Electron emission from bulk and thin film ceramic materials

Abstract

Summary form only given, as follows. 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 utilization of ferroelectric materials as an electron source. Ferroelectric emission is initiated by rapidly reversing or altering the spontaneous polarization of the dielectric with a pulsed electric field. This excitation produces a charge imbalance at the surface of the material. The intense field resulting from this charge imbalance accelerates screening charges into the HPM diode. The investigation will include the performance of thin film ferroelectric materials which can be switched with lower excitation voltage (2-30 V) and can be coated onto any shape. Experiments are conducted using a demountable high vacuum triode with both bulk (300 μm-1 mm thickness) and thin film (≈10 μm thickness) polar dielectrics. The material compositions investigated include 9/65/35 (Pb, La)(Zr, Ti)O/sub 3/(PLZT) soft ferroelectrics, 2/95/5 PLZT antiferroelectrics, and hard ferroelectric Pb(Zr, Ti)O/sub 3/ materials. The samples under test are gridded with an evaporated gold film that is patterned with circular apertures. This grid electrode is excited by a solid state drive circuit that erects a field of 15 kV/cm with a risetime of about 1 μs. Additional tests are performed with samples electroded with thin metal films (≈500 A) with no grid pattern. 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. Results to be presented include peak emitted charge and current and hysteresis measurements for both 60 Hz and pulsed excitation.