) to produce x-rays. By shining the x-rays through the liner at various stages in its implosion, the researchers could image what was going on. They found that at the sweet-spot thickness, the liner held its shape right through the implosion. "It performed as predicted," McBride says. The team aims to test the other two enhancements—the laser preheating and the magnetic blanket—in the coming year, and then put it all together to take a shot at break-even before the end of 2013.
Earlier this year, Slutz and his team published other simulations in PRL that showed that if a more powerful pulse generator was built to produce higher currents—say, 60 million amps—the system could achieve not just break-even, but high gain. In other words, the MagLIF could produce the kind of energy needed for a commercial fusion power plant.
"I am excited about Sandia discovering that magnetized target fusion … is a pathway to significant gain on the Z machine. We agree, and hope that their experiments get a chance to try it out," says Glen Wurden, the magnetized plasma team leader at Los Alamos National Laboratory in New Mexico.