ALBUQUERQUE, N.M. — For periods of ten-billionths of a second this fall, a massive accelerator at Sandia National Laboratories consistently emitted intense bursts of more than 40 trillion watts of X-ray power. The highest power pulse was more than 160 trillion watts — more than 30 times the combined output of the Earth’s utility plants.
The accelerator, known as PBFA-Z (Particle Beam Fusion Accelerator, Z-Pinch version), produced increasingly powerful bursts of energy — 1.0 megajoules in early October, 1.2 megajoules in mid-October, and 1.8 megajoules in November. A megajoule is a million joules — a unit of energy.
The powerful “shots” provide data for computer simulations used to predict the physics within, and effects of, a nuclear blast. Scientists hope to substitute this laboratory data for some of that obtained from underground nuclear explosions, which have been halted.
The blasts also move Sandia researchers closer to reaching sustainable nuclear fusion in the laboratory using the technique of inertial confinement, something that has never been achieved. Sustainable refers to a condition when the amount of energy produced by the fusion reaction exceeds the amount of energy used to create it.
“So, we’ll use the PBFA-Z facility to conduct experiments on nuclear fusion over the next year as part of the national ICF [Inertial Confinement Fusion] Program,” said Rick Spielman, physicist and project manager. “We’re much closer than we were a year ago.”
And during the past year, study of the controlled bursts also have been useful for astrophysicists interested in getting a handle on processes that take place in stellar atmospheres.
Sandia, with machines like the Department of Energy-funded PBFA-Z, is a world leader in generating large pulses of power. PBFA-Z uses its tremendous electrical energy to create a powerful magnetic field that rapidly implodes a plasma within what can be visualized as a tiny, gold-plated soup can the size of a thimble. Stopping the motion of the plasma’s atoms as the magnetic field shrinks to nearly zero creates tremendous heat within the field’s confines.
The z-pinch
The extremely large output of power and energy was accomplished by converting the accelerator’s electrical output into a dense, ionized gas (plasma) called a z-pinch, which efficiently produces X-rays.
A z-pinch is so named because it creates a magnetic field that, as it contracts around ionized gas, pinches it vertically along (to a mathematician) the z-axis.
Sandia’s technology is the world’s best for generating the ultra-high pulsed power needed for z-pinch experiments. Achievement of its high-energy goals have advanced dramatically from a year ago, when Sandia generated 0.5 megajoules with the Saturn accelerator, an earlier version of PBFA-Z.
“The concept of a z-pinch as a radiation source is qualitatively like that of an old-fashioned flash bulb energized by a capacitor discharge, but emanating X-rays instead of visible light. The other difference is that our z-pinch puts out as much radiated power as 100 billion ordinary flashbulbs,” said Gerry Yonas, Sandia Vice President for Information and Pulsed Power Research and Technology.
In one of life’s ironies, Sandia researchers ultimately lit upon using tiny tungsten filaments — similar to those used in an ordinary incandescent light bulb — for their ‘flash’ material to create the powerful effect. The filaments, rather than getting hot and radiating light as in a light bulb, instead vaporize, become a plasma, and implode.
Security of the Nuclear Stockpile and Nuclear Fusion
With the nation’s decision to stop all underground nuclear testing, Vic Reis, U.S. Department of Energy Assistant Secretary for Defense Programs, challenged the three nuclear weapons laboratories to devise new technologies to help offset the loss of testing and continue to retain a credible nuclear deterrent. “These new results are one of the ways that Sandia is responding to the Reis challenge,” said Yonas.
High-energy laboratory sources of X-rays are necessary because “simulations based upon inaccurate, low-energy data could mislead us,” said Yonas. “The danger is that without the validating experiments, simulations might provide decision makers with an unrealistic basis for action.”
For national defense purposes, Sandia scientists are interested in understanding the effects of nuclear explosions, and scientists at Los Alamos National Laboratory and Lawrence Livermore national laboratories are faced with the need to use laboratory sources of X-rays to replace experiments they can no longer carry out using nuclear explosions. So, all the accelerator’s power is focused into a tiny cylindrical target to generate X-ray environments.
