A pinch (or: Bennett pinch (after Willard Harrison Bennett), electromagnetic pinch, magnetic pinch, pinch effect, or plasma pinch.) is the compression of an electrically conducting filament by magnetic forces, or a device that does such. The conductor is usually a plasma, but could also be a solid or liquid metal. Pinches were the first type of device used for experiments in controlled nuclear fusion power. Pinches occur naturally in electrical discharges such as lightning bolts, planetary auroras, current sheets, and solar flares.
Basic mechanism
Types
Pinches exist in nature and in laboratories. Pinches differ in their geometry and operating forces. These include:
Uncontrolled – Any time an electric current moves in large amounts (e.g., lightning, arcs, sparks, discharges) a magnetic force can pull together plasma. This can be insufficient for fusion. Sheet pinch – An astrophysical effect, this arises from vast sheets of charged particles. Z-pinch – The current runs down the axis, or walls, of a cylinder while the magnetic field is azimuthal Theta pinch – The magnetic field runs down the axis of a cylinder, while the electric field is in the azimuthal direction (also called a thetatron) Screw pinch – A combination of a Z-pinch and theta pinch (also called a stabilized Z-pinch, or θ-Z pinch) Reversed field pinch or toroidal pinch – This is a Z-pinch arranged in the shape of a torus. The plasma has an internal magnetic field. As distance increases from the center of this ring, the magnetic field reverses direction. Inverse pinch – An early fusion concept, this device consisted of a rod surrounded by plasma. Current traveled through the plasma and returned along the center rod. This geometry was slightly different than a z-pinch in that the conductor was in the center, not the sides. Cylindrical pinch Orthogonal pinch effect Ware pinch – A pinch that occurs inside a Tokamak plasma, when particles inside the banana orbit condense together. Magnetized liner inertial fusion (MagLIF) – A Z-pinch of preheated, premagnetized fuel inside a metal liner, which could lead to ignition and practical fusion energy with a larger pulsed-power driver.
Common behavior Pinches may become unstable. They radiate energy across the whole electromagnetic spectrum including radio waves, microwaves, infrared, x-rays, gamma rays, synchrotron radiation, and visible light. They also produce neutrons, as a product of fusion.
Applications and devices Pinches are used to generate X-rays and the intense magnetic fields generated are used in electromagnetic forming of metals. They also have applications in particle beams including particle beam weapons, astrophysics studies and it has been proposed to use them in space propulsion. A number of large pinch machines have been built to study fusion power; here are several:
MAGPIE A Z-pinch at Imperial College. This dumps a large amount of current across a wire. Under these conditions, the wire becomes plasma and compresses to produce fusion. Z Pulsed Power Facility at Sandia National Laboratories. ZETA device in Culham, England Madison Symmetric Torus at the University of Wisconsin, Madison Reversed-Field eXperiment in Italy. Dense plasma focus in New Jersey University of Nevada, Reno (USA) Cornell University (USA) University of Michigan (USA) University of California, San Diego (USA) University of Washington (USA) Ruhr University (Germany) École Polytechnique (France) Weizmann Institute of Science (Israel) Universidad Autónoma Metropolitana (Mexico). Zap Energy Inc. (USA)
Crushing cans with the pinch effect
Many high-voltage electronics enthusiasts make their own crude electromagnetic forming devices. They use pulsed power techniques to produce a theta pinch able to crush an aluminium soft drink can using the Lorentz forces created when large currents are induced in the can by the strong magnetic field of the primary coil. An electromagnetic aluminium can crusher consists of four main components: a high-voltage DC power supply, which provides a source of electrical energy, a large energy discharge capacitor to accumulate the electrical energy, a high voltage switch or spark gap, and a robust coil (capable of surviving high magnetic pressure) through which the stored electrical energy can be quickly discharged in order to generate a correspondingly strong pinching magnetic field (see diagram below).
In practice, such a device is somewhat more sophisticated than the schematic diagram suggests, including electrical components that control the current in order to maximize the resulting pinch, and to ensure that the device works safely. For more details, see the notes.
History
The first creation of a Z-pinch in the laboratory may have occurred in 1790 in Holland when Martinus van Marum created an explosion by discharging 100 Leyden jars into a wire. The phenomenon was not understood until 1905, when Pollock and Barraclough investigated a compressed and distorted length of copper tube from a lightning rod after it had been struck by lightning. Their analysis showed that the forces due to the interaction of the large current flow with its own magnetic field could have caused the compression and distortion. A similar, and apparently independent, theoretical analysis of the pinch effect in liquid metals was published by Northrup in 1907. The next major development was the publication in 1934 of an analysis of the radial pressure balance in a static Z-pinch by Bennett (see the following section for details). Thereafter, the experimental and theoretical progress on pinches was driven by fusion power research. In their article on the "Wire-array Z-pinch: a powerful x-ray source for ICF", M G Haines et al., wrote on the "Early history of Z-pinches".
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