Inductively coupled plasma (ICP) 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 two types of ICP geometries: planar and cylindrical. In planar geometry, the electrode is a coil of flat metal wound like a spiral. In cylindrical geometry, it is like a helical spring. When a time-varying electric current is passed through the coil, it creates a time-varying magnetic field around it, which in turn induces azimuthal electric currents in the rarefied gas, leading to the formation of a plasma. Argon is one example of a commonly-used rarefied gas.