Pinch effect principle.

Compression of the plasma by an electric current flowing through it. When current flows in the same direction through two parallel wires, they will be pulled toward each other due to the Lorentz force. The plasma can be thought of as many parallel wires, so when a current flows through the plasma, a magnetic field begins to arise and compresses the plasma. The effect is called a pinch. Two major principles could be used: the Z-pinch and the theta-pinch. In the Z-pinch, the current runs down the axis of the device (e.g., a cylinder filled with gas) while the magnetic field is azimuthal. In theta-pinch, the magnetic field runs down the device axis while the electric field is in the azimuthal direction. Pinch was used in the early days of thermonuclear fusion research.