What makes a plasma a plasma?

Answer 1

The three criteria to define a plasmaThere needs to be enough charged particles in a given volume so that the plasma can act to shield an external potential. This means there needs to be many thousands of charged particles of both charges (positive and negative) inside a radius which is equal to the Debye length, typically less than a millimeter. The volume of the plasma needs to be big enough so that is covers many Debye lengths in each direction. If it takes a Debye length to shield a potential and the plasma is just one Debye length long then the shielding will not work. The number of collisions that the shielding particle has with neutral gas particles has to be small enough as not to prevent the shielding being effective. It is normally the electrons that do the shielding so that the time between electron-neutral collisions has to be longer that the response time of an electron to an electromagnetic field.