STAR POWER
In fusion power research, the 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 (see Pinch). 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

The Z machine is the largest X-ray generator in the world and is designed to test materials in conditions of extreme temperature and pressure. It is operated by Sandia National Laboratories to gather data to aid in computer modeling of nuclear weapons. The Z machine is located at Sandia's main site in Albuquerque, New Mexico.



Operation overview
When the Z machine fires, the energy from a 20-million-ampere electrical discharge vaporizes an array of thin, parallel tungsten wires, creating plasma. Simultaneously, the electrical current creates a powerful magnetic field that compresses and implodes the plasma by means of a z-pinch process. The imploding cylindrical plasma produces an X-ray pulse which can create a shock wave in a target structure. The target structure is placed in a cavity inside the wires called a hohlraum. The powerful fluctuation in the magnetic field (an "electromagnetic pulse") also generates electric current in all of the metallic objects in the room (see picture at right). The cylinder's axis is conventionally termed the z-axis.

Originally designed to supply 50 terawatts of power in one fast pulse, technological advances resulted in an increased output of 290 terawatts, enough to study nuclear fusion. Z releases 80 times the world's electrical power output for a few billionths of a second; however, only a moderate amount of energy is consumed in each test (roughly equal to the electrical energy consumed by 100 U.S households in two minutes). Marx generators are slowly charged with energy prior to firing.

Sandia announced the fusing of deuterium in the Z machine on April 7, 2003. This application could result in an efficient method to ignite a nuclear fusion reaction starting from a small capsule of deuterium. Unfortunately many technical difficulties for instance the small quantity of deuterium that can be contained in the hohlraum and the practical impossibility to transfer the compressed capsule to a larger nuclear fuel reservoir prevent the machine to be used this way, for the moment.

A $60 million refurbishment program was announced in 2004 that will raise the power output to 350 terawatts. The refurbishment, which started in July of 2006, includes the installation of newly designed hardware and components and more powerful Marx generators. The de-ionized water section of the machine has been reduced to about half its old size while the oil section has been expanded significantly in order to house larger intermediate storage lines (i-stores) and new laser towers, which used to sit in the water section. The refurbishment is estimated to be finished in May of 2007. The X-ray output will be 2.7 megajoules.

The Z machine is now able to propel small plates at 34 kilometers a second, faster than the 30 kilometers per second that Earth travels in its orbit around the Sun, and three times Earth's escape velocity.

In 2006, Z produced plasmas with temperatures in excess of 2 billion kelvins (2 GK, 2.0x109 K) or 3.6 billion F. It was achieved in part by replacing the tungsten wires by thicker steel wires. This temperature, which enables a 10% to 15% efficiency in converting electrical energy to soft x-rays, was much higher than anticipated. Thus far, it is currently the highest man-made temperature ever achieved according to The Guinness Book Of Records. It is theorized that small-scale turbulence and viscous damping are converting magnetic energy into thermal energy of the ions, which then transfer their energy to the electrons through collisions.



Number 767 #3, February 28, 2006 by Phil Schewe and Ben Stein
America's Hottest Lab

A temperature of 2 to 3 billion degrees Kelvin -- hotter than the interior of any known star -- has been achieved in a lab in New Mexico.

The temperature record was set recently in a test shot at the Z Pinch device at Sandia National Laboratory, where an immense amount of electrical charge is stored in a device called a Marx generator. Many capacitors in parallel are charged up and then suddenly switched into a series configuration, generating a voltage of 8 million volts. The process captured in a famous photograph, see Physics News Graphics.

This colossal electrical discharge constitutes a current of 20 million amps passing through a cylindrical array of wires, which implodes. The imploding material reaches the record high temperature and also emits a large amount of X-ray energy (see PNU 702).

Why the implosion process should be so hot, and why it generates X-rays so efficiently (10-15 percent of all electrical energy is turned into soft X-rays), has been a mystery.

Now Malcolm Haines of Imperial College, in London, and his colleagues, think they have an explanation. In the hot fireball formed after the jolt of electricity passes through, they believe, the powerful magnetic field sets in motion a myriad of tiny vortices (through instabilities in the plasma), which in turn are damped out by the viscosity of the plasma, which is made of ionized atoms.

In the space of only a few nanoseconds, a great deal of magnetic energy is converted into the thermal energy of the plasma. Last but not least, the hot ions transfer much energy to the relatively cool electrons, energy which is radiated away in the form of X-rays.


Haines et al., Physical Review Letters, 24 February 2006
Contact Malcolm Haines, m.haines@imperial.ac.uk
Image at Physics News Graphics

http://www.aip.org/enews/physnews/2006/split/767-3.html