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RAW POWER: electrical discharges illuminate the surface of the Z machine, the world's most powerful X-ray source, during a recent accelerators shot.
The Z machine created a hot dense plasma that produces neutrons associated with nuclear fusion. Compressing hot dense plasmas that produce neutrons is an important step towards realizing ignition, the level at which the fusion reaction becomes self-sustaining.
According to Sandia's Ray Leeper, the neutrons emanate from fusion reactions within a BB-sized deuterium capsule placed within the central target in the Z facility, itself about a third of a football field in diameter. While tokamaks cause fusion reactions to occur by confining plasmas in large magnetic fields, and laser facilities focus intense beams on or around a target, Z applies a huge pulse of electricity (about 12 million joules) with very sophisticated timing.
The pulse creates an intense magnetic field which crushes an array of tungsten wires into an ultra-light foam cylinder to produce x-rays. Striking the surface of a fuel capsule embedded in the cylinder, the x-ray energy produces a shock wave that compresses deuterium gas within the capsule, fusing enough deuterium to produce neutrons. All this action takes place within a container the size of a pencil eraser, called a hohlraum, at the center of the Z machine, itself a circular device about 120 feet in diameter.
Sandia researchers measured a yield of approximately 10 billion neutrons, around the expected energy of 2.45 MeV, corresponding to a very modest level of nuclear fusion (about 4 millijoules of energy). The deuterium capsule reached a temperature of about 11.6 million Kelvin and was compressed from a diameter of 2 mm to 160 microns. The whole compression took about 7 nanoseconds. "Pulsed power electrical systems have always been energy-rich but power-poor," said Leeper. "That is, we can deliver a lot of energy, but it wasn't clear we could concentrate it on a small enough area to create fusion. Now it seems clear we can do that."
Providing outside commentary, Cornell University's David Hammer said that the Sandia group performed a full set of tests to verify that they had achieved nuclear fusion. A partial confirmation of the result came about when theoretical predictions and lab outcomes were determined to be of the same order of magnitude, on the order of 10 billion neutrons. The predicted neutron yield depends on the ion density temperature and volume. Those quantities were independently confirmed by X-ray spectroscopy measurements.
While deuterium-filled capsules driven by lasers have long ago produced neutrons, this experiment represents the first time that the straightforward, relatively inexpensive and potentially robust technology of pulsed power has been able to achieve the conditions of high temperature and density needed to produce measurable thermonuclear neutrons.
The ZR (Z-Refurbished) facility, an upgrade slated to go online in 2006, is expected to scale up fusion experiments. The amount of energy this larger successor could bring to bear offers the eventual possibility of high-yield fusion?the state in which much more energy is released than is needed to provoke the reaction initially to occur. The excess energy could be used for applications such as the generation of electricity. However, while the Z approach to fusion is a promising method, researchers caution that they are at the start of a very long road in terms of investigating its feasibility as a fusion power source.
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