Commonly called as the forth state of matter is a mixture of ions, electrons and neutral gas molecules. In nature it can be observed in atmosphere in form of lightning, polar aurorae, stars, solar winds etc. In laboratory, a plasma can be generated by heating a neutral gas by subjecting to high electromagnetic fields. The term Plasma was first coined by Irving Langmuir in 1920 with words:
Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons.


When a positively charged particle with charge +Q is placed in the plasma, it redistributes the ions and electrons surrounding in such a way that Coulomb electrostatic potential is attenuated at distance beyond Debye length. This effect is known as Debye screening.

Plasma is in the constant state of vibrations due to small disturbances that gives rise to plasma oscillations. The vibration frequency is called plasma frequency.

From the characteristics of the plasma, there are different ways to classify plasma. Cold plasma, warm plasma, neutral plasma, non-neutral plasma and many other ways. In general the plasma can be identified by coordinates on temperature-density graph.

In the ion source plasma is ignited inside the chamber and the ions are extracted from plasma sheath. The fuel gas or the neutral gas is injected through small tubes or capillaries into the plasma chamber. It has been known that the ions would also back drifted in this small channel which can damage the material.

Grid erosion is the major concern for gridded ion thrusters. Here is an example of erosion of metal rod exposed to ion beam. These snapshots span over a year of time.