Polarizability model of emission from ceramic cathodes

Abstract

We show that properly electroded ceramic disks are strong and robust electron emitters when excited with short voltage pulses. They can operate in two different regimes: (I) a regime at low exciting voltage, ∼0.7 kV per millimeter thickness and (II) a regime at high exciting voltage, ∼2 kV/mm. In the first regime the excitation pulse must be bipolar, positive–negative, the output current results in several tens of milliamperes per cm2. The first positive semiwave charges the cathode surface and the negative semiwave expels the previously accumulated electrons. This behavior of ceramic electroded disks, before as sink and next as source of electrons, is due to the succession of an attractive and then repulsive electron field on the unelectroded zones of the front surface. The two opposite oriented fields are generated by the voltage applied at the rear electrode. In the second regime, the excitation pulse can be either monopolar or bipolar, the emission process is governed by the building up of a plasma sheet as a consequence of the excitation pulse. The high-voltage exciting pulse initiates a discharge, and so the plasma, at the triple metal–insulator–vacuum point because of the strong longitudinal component of the electric field. The plasma cloud expands over the front surface becoming a dynamical electrode. Screening electrons accumulate in front of the sample, either in the plasma sheet or on the front surface of the material. The component of the electric field perpendicular to the surface (due to either the negative semiwave of a bipolar pulse or to the space charge of the huge amount of electrons no longer attracted by bound positive charge) pushes screening electrons out from the cathode.