Electron emission and beam generation using ferroelectric cathodes

In 1989, researchers at CERN published the discovery of significant electron emission (1--100 A/cm2) from Lead-Lanthanum-Zirconate-Titanate (PLZT). The publication of these results led to international interest in ferroelectric cathodes studies for use in pulsed power devices. At Cornell University in 1991, experiments with Lead-Zirconate-Titanate (PZT) compositions were begun to study the feasibility of using this ferroelectric material as a cathode in the electron gun section of High Power Traveling Wave Tube Amplifier Experiments. Current-voltage characteristics were documented for diode voltages ranging from 50--500,000 V with anode cathode gaps of .5--6 cm. A linear current-voltage relation was found for voltages less than 50 kV. For diode voltages ≥200 kV, a typical Child-Langmuir V3/2 dependence was observed. Additional experiments have demonstrated repetition rates of up to 50 Hz with current densities of ≥20 A/cm2. These results have been used in the ongoing design and construction of the electron gun for a 500 kV pulse modulator capable of repetitive operation at 1 Hz. The electron gun uses a PZT 55/45 (Pb(Zr.55,Ti.45 )O3) cathode to produce a ≤400 A electron beam focused by a converging magnetic field.; Studies of the emission process itself indicate the initial electrons are produced by field emission from the metallic grid applied to the front surface of the cathode. The field emission is induced by the application of a fast rising 1--3 kV, 150 ns pulse to the rear electrode of the 1 mm thick ferroelectric. Field emission can lead to explosive emission from microprotrusions and metal-ferroelectric-vacuum triple points forming a diffuse plasma on the surface of the sample. Under long pulse experiments (1--5 mus), plasma velocities of ∼2 cm/mus were measured from gap closure rates. Results from an ion Faraday cup experiment showed ion velocities of 1--2 cm/mus. Experimental evidence indicates the electron emission is dependent on the field emission initiated by the voltage applied to rear surface of the ferroelectric; however, for current pulse durations on the order of microseconds, the surface plasma expansion into the gap can dominate current flow.