An inductively coupled plasma (ICP) or transformer coupled plasma (TCP) is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.

There are three types of ICP geometries: planar (Fig. 3 (a)), cylindrical (Fig. 3 (b)), and half-toroidal (Fig. 3 (c)).

In planar geometry, the electrode is a length of flat metal wound like a spiral (or coil). In cylindrical geometry, it is like a helical spring. In half-toroidal geometry, it is a toroidal solenoid cut along its main diameter to two equal halves.

When a time-varying electric current is passed through the coil, it creates a time-varying magnetic field around it, with flux

${\displaystyle \Phi =\pi r^{2}H=\pi r^{2}H_{0}\cos \omega t}$,

where r is the distance to the center of coil (and of the quartz tube).

According to the Faraday–Lenz's law of induction, this creates azimuthal electromotive force in the rarefied gas:

${\displaystyle U=-{\frac {d\Phi }{dt}}}$,