One Step Closer to Nuclear Fusion

Wednesday, November 17, 2010 @ 03:11 PM gHale


New X-ray imaging capability can take pictures of a critical instability at the heart of Sandia’s huge Z accelerator, which may help remove a major impediment in the worldwide effort to harness nuclear fusion to generate electrical power from sea water.
“These are the first controlled measurements of the growth of magneto-Rayleigh-Taylor [MRT] instabilities” in fast Z-pinches, said project lead Daniel Sinars.
MRT instabilities are spoilers that arise wherever electromagnetic forces contract (pinch) a plasma, which is essentially a cloud of ions. The pinch method is the basis of the operation of Z.
A pinch contracts plasma so suddenly and tightly that hydrogen isotopes available from sea water, placed in a capsule within the plasma, should fuse.
That’s the intent. Instead, the instability rapidly crimps the cylindrically contracting plasma until it resembles a string of sausages, or shreds the plasma into more fantastic, equally useless shapes. This damaged contraction loses the perfect symmetry of forces necessary to fuse the material.
Fast pinches at Z, which take place in less than 100 nanoseconds, already have produced some neutrons, a proof of fusion. But a major reason they have not been able to produce enough neutrons to provide a source of peacetime electrical power is the MRT instability.
Sinars led seven experimental shots to map the disturbance. The experiments came as a result of a concept proposed last year by Sandia researcher Steve Slutz.
Traditionally, scientists would use an array of spidery wires to create a compressed, X-ray-generating ion cloud. The X-rays then compressed the fusion fuel.
Slutz suggested the magnetic pinching forces could directly fuse fuel by compressing a solid aluminum liner around fusion material preheated by a laser.
Because the new concept would not produce X-rays as a heating tool but instead relied on directly compressing the fuel with magnetic pressure, the MRT instability was the primary threat to the concept.
“Once we started looking at solid liners it was easy to conceive of doing a controlled experiment to study the growth of the instability,” Sinars said.
This is because experimenters could etch the solid tubes, creating instabilities to whatever degree they desired. Accurate etching is not an option with fragile wire arrays.
The MRT problem occurs because even small dips in a current-carrying surface — imperfections merely 10 nanometers in amplitude — can grow exponentially in amplitude to millimeter scales. In the experiments by Sinars and others, they scored the tubes with a sinusoidal perturbation to intentionally start this process.
“The series of pictures over a time scale of 100 nanoseconds brought the life of the MRT into focus,” Sinars said.
Previously, competing computer simulation programs had given conflicting predictions as to the extent of the threat posed by the MRT instability, leaving researchers in the position, Sinars said, of “a man with two watches: He never really knows what time it is.”
The more accurate simulations will enable researchers to better tweak the conditions of future Z firings, more effectively combating the effect of the instability.
Researchers believe with thick liners and control of the MRT, the Z machine could achieve an output of 100 kilojoules to match the 100 kilojoules input to the fuel to start the fusion reaction. “That would be scientific breakeven,” Sinars said. “No one has achieved that.”
That day, he said, may be just two to three years away.



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