Sandia's Z-Beamlet laser confirms even pellet compression

Sept. 5, 2001
In its first try as a diagnostic tool, the third-biggest laser on earth, Z-Beamlet at Sandia National Laboratories, confirmed that the lab's Z machine – the most powerful laboratory producer of X-rays in the world – spherically compressed a simulated fusion pellet during a firing, or shot, of the giant accelerator.

In its first try as a diagnostic tool, the third-biggest laser on earth, Z-Beamlet at Sandia National Laboratories, confirmed that the lab's Z machine – the most powerful laboratory producer of X-rays in the world – spherically compressed a simulated fusion pellet during a firing, or shot, of the giant accelerator.

These results are significant because researchers must be able to evenly compress a BB-sized pellet so that its atoms are forced to fuse to create high-yield nuclear fusion that will ultimately produce cheap electric power from sea water. According to John Porter, leader of the related Sandia project, “The beam compressed the pellet by a factor of two, and demonstrated an encouraging uniformity. Our results show we're moving in the right direction.”

Uniform 3-D compression is an essential step in creating controlled nuclear fusion. It means that almost none of the X-ray energy delivered to the pellet squirts uselessly away. Weapons simulation work conducted on supercomputers by Sandia for the US Department of Energy is expected to benefit from data from high-yield explosions, as should, further down the pike, energy production.

Until now, Z researchers had to be content with electronic images of smoother and smoother Z pinches (with a pinch being the tool of compression). With increasing smoothness, the pinch – a vertical magnetic cylinder – impels ions of tungsten toward its vertical axis at a considerable fraction of the speed of light. But knowing that the tool is good and getting better does not confirm that all is well regarding the pellet upon which the tool is operating. Only direct data is entirely convincing.

Z-Beamlet images the pellet in a kind of giant dental X-ray, says Porter. In a burst of energy only a fraction of a billionth of a second long, it takes a snapshot by creating a shadow on a piece of X-ray film placed behind the BB-sized pellet inside the central chamber of the firing Z machine. The shadow, like the picture taken of a tooth, accurately depicts what is going on in the mouth of Z.

The comparison with the dental X-ray process is closer than it might appear. The laser's light itself is not used to create the pellet image. Higher frequencies of light are needed to produce better information. So the beam, after traveling horizontally 75 yards from a former warehouse adjacent to the Z building, is turned downward 90 degrees into the maw of Z, where it is focused to a small spot about the diameter of a human hair. Because the duration of the pulse is about 300 ps, an extremely powerful beam is created during the short time its energy is expended. The powerful beam striking the metal plate causes the plate to release X-rays. It is these X-rays, as they emanate from a single point, that have the accuracy and intensity to image the pellet.

While pulsed lasers are not new, they normally produce mere millijoules of energy in university research labs. According to Porter, though, the DOE wants lots of energy, and Z-Beamlet delivers kilojoules of laser energy for its diagnostic work. (Z itself, upon firing, delivers megajoules.)

Light starts its voyage humbly enough in Z-Beamlet with picojoules of energy in its initial beam. On a simple metal table – using an assortment of small mirrors, lenses, beam splitters, and polarizers – researchers develop as perfect a seed beam as possible. Then the beam is amplified and smoothed to clear up any spatial nonuniformity. Then it is passed through a vacuum chamber in which it is focused into a point source

from which it opens again. The entire laser system is run and monitored by an elaborate computer control system residing on five desktop computers. (This is one enhancement of many incorporated into Z-Beamlet to modernize the mid-1990s vintage laser.)

After a final smoothing from an adaptive optics system (a flexible mirror that is continuously pushed and pulled by an array of 39 electromechanical actuators), yet more energy is added to the laser pulse by flash lamps.

Lawrence Livermore National Laboratory originally built the Beamlet laser to serve as the scientific prototype of the National Ignition Facility. The California lab decided to remove the laser to make room for those of the NIF. The entire project to reassemble the recycled Livermore laser cost $12.875 million, took three years to complete, and required the talent and dedication of scores of individuals from Lawrence Livermore and Sandia, adds Porter.

“Now we're more optimistic than ever,” he adds. “Instead of seeing the outside of Z science – the instabilities in the compressing magnetic field – we can now see the inside, the pellet at the center of the million-degree furnace, and we can accurately describe what's happening there.”

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