"Dear Arjun"

Dear Arjun:

What is a "z-pinch" and can it contribute to the development of new nuclear weapons?
--Nervous in Napoli

Dear Nervous,
Many years ago in Italy there lived a cavalier young lobster who was fond of wearing masks. He was most known for the disproportionate pleasure he derived from using his powerful claws on the vulnerable ankles of swimmers in the Mediterranean Sea. Local doctors, treating the lobster's victims, were puzzled. But one day, a bright intern (and part-time swimmer) realized with horror that the mark was actually a "Z," and the masked crustacean was none other than Zorro the Lobster. After that, Italian swimmers who suffered at the claws of Zorro were said to have gotten "z-pinch."

The name of the device derives from the fact that it is a cylindrical array of wires. The vertical direction of a cylinder is usually denoted by the letter "z," (for z axis), and the cylinder is "pinched" to a very small diameter - thus the name "z-pinch." In the z-pinch wire-array experiments a large current is passed through a large number of very thin wires arranged in a cylindrical bundle. As the current rises, the magnetic field associated with it increases. This in turn compresses the array of wires into a cylinder of progressively smaller diameter. At the same time, the high current is rapidly heating the wires, evaporating the wire material, and turning it into a plasma.¹ As this plasma is compressed further by the magnetic field, the electrons and ions forming the plasma come to an abrupt stop (this is called stagnation). This abrupt stop converts the kinetic energy of the particles into x-rays. The process is somewhat analogous to the conversion of the kinetic energy of a car into heat during sudden braking.

In order for atoms to fuse together and release huge amounts of energy, extremely high temperatures and pressures must be exerted in a very precise way on a fuel pellet (usually made up of deuterium and tritium.) Since x-rays can be used to compress a fusion fuel pellet, the high level of x-ray energy achieved by the wire-array z-pinch makes it very interesting to fusion researchers. Furthermore, unlike lasers and ion-beams (other "drivers" that can be used to compress fuel pellets), the wire-array z-pinch could possibly be miniaturized, increasing its suitability for weapons applications.

Significant improvements in the wire-array z-pinch have occurred at Sandia over the past few years. Recent experiments on the device have generated x-rays with an energy output of 2-megajoules,² a level comparable with that planned for the National Ignition Facility (NIF).

A large capacitor bank is used as the energy source for creating the current in the wires that are pinched.³ The recent performance level announced for the wire-array z-pinch (290 trillion watts) demonstrates the potential of this technology for contributing to pure fusion weapons development, since levels of power only a few times greater than this would be needed to establish their scientific feasibility. The experiments have exceeded most of the milestones that have been set in a relatively short period of time.

Sandia has officially requested permission from the US Department of Energy to design the next generation of x-ray facility, the X-1. While no official design has been produced, there are articles indicating that conceptual designs have been completed, indicating that X-1 would produce x-rays of approximately 16 megajoules.⁴

Z-pinch technology goes hand in hand with DOE's other existing and planned explosive fusion research. For example, z-pinch experiments complement magnetized target fusion (MTF) experiments being conducted jointly by DOE and scientists from the Russian Ministry of Atomic Energy, as both technologies use a conductor carrying a high current in order to electromagnetically compress a plasma. Results of experiments at laser facilities like NIF and NOVA can study the shape of energy pulses that could be used to help design optimal pellets for x-ray technologies like z-pinch. According to Donald Cook, director of Sandia's Pulsed Power Sciences Center, "Without the knowledge of target experiments at NIF, it would take considerably longer to achieve high yield on X-1, and the risk of failure would be greater."⁵

Besides its potential for assisting the development of pure fusion weapons, z-pinch technology can also be used for design of fission-triggered thermonuclear weapons.

Ghost-written by Hisham Zerriffi and Pat Ortmeyer