A cold plasma is a gas in which the electrons are excited to very high energies, but the ions are only slightly excited. It is called “cold” because the electrons have much more energy than the ions, and so they collide frequently with each other, transferring their energy to the ions. The ions then collide with neutrals, exciting them too. In a cold plasma, all of the particles are at approximately the same temperature.
A cold plasma can be generated in several ways. One way is to pass an electric current through a gas. The electrons will be accelerated by the electric field and will collide with atoms, exciting them. Another way is to shine intense ultraviolet light on a gas. The photons will ionize atoms, creating electrons and ions that will then collide with each other and with neutrals.
Cold plasmas have many applications. They can be used for making thin films of materials such as semiconductors or metals. They can also be used for etching patterns into these materials. Cold plasmas are also used in medical devices such as blood coagulators and cancer treatments (where they kill cancer cells).