Study of the surface discharge on ferroelectrics

Ferroelectric plasma sources are known as sources of surface discharge plasma. Main advantage of these sources is the ability to create layers of plasma on surfaces with large area and arbitrary shape. In vacuum, numerous discharges on the surface of ferroelectric ceramics appear in response of a driving pulse applied across a sample of ferroelectric ceramics. Electron avalanches can be initiated by field emission from triple junctions at the edges of patterned electrodes in electric fields, induced by inhomogeneities of the polarization charge. These avalanches along the surface are driven by secondary electron emission form the ferroelectric ceramics. Surface ablation and gas desorption supply neutrals, which are further ionized by avalanching electrons, resulting in a surface plasma. The depicted scenario is the same as for surface flashover; however, discharge on ferroelectrics does not turn to vacuum arc because of separation of electrodes by the ceramic bulk. This feature makes the discharge on ferroelectrics very interesting for study of the plasma formation process in the surface discharge. For instance, initial plasma density and electron temperature in the stem of the discharge can be deduced from the temporal profiles of the density in expanding plasma. For PZT ceramics, the initial plasma density was found in the range of (0.4-4)×10 ¹⁷ cm ^-3 and the initial electron temperature of about 16-20 eV for the driving electric fields of 16-30 kV/cm. It is shown that the discharge on ferroelectrics can be realized also in a gas mode. Transition from vacuum surface discharge to the gas discharge begins at the background pressure of 1-20 Torr, and the surface discharge in this pressure range sustains the gas discharge. At the pressures between 20 and 80 Torr, formation of the discharge stems is completely suppressed, which indicates the range of neutral pressure in the stem of the surface discharge in vacuum. The gas discharge is uniform and it appears on the open surface of the ferroelectric ceramics. In Argon at a distance of 5.5 mm from the surface, the maximal plasma density of ∼10 ¹³ cm ^-3 was observed at pressure of ∼8 Torr. Preliminary study of secondary electron emission from ferroelectrics shows that at low energies of primary electrons the total yield of electron-induced SEE from ferroelectrics is high comparing with other dielectrics and it depends on polarization of the ferroelectric material. Low value of energy which corresponds to the unity yield of SEE from ferroelectrics leads to higher desorption rate and consequently to lower threshold of the surface discharge on ferroelectrics.