Z-pinch (Redirected from Z-Pinch)
In fusion power design, Z-pinch, or zeta pinch is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it. The name refers to the direction of the earliest experimental devices in England, where the current flowed down a vertical quartz tube, the Z-axis on a normal mathematical diagram.
Specifically, Z-pinch relies on Lenz's Law, that a magnetic field will induce a current in a conductor that itself creates a magnetic field in the opposite direction. That is, if a magnet is approaching a conductor, current will be induced in the conductor that creates a field to push the magnet away. Since plasma itself is electrically conducting, in the Z-pinch system an external magnetic field induces a current into the plasma, in turn creating a magnetic field that opposes the external one. However the external magnetic field is "fixed" to the equipment, while the plasma itself can move, and therefore is compressed away from the external magnetic field.
Most early Z-pinch research was carried out in England, where a large toroidal device known as ZETA went into operation in 1954. In ZETA the external field was generated in a large magnet which was fed by a huge bank of capacitors, hoping to quickly pinch the plasma to fusion temperatures. Instead they found that the plasma quickly became unstable and "broke up" before it was compressed to these levels, applying the current more quickly simply made it break up faster. After some study a reason for this was offered, and it appeared basically impossible to avoid. They did notice several neutron production spikes that the researchers initially attributed to fusion, which generated some news for a time, but later it was realized these were due to the instabilities themselves. The last few firings showed an odd "quiet period" of long stability in the system, but the nature of these quiet periods, or quiescence, was not fully researched. ZETA seemed to suggested that the pinch concept was simply unworkable, and the efforts with ZETA ended in 1958.
The concept was picked up again at LLNL by Jim Tuck, who felt that it might be possible to avoid the instabilities by slowly increasing the external current, instead of doing so rapidly as in the British experiments. In early 1952 they built a small device they called the Perhapsatron, but found the same sorts of problems as ZETA. A second attempt with fast pinch again ended with similar results as the British teams, namely the faster you squeezed, the faster it broke up. Efforts to add additional stability with external magnetic fields, referred to as "giving the plasma a backbone", also failed, albeit at a somewhat higher density.
The z-pinch efforts were by no means outright failures. Efforts with other styles of pinch fields continued until the 1970s, notably theta-pinch, and for many years the pinch devices were the only ones able to generate fusion reactions, even at a low rate.
More importantly, the "quiet period" seen in the later ZETA runs turned out to be far more interesting than anyone first realized. In 1974 Ted Taylor examined the results and found a new class of self-stabilizing plasmas known as the reversed-field pinch. Research into these class of plasmas became a major effort in the 1980s and '90s.
An entirely new concept is the use of Z-pinch started in the 1980s. Instead of using an external magnet to generate the induction field, a set of very fine tungsten wires running around the fuel would be "dumped" with the current instead. The wires would quickly vapourize into a plasma, which is conductive, and the current flow would then cause the plasma to pinch as in prior experiments. The key difference is that the plasma would not be the fuel, as in previous experiments, but used solely to generate very high-energy X-rays as the metal plasma compressed and heated. The x-rays would compress a tiny fuel cylinder containing deuterium-tritium mix, in the same fashion that the x-rays generated from a nuclear bomb compress the fuel load in an H-bomb. This sort of Z-pinch system has much more in common with inertial confinement fusion (ICF) systems, and is generally referred to as one. For details on the reaction within the pellet, see the main ICF article.
To date the only serious effort to build such a device is Sandia National Laboratories' Z machine. Early tests were limited by instabilities in the compression to about .7 million degrees, far below the 4.5 needed for the fusion ignition temperature, but careful target design was able to increase this dramatically. The milestones for the project were to be able to produce 150 terawatts of X-rays of 1.5 megajoules, but the project has since delivered 290 terawatts at 2.0 megajoules.
The success of the Z machine has led to the design of a somewhat larger, but much more powerful version known as X-1. X-1 would be able to reach 3 million degrees, enough for the reaction to reach break even, although not enough for ignition. Whether or not the increased power will lead to new instabilities, as has been the case in most fusion research, remains to be seen.
In Ocean's Eleven (2001), a Z-pinch system is used to disrupt power in Las Vegas so that the gang could break into the casino vault without tripping the alarm.
External link
- Z Machine (http://www.prod.sandia.gov/zmachine/)
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