How do Z pinches contribute to a variety of scientific research projects?
Z provides the fastest, most accurate, and cheapest method to determine how materials will react under high pressures and temperatures, characteristics that can then be expressed in formulas called “equations of state.” Equations of state tell researchers how materials will react if basic conditions like pressure and temperature are changed by specific amounts. Dovetailing theoretical simulations with laboratory work, Sandia researchers have been able to perform equation-of-state measurements more precisely than ever before. Exposing targets to the high power levels of Z also allows scientists to study extreme states of matter, such as plasmas, and it may produce unexpected reactions and generate responses of great interest to many areas of science.
Fusion research on Z, too, contributes to broader scientific insight. Because near-perfect symmetry is necessary to ignite fusion (so the imploding particles will be forced to collide by not having room to escape), a persistent challenge in fusion science has been to heat the target evenly, so it will implode symmetrically. The capsule and container holding the target have to work together to produce the desired outcome, and their configurations and interactions have been the focus of intense theory and experimentation.
Diamond, for example, has been the object of much study as a potential capsule material. In melting diamond to a puddle, Z scientists have been able to understand the material’s various states – from solid to liquid, with a mixed state in-between. Thanks to Z, researchers now have a better understanding of the mixed state, which is not ideal to ignite a fusion reaction, and they can avoid it as they continue to experiment with diamond. In this and other ways, research on Z provides a roadmap for potential problems and opportunities on the path to fusion.
Beyond the fabrication of fusion pellets and the careful design of targets, achieving fusion requires work on many other interdependent elements including the machines, the mechanisms for delivering power onto a target, implementing detailed diagnostics for experiments, and creating computer codes to understand and then predict what the diagnostics revealed. Fusion is conducted in extremely complicated systems that involve complicated radiation dynamics as well as densities and temperatures not otherwise seen in nature. Trying to understand all the elements involved requires large computer codes, and tests conducted on the Z machine are very useful for testing and refining those codes. All of this work is crucial especially in conjunction with the National Ignition Campaign, which is the program to reach ignition of an inertial confinement fusion target at the National Ignition Facility.
How do Z pinches work?
The Z machine uses electricity to create radiation and heat, which are both applied to a variety of scientific purposes ranging from weapons research to the pursuit of fusion energy.
The process starts with wall-current electricity, which Z uses to charge up large capacitors (structures designed to store an electric charge). The electricity is supplied by a local utility company, and in every shot the machine consumes only about as much energy as it would take to light 100 homes for a few minutes. Metal cables arranged like the spokes of a wheel connect the capacitors to a central vacuum chamber, 10 feet in diameter and 20 feet high. The cables, some insulated by water and some by oil, are each as big around as a horse and 30 feet long.
When the accelerator fires, powerful electrical pulses strike a target at the center of the machine. Each shot from Z carries more than 1,000 times the electricity of a lightning bolt, and it finishes 20,000 times faster. The target is about the size of a spool of thread, and it consists of hundreds of tungsten wires, each thinner than a human hair, enclosed in a small metal container known as a hohlraum (German for hollow space). The hohlraum serves to maintain a uniform temperature.
The flow of energy through the tungsten wires dissolves them into a plasma and creates a strong magnetic field that forces the exploded particles inward. The speed at which the particles move is equivalent to traveling from Los Angeles to New York – about 3,000 miles – in slightly less than one second. The particles then collide with one another along the z axis (hence the name “Z pinch”), and the collisions produce intense radiation (in physics terms, 2 million joules of X-ray energy) that heats the walls of the hohlraum to approximately 1.8 million degrees Celsius.
As fast and powerful as the implosion is, the instruments that measure it must be even faster in order to record the details of the process. This is an added challenge for researchers who rely on the accuracy of these measurements to understand Z results and nuclear events in general. Accurate measurements also allow scientists to experiment with a variety of target arrangements to create conditions useful for many different purposes.