Pinch effect (electrodynamics)

The pinch effect is the reason why narrow, only a few centimeters wide channels for the current form in lightning .

As pinch effect (of English. To pinch squeeze) is a phenomenon of the electric dynamics referred to, which in the plasma physics and the welding equipment is important and on the Lorentz force is based.

Plasma physics

In plasma physics, the term refers to the contraction of a plasma through which a sufficiently large electric current flows to form a thin, compressed plasma tube or thread as a result of the interaction of the plasma current with the magnetic field it generates. The pinch effect was previously used to heat up and limit a plasma to extremely high temperatures, for example in nuclear fusion in the Z machine . The arrangement of a cylindrical plasma with the flow along the cylinder axis, with the plasma being compressed radially, is also referred to as a linear pinch or z-pinch . If, in addition, an external axial magnetic field is applied, as the arrangement is Screw-pinch ( English screw ) designated which is more stable than the linear pinch. Another possible arrangement in which the plasma is arranged in the form of a torus is the reversed field pinch .

The lifetime of pinch plasmas is only a few microseconds. The pinch effects have therefore receded into the background for the development of a fusion reactor.

Bennett equation

The Bennett equation (based on Willard Harrison Bennett ) specifies the current required to compress a plasma column in the z-pinch. The following applies to the discharge current:

${\displaystyle I^{2}={\frac {8\pi }{\mu _{0}}}Nk_{\mathrm {B} }T}$

with : discharge current : magnetic field constant , : charge carrier density per unit length , : Boltzmann constant and : plasma temperature ${\displaystyle I}$ ${\displaystyle \mu _{0}}$ ${\displaystyle N}$ ${\displaystyle k_{\mathrm {B} }}$ ${\displaystyle T}$

Arc welding

Material detachment due to the pinch effect during arc welding with a melting electrode

In welding technology, the detachment of the drop at the end of the wire due to the constriction effect is referred to as the pinch effect.

The current-carrying, melting wire electrode is surrounded by a magnetic field. The magnetic field exerts a radial force ( Lorentz force ) on the wire, which increases with the square of the current density. At the liquid end of the wire, it is immediately constricted where a smaller cross-section and thus a higher current density is established. This leads to the detachment of the liquid material.

Further information

• Ulrich Stroth: Plasma Physics . 1st edition. Vieweg + Teubner, Wiesbaden 2011, ISBN 978-3-8348-1615-3 .
• Horst Stöcker: Pocket book of physics . 6th edition. Harri Deutsch, Leck 2010, ISBN 978-3-8171-1860-1 .

credentials

1. ↑ M. Schellhase: The welding arc as a technological tool. Verlag Technik, Berlin 1985, ISBN 3-87155-100-7 , p. 135.