Aug. 6, 1997 ALBUQUERQUE, N.M. --If the power and temperatures generated recently by Sandia's Z accelerator were graphed like stock prices, brokers would describe them as going through the roof and still climbing.
The remarkable test results lay the groundwork to achieve sustainable fusion reactions, provide data to help test US defenses without physically exploding large-scale devices (the concept of so-called science-based stockpile stewardship), and advance basic scientific research.
Z (formerly called PBFA-Z) is the most powerful generator of X-rays in the world. In the past ten months, the machine, located at Sandia National Laboratories, has more than quintupled its output from 40 to 210 trillion watts (terawatts). It took 25 years for a succession of Sandia accelerators to reach 40 terawatts.
Z's output is now 60 times the world's usage of electrical power at a given moment.
In the most recent development, Z in mid July achieved a temperature of 1.5 million degrees Celsius after languishing for many years at a mere 0.5 million degrees. Nuclear fusion requires temperatures from 2 to 3 million degrees.
"The progress the Z-team continues to make is frankly astounding," says Gerry Yonas, Sandia vice president of Information and Pulsed Power Research & Technology Division. "Time and time again, the team has made theoretical projections, done experiments faster than expected, and made improvements along the way that gives even better results than predicted. This is world-class science and technology."
Says Vic Reis, Assistant Secretary for Defense Programs for the US Department of Energy, "With this world-record result, the Z machine proves once again that the people at Sandia are up to the challenge of science-based stockpile stewardship. My heartiest congratulations!"
Reis calls stockpile stewardship, to which the Z machine contributes, "perhaps the greatest scientific challenge of the next decade. It is our foundation for maintaining nuclear deterrence, as well as the basis for much of our arms control and nonproliferation objectives in the post cold war era. It clearly requires the best and the brightest."
Stewardship requires studying the physics of nuclear explosions, and creating in miniature the effects of these explosions.
Carl Ekdahl, program manager of the high-energy density physics program at Los Alamos National Laboratory, says, "It's wonderful work. Sandia researchers are way ahead of schedule. They fill a niche that no other facility in the world fulfills. My program is and will continue to be one of the largest users of Z for weapons physics."
David Hammer, a physics professor at Cornell University, was quoted in the July 18 issue of Science: "I think it's spectacular what they [at Sandia] have done. The implications are only beginning to dawn on people."
Says Jeff Quintenz, manager of the Inertial Confinement Fusion Program at Sandia's Pulsed Power Sciences Center, "There's a band in the spectrum - a profile of X-ray energies - that we are not able to adequately reproduce with today's X-ray generators. We lost that capability when we ceased underground testing. We need that band to certify our microchips are hardened against the effects of a nuclear explosion. These new results from Z bring us closer to producing the required output in the appropriate energy band."
Sensors produce data to be used in Sandia's supercomputer -- the fastest in the world -- and in computers at Los Alamos and Lawrence Livermore national laboratories so that computer codes realistically can portray variations in the complex physics related to high-intensity emissions. Realistic data are needed to check the properties of materials, using an iterative series of prediction and experiment that close in on physical properties as yet unknown. These checks ensure that computers accurately model the physical world rather than produce logical but wrong conclusions. P> "The best part," says Don Cook, director of the Pulsed Power Sciences Center, "is that if fusion can be made to work in a very cost-effective way, there will not be future wars over oil in the Persian Gulf or anywhere else, and the injury to the environment caused by civilization will be reduced. Fond hopes, but these were, and are, some of the dreams of fusion scientists and engineers."
Closer to high-yield fusion
Sandia's inertial confinement approach uses massive bursts of electricity to superheat a miniature oven, called a hohlraum, that is about the size of a sugar cube.
Numerical figures on Z's achievement are necessarily approximate because a change in any factor influences the others. But it could be said that Z now produces approximately 20 percent of the energy, 40 percent of the power, and 33 to 50 percent of the temperature to achieve high-yield fusion - a state in which much more energy is created than used. Of particular importance is temperature, because the pressure that drives the basic reaction increases as the fourth power of the temperature.
While temperature is the hardest quantity for the Z accelerator to increase, "By optimizing the configuration of the hohlraum further, we believe we can increase its temperature still more," Cook says. "If all goes well with these tests on Z over the next several months -- and so far, we have exceeded all milestones -- at 1.7 million degrees, we will submit a formal request, first to design and then to build the next-generation X-1 accelerator."
The next-generation machine's energy, power, and temperature outputs would be sufficient to create the fusion energy required to start the reaction in the accelerator.
"In X-rays, this new machine would yield 1,000 terawatts of power, 16 megajoules of energy, 2 million to 3 million degrees Centigrade, and cost about $300 million," Cook says.
One eventual use of the Z technique may be for a rocket propulsion system, says Sandia researcher Rick Spielman. "Z generates tremendous pulsed thrusts from a portion of the machine an inch high." Every time one drive pellet is burnt up, another would be dropped into place and lit. ------------------------------------------------------------------------
Why the breakneck breakthrough pace?
The first breakthrough came when Sandia scientists realized that a nearly discarded forty-year-old technique -- the passage of a huge electric current through a wire cage the size of a thimble -- could produce dramatically more power in the form of X-rays if scientists greatly increased the number of thimble-wires -- from 30 to 300 -- through which the current passes.
Within limits, the more wires available, the more uniform the magnetic field. The field evenly collapses at tremendous speed as the wires vaporize and become plasma. Atoms caught within the collapsing field speed up and then are braked suddenly to a stop - nowhere to go - as the uniformly shrinking magnetic field reaches a diameter about the thickness of a mechanical pencil lead. The sudden stoppage generates heat, much like the tires of a fast-moving car get hot when suddenly braked. While tire heat is generated at frequencies in the infrared range, the much faster deceleration of plasma in the shrinking magnetic field produces heat at higher radiation frequencies -- as it happens, in the X-ray range.
While tweaking input energies and wire arrays could boost output even further than the 1.5 million degrees, the present arrangement is not certain to achieve fusion temperatures, says Cook.
But by putting a very thin-walled gold cylinder inside the wire cage, Cook anticipates heat of approximately 1.7 million degrees, because of the heating effects of the imploding magnetic field and plasma striking the three-microns-thick walls of the cylinder. Says Cook, "The temperature goes up as the radiation container size decreases."
If that works, "We'll want to do experiments to show we can get symmetry in produced X-ray flux -- adequate symmetry to drive a high-yield fusion reaction when scaled up to X-1 levels." Symmetry is important because without it, not all the energy arrives at the same location at the same time, thus diluting the impact.
Successful conclusion of these experiments would mean that every contingency has been examined on the Z machine and found to be working correctly.
At that point, a request to actually build the X-1 machine will be submitted to DOE.
If granted, the Sandia team will move ahead on the road to fusion.
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