A quarterly research and development magazine.Winter 2006/2007
Volume 8, No. 4
The power Z emits in X-rays when it fires is equivalent to many times the entire world’s generation of electricity — but only for a few nanoseconds. The machine creates high temperatures and pressures in water because of the 20-million-amp electrical pulses it sends through a row of water switches. First, the water acts as an insulator, restraining the incoming electric charge. Then, overcome by the buildup, water transmits the pulse, shortening it from microseconds to approximately 100 nanoseconds. This compression in time is a key element of what makes the Z accelerator so powerful.
The concern was that the Z refurbishment might go beyond the ability of a water switch to function as designed and carry the required current, says Keith Matzen, director of Sandia’s Pulsed Power Sciences Center. “More efficient, larger machines may run into a limit and their switches not meet design requirements. So the question is: how does a water switch really work from first principles?”
One aspect of this knowledge is to model water to get a better understanding of its behavior under these extreme conditions, he says. Mattsson and Desjarlais first found deviations from the standard water-phase diagram when they ran an advanced quantum molecular simulation program on Sandia’s Thunderbird supercomputer that included “warm” electrons instead of unrealistic “cold” ones, says Desjarlais.
Desjarlais extended a molecular modeling code, written in Austria, to model electrical conductivity, and Mattsson developed a model for ionic conductivity based on calculations of hydrogen diffusion. An accurate description of water requires this combined treatment of electronic and ionic conductivity.
As it turns out, the newly discovered regime will not adversely affect Sandia’s water switches on the renovated Z machine. But water switches not yet designed for future upgrades may require the more accurate understanding of the phases of water discovered by the researchers.
Because of Z’s success in provoking fusion neutrons from deuterium pellets, it is thought of as a possible (if dark-horse) contender in the race for high-yield controlled nuclear fusion, which would provide essentially unlimited power to humanity. Z is more immediately useful for U.S. defense purposes — data from its firing is used to validate physics models in computer simulations to certify the safety and reliability of the nuclear weapons stockpile.
Technical Contact: Thomas Mattsson
(505) 844-9215, trmatts@sandia.gov
Media Contact: Neal Singer
(505) 845-7078, nsinger@sandia.gov