Z-pinch

# Z-pinch

Discussion

Encyclopedia
In fusion power
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...

research, the Z-pinch, also known as zeta pinch or Bennett pinch (after Willard Harrison Bennett
Willard Harrison Bennett
Willard Harrison Bennett was a scientist and inventor, born in Findlay, Ohio. Bennett conducted research into plasma physics, astrophysics, geophysics, surface physics, and physical chemistry...

), is a type of plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it (see pinch
Pinch (plasma physics)
A pinch is the compression of an electrically conducting filament by magnetic forces. The conductor is usually a plasma, but could also be a solid or liquid metal...

). These systems were originally referred to simply as "pinch", but the introduction of the theta-pinch concept led to the need for increased clarity. The name refers to the direction of the current in the devices, the Z-axis on a normal mathematical diagram
Mathematical diagram
Mathematical diagrams are diagrams in the field of mathematics, and diagrams using mathematics such as charts and graphs, that are mainly designed to convey mathematical relationships, for example, comparisons over time.- Argand diagram :...

.

## Physics

The Z-pinch is an application of the Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...

, in which a current-carrying conductor in a magnetic field experiences a force. One example of the Lorentz force is that, if two parallel wires are carrying current in the same direction, the wires will be pulled toward each other. The Z-pinch uses this same effect; the entire plasma can be thought of as many current-carrying wires, all carrying current in the same direction, and they are all pulled toward each other by the Lorentz force, thus the plasma contracts. The contraction is counteracted by the increasing gas pressure of the plasma.

In Z-pinch machines the current is generally provided from a large bank of capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s and triggered by a spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...

. The power flows into electrodes on either end of a cylindrical vacuum tube. The experimental gas in the tube is pre-ionized to make it electrically conductive. When the current flows into the plasma, it pinches into a smaller cylinder, until the gas pressure and magnetic force balance. As the conductivity of plasma is fairly good, about that of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, the energy stored in the power source is quickly depleted by running through the plasma. Z-pinch devices are inherently pulsed in nature.

Z-pinch machines contrast with their better studied toroidal counterparts. As the plasma is electrically conductive, a magnetic field nearby will induce a current in it. In practical devices this was normally arranged by placing the plasma vessel inside the core of a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

, arranged so the plasma itself would be the secondary side. When current was sent into the primary side of the transformer, the magnetic field induced a current into the plasma. As induction requires a changing magnetic field, and the induced current is supposed to run in a single direction in most reactor designs, the current in the transformer has to be increased over time to produce the varying magnetic field. This places a limit on the product of confinement time and magnetic field, for any given source of power.

### Early machines

Pinch devices were among the earliest efforts in fusion power. The concept traces itself to work in the UK in the immediate post-war era, but a lack of interest led to little development until early 1950. Then, the announcement of the Huemul Project
Huemul Project
The Huemul Project was a secret project proposed by the German scientist of Austrian origin Ronald Richter to the government of Argentina during the first presidency of Juan Domingo Perón....

in early 1951 led to fusion efforts around the world, notably pinch devices in the UK and pinch and stellarator
Stellarator
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma...

s in the US. A number of small experiments were built at labs as various practical issues were addressed, but all of these machines demonstrated unexpected instabilities of the plasma that would cause it to hit the walls of the container vessel. The problem became known as the "kink instability".

### Stabilized pinch, the race to fusion

A number of solutions were proposed, and by 1953 the "stabilized pinch" seemed to solve the problems encountered on earlier devices. Stabilized pinch machines added a set of external magnets that created a toroidal magnetic field inside the chamber. When the device was fired, this field added to the one created by the current in the plasma. The result was that the formerly straight magnetic field was twisted into a helix, which meant that particles attempting to kink outward would soon find themselves on the inside of the vessel, where the outward force was the "proper direction".

Researchers in the UK planned a major assault on the stabilized pinch field, and started construction of ZETA in 1954. ZETA was by far the largest fusion device of its era, and equipped with all of the latest equipment. At the time, almost all fusion research was classified, so progress on ZETA was generally unknown outside the labs working on it. However, in 1956 the walls started to come down, and when they visited ZETA at Harwell, US researchers became aware that they were about to be trumped. A race broke out as teams on both sides of the Atlantic rushed to be the first to complete their stabilized pinch machines.

ZETA won the race, and by the summer of 1957 it was producing bursts of neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s on every run. Although the scientists working on the device, and similar ones in the US and UK, were careful to point out that it was not proven, the results were nevertheless released with great fanfare as the first successful step on the path to commercial fusion energy. However, further study soon demonstrated that the measurements were misleading, and none of the machines were near fusion levels. Interest in pinch devices faded, although ZETA and its cousin Sceptre
Sceptre (fusion reactor)
Sceptre was an early fusion power device based the Z-pinch concept of plasma confinement, built in the UK starting in 1957. They were the ultimate versions of a series of devices tracing their history to the original pinch machines, built at Imperial College London by Cousins and Ware in 1947...

would serve for many years as experimental devices.

### Tokamak

Although it remained relatively unknown for years, the Soviets used the pinch concept to develop the tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...

device. Unlike the stabilized pinch devices in the US and UK, the tokamak used considerably more energy in the stabilizing magnets, and much less in the plasma current. This reduced the instabilities due to the large currents in the plasma, and led to great improvements in stability. The results of their experiments were so good that other researchers were skeptical of them when they were first announced in full force in 1968. Members of the still-operational ZETA team were called in to verify the results. The tokamak has since gone on to become the most studied approach to controlled fusion.

### Self-stability

While much of the fusion research world rushed to build new tokamaks, ZETA still had new concepts to offer up. In 1974 John Bryan Taylor
John Bryan Taylor
John Bryan Taylor is a British physicist known for his important contributions to plasma physics and their application in the field of fusion energy. Notable among these is the development of the "Taylor state", describing a minimum-energy configuration that conserves magnetic helicity...

was examining an odd result that had been seen many times in ZETA with no explanation; often when the machine was "turned off", the plasma would settle into a stable state he called "quiescence". Studying the problem, he developed the Taylor state
Taylor state
In plasma physics, a Taylor state is the minimum energy state of a plasma satisfying the constraint of conserving magnetic helicity.- Derivation :...

concept, and from this came the reversed-field pinch concept. Research into these class of plasmas became a major effort in the 1980s and '90s.

## Various Z-pinch machines

They can be found in various institutions such as Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

(USA), University of Michigan
University of Michigan
The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of the University of Michigan...

(USA), Sandia National Laboratories
Sandia National Laboratories
The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....

(USA), Nevada Terawatt Facility (USA), Ruhr University
Ruhr University
Ruhr University Bochum , located on the southern hills of central Ruhr area Bochum, was founded in 1962 as the first new public university in Germany since World War II...

(Germany), Imperial College (United Kingdom), Ecole Polytechnique
École Polytechnique
The École Polytechnique is a state-run institution of higher education and research in Palaiseau, Essonne, France, near Paris. Polytechnique is renowned for its four year undergraduate/graduate Master's program...

(France), and the Weizmann Institute of Science
Weizmann Institute of Science
The Weizmann Institute of Science , known as Machon Weizmann, is a university and research institute in Rehovot, Israel. It differs from other Israeli universities in that it offers only graduate and post-graduate studies in the sciences....

(Israel).