Cautions Yonas, “The technical foundation for our moving forward is not just energy output, but also power output in a useful geometry, and although that has yet to be shown, we are moving closer to our goal every day as the new results have already validated much of the theoretical basis for our work.”
How the machine works
PBFA-Z uses its tremendous electrical energy to create a powerful magnetic field that rapidly implodes a plasma within what can be visualized as a small soup can with gold-plated innards.
The intense magnetic field squeezes the plasma down to the thickness of the wire in a paper clip, and creates tremendous heat by very quickly stopping the plasma’s rapid motion within the field’s confines. The lines release the current — 17 million amperes — over a very small time interval (just 100 billionths of a second) and into a very small target, thereby magnifying the power exerted in that mini-moment.
The extraordinarily large burst of electricity passes vertically through a cylindrical container four centimeters in diameter and two centimeters long. Inside the can, whose inner surface is gold plated, is a network of 120 fine tungsten wires in a cylindrical array. The entire assembly is in vacuum.
The electric current passes downward through the can wall and runs back through the tungsten wires inside the can, immediately vaporizing the wires and generating an ionized tungsten gas or plasma, and simultaneously creating an enormous magnetic field. Driven by the magnetic field located between the lining of the can and the periphery of the wires, the wire plasmas are pushed rapidly inward by the field’s intense pressure. This compresses the tungsten plasma, increasing its density and temperature.
By compressing the plasma suddenly, the contracting magnetic field rams tungsten atoms from one side of the can into tungsten atoms driven in by the contraction of the far side. These collisions bring the ionized atoms to a sudden halt and generate heat, just as applying brakes to bring a speeding car to a screeching halt produces tires and brakes hot to the touch. The heat is released in the form of X-rays.
Exceeding the production of 1.8 megajoules of X-rays, while an important milestone, is simply one more step in a series of accelerator improvements based on continually improving Sandia expertise in high-energy density physics and pulsed-power engineering that stretches back for more than a decade.
The study of z pinches driven by pulsed power devices goes back about 30 years when several labs considered exploring such sources of X-rays and began theoretical and experimental research, but many limitations existed. The pinches were so unstable in most cases that results were difficult to reproduce and fell far below expectations.
In 1986, Sandia accelerators could generate only 0.1 megajoules. By 1989, Sandia generated 0.4 megajoules but with relatively low power levels, and in 1995 Sandia discovered, using its Saturn accelerator operating first with an array of fine aluminum wires and subsequently with fine tungsten wires, that the powerful electrical pulses at seven million amperes could be used to reproducibly create much more powerful x-ray pulses than ever had been achieved.
In October, after a cost of $13 million to upgrade an accelerator known as PBFA-II in order to more than double the z-pinch current to 17 million amperes, Sandia first passed a milestone in exceeding generation of 1.0 megajoules. The design goal for PBFA-Z — a bit over 1.5 megajoules — was reached early in November.
These results were publicly presented for the first time on November 11, in a paper by Keith Matzen, department manager of Target and Analysis Theory at Sandia, at the annual meeting of the American Physical Society Division of Plasma Physics.
For more information:
Z, which reaches temperatures of the Sun, to help astronomers interpret Chandra data (November 5, 1999) (November 5, 1999)
Concept for rapid-fire thermonuclear explosions proposed by Sandia scientists (September 13, 1999)
Sandia researchers push Z machine to new limits to test radiation effects (June 16, 1999)
Sandia formally proposes to design accelerator expected to produce high-yield fusion (April 8, 1998)
Another dramatic climb toward fusion conditions for Sandia Z accelerator (March 2, 1998)
Output of Sandia Z Accelerator Climbs Closer to Fusion (August 1, 1997)
High-Output Sandia Accelerator Able to Predict Nuclear Blast Physics (December 2, 1996)