The Scylla theta-pinch device (white tube on the right).

In 1905, a lightning strike crushed a hollow copper lightning conductor. James Pollock and Samuel Barraclough investigated the damage and concluded that the enormous current flowing through the tube had generated a magnetic field whose inward electromagnetic force compressed the conductor. This phenomenon later became known as the pinch effect and played an important role in early fusion research.

Plasma Squeezing

When a strong electric current flows through a plasma, it generates a magnetic field around the current. The interaction between this magnetic field and the current produces an inward Lorentz force that compresses, or “pinches”, the plasma into a narrower column. If the compression is sufficiently rapid and strong, the plasma can be heated to temperatures relevant for fusion.

Z-pinch and Theta-pinch

Two principal configurations were developed: the Z-pinch and the theta-pinch. In a Z-pinch, an electric current flows along the axis of the plasma column and generates an azimuthal magnetic field that compresses the plasma inward. In a theta-pinch, a rapidly changing axial magnetic field induces an azimuthal current in the plasma, producing radial compression. Both linear and toroidal configurations were investigated; linear devices also had to contend with particle and energy losses through their open ends.

Lightning conductor deformed by the pinch effect of a lightning strike.

Perhapsatron

Glowing plasma inside a laboratory Z-pinch device.

Experimental and theoretical work on pinches accelerated in the early 1950s as fusion research received greater support in the United Kingdom, the United States and the Soviet Union. At Los Alamos, James Tuck developed the Z-pinch device known as the Perhapsatron. The first machine was built in 1952–1953 using relatively simple components, including a toroidal glass chamber made by a local glass shop and a large iron transformer core that induced current in the plasma. The experiments soon revealed severe plasma instabilities that became a central problem for Z-pinch research.

In 1951, Tuck and Lyman Spitzer separately sought support from the U.S. Atomic Energy Commission for the pinch and stellarator concepts. Spitzer received USD 50,000 for the stellarator programme, while Tuck later obtained a similar amount from discretionary funds at Los Alamos to pursue the Perhapsatron.

ZETA

The British ZETA Z-pinch experiment.

Another major Z-pinch experiment was ZETA (Zero Energy Thermonuclear Assembly), which began operation in the United Kingdom in 1957. Its aluminium torus had a major radius of about 1.6 metres, and its 150-tonne pulse transformer was among the largest then built in Britain. ZETA soon produced bursts of up to about one million neutrons per discharge, initially raising hopes that thermonuclear fusion had been achieved. Subsequent measurements showed, however, that most of the neutrons were produced by non-thermal processes associated with plasma instabilities rather than by a uniformly hot thermonuclear plasma. Despite this setback, ZETA made important contributions to plasma physics. It helped clarify kink and sausage instabilities, contributed to the development of the reversed-field-pinch concept and became an important platform for early laser Thomson-scattering measurements of plasma temperature.

Scylla-I

In 1958, the Scylla-I theta-pinch experiment at Los Alamos produced hot deuterium plasma and neutron emission consistent with thermonuclear fusion, becoming one of the first magnetic-confinement devices to demonstrate genuine thermonuclear reactions. The early Scylla devices used linear plasma columns and reached temperatures of the order of 15 million K, but confinement times remained very short because of instabilities and particle losses through the open ends.

A later toroidal theta-pinch experiment, Scyllac, was developed in an attempt to overcome end losses and move towards reactor conditions, but it did not achieve the expected performance and the programme was eventually discontinued in 1977.

During the 1960s and 1970s, pinch concepts gradually lost their leading role in controlled-fusion research as tokamaks achieved much better confinement. Pinch research nevertheless continued in specialised forms and remains an active area of fusion and high-energy-density plasma research today.