PORTLAND, Ore. Massachusetts Institute of Technology researchers have described a new method of imaging nuclear fusion reactions. The technique uses a second fusion reaction as a "flash" to photograph a reaction designed to generate energy. As a result, the researchers now have a way of measuring their success as they proceed toward clean, safe nuclear fusion reactors.
The "flash" camera methodology uses matter (protons), instead of light (photons). Unlike photons, protons have a charge, and thus can image the electrical and magnetic fields surrounding a nuclear fusion reaction.
"What we are doing is very much like taking an X-ray, except that instead of using photons we are using protons, which has never been done before," said Richard Petrasso, senior research scientist at MIT's Plasma Science and Fusion Center. "Because protons have a positive charge, they are deflected by the magnetic and electrical fields surrounding the nuclear implosion, helping us learn about its dynamics, giving us new insights into what is taking place and hopefully getting us closer to the ultimate goal of nuclear fusion using ignition."
|Doctoral candidates Dan Casey and Mario Manuel along with MIT professor Richard Petrasso (left to right) work on the detector used to study nuclear implosions. (Photo by Sean McDuffee).
Nuclear fusion is the process of fusing deuterium and tritium, forming helium-5, which immediately decays into helium-4 and a neutron, thereby releasing vast amounts of energy. Ignition is an alternative method of inducing nuclear fusion reactions, and is finding favor after years of only marginal success using magnetic confinement for fusion reactors.
Ignition uses laser beams, instead of magnetic fields, to induce nuclear fusion reactions. By shining 40 or more laser beams on a tiny pellet of deuterium-tritium, inertia causes the atoms to fuse. MIT's flash-camera technique uses a second set of 20 lasers to implode a second pellet of helium-3, which is located about a centimeter away. The second implosion releases a uniform wave of protons all with a single energy level--15
million electron volts--which are deflected by the first implosion, in effect taking a flash photograph when imaged by a detector.
The deuterium-tritium pellet is about two millimeters in diameter with a hollow core shell measuring about 200-microns-thick. By using laser beams directly, or by creating high energy X-rays from them in indirect inertial confinement, the force implodes the pellet, squeezing it up to 30 times smaller.
If inertial confinement can squeeze the pellet down to less than 66 microns, raising the temperature inside it to 100 million degrees, or about seven times hotter than the center of the sun, then nuclear ignition results, fusing the pellet as nuclear fuel and releasing abundant energy.
Unfortunately, nuclear ignition has never been achieved.
"What we can do so far is a little like holding a match to a log; you can get it to smolder a little bit, with smoke coming off, but you can't get it to really burn. And until then, we haven't ignited it," said Petrasso.
However, Lawrence Livermore National Laboratory is currently constructing a fusion reactor based on inertial confinement that it hopes will achieve ignition of deuterium-tritium pellets. Its National Ignition Facility (NIF) will start ignition experiments in 2010. MIT's work on imaging the reactions is an attempt to pave the way for NIF's efforts.
The main requirement for success, according to Petrasso, is maintaining an almost perfect spherical shape as the pellet implodes. Even the slightest perturbations can cause the pellet to change shape, thus spoiling the implosion and preventing ignition.
The principle used to compress the pellet is Newton's third law of reciprocal actions: As a laser beam ablates material off the surface of the pellet an equal and opposite force compresses the remaining material inward. If an almost perfectly spherical shape can be maintained during the few nanoseconds it takes the pellet to compress 30 times, then ignition theoretically will occur, causing the pellet to burn in a controlled manner rather than in a uncontrolled chain reaction.
If ignition can be demonstrated by NIF, then theoretically a reactor could be fed with a constant stream of nuclear pellets. As little as one megajoule of laser energy can release up to 150 megajoules from the reaction.
Now that the MIT scientists have invented a way to observe the electric and magnetic fields around the pellets, they will fine tune the technique by observing spherical shape and controlling the process with enough precision to induce nuclear ignition.
Funded for the fusion research was provided by the Fusion Science Center for Extreme States of Matter and Fast Ignition at the University of Rochester and the U.S. Energy Department's Office of Inertial Confinement.