Ion formation on the surfaces of electrodes

The initial phase of the onset of electrical breakdown in a vacuum discharge is characterized by very rapid ionization of surface material which leads to a kind of ''explosive'' plasma formation on the electrodes. As an increasing electric field is applied between the two electrodes of a vacuum diode the ionization process is initiated by the field emission of electrons from highly localized spots on the cathode surface. Despite the fundamental importance of cathode spots for the breakdown process, the structure of cathode spots and the fast ionization rates of surface layers were not fully understood. Besides the Joule heating of the field emitting spot, the electrons also desorb contaminants and ionize some of the released neutrals. Ions produced a short distance (∝1 μm) from the spot are accelerated back towards the cathode. This ion bombardment leads to surface heating of the spot and calculations of the power deposition show that ion surface heating is initially orders of magnitude larger than Joule heating. Ion bombardment is especially important at low initial current densities since it is efficient in producing desorption and sputtering of neutrals from the surface and hence increases the neutral density which in turn increases the ionization rate. As more ions are produced, a positive space charge layer forms which enhances the electric field and thus strongly enhances the field emitted electron current. Surface heating and the buildup of positive space charge rapidly lead to further enhanced field emission and finally thermionic electron emission. The localized buildup of an ion sheath above the electron emitting spot naturally leads to pressure and electric field distributions which provide such a large electron flow and ion sputtering rates that the spot explodes into a dense plasma within a few nanoseconds. (orig.)