Investigation of electron emission from bulk ferroelectric ceramic materials

Abstract

One means of enhancing high power microwave (HPM) tube performance is to improve the cathode technology. Of particular interest are low temperature cathodes that provide high current density electron beams without the problem of plasma closure. Our research focuses on the application of ferroelectric materials as an electron source. The mechanism of ferroelectric emission is initiated by rapidly altering or reversing the polarization of the material with a pulsed electric field. This excitation produces a charge imbalance at the surface of the material. The charge imbalance then accelerates screening charges into the HPM diode. Experiments are conducted using a demountable high vacuum triode with bulk (300 /spl mu/m-1 cm thickness) polar dielectrics. The material compositions investigated include 9/65/36 (Pb,La)(Zr,Ti)O/sub 3/ (PLZT) soft ferroelectrics, 2/95/5 PLZT antiferroelectrics, and hard Pb(Zr,Ti)O/sub 3/ materials. The samples under test are gridded with an evaporated metal film that is patterned with circular apertures. This grid is excited by a solid state driver that erects a field of 15 kV/cm in about 1 /spl mu/s. A DC bias voltage is used to reset the polarization of the material after each shot. Preliminary results indicate that this reset voltage greatly enhances the reliability of the electron emission. A DC voltage (up to 20 kV) applied to the anode accelerates any carriers liberated from the material. The materials have been operated as a repetitively pulsed electron source. Hysteresis measurements are also conducted in situ to determine the dielectric properties of the candidate materials.