D. Flechtner

Characteristics of electron emission from PZT ferroelectric cathode under strong accelerating field

We have studied emission characteristics of a PZT ferroelectric cathode under the influence of a strong accelerating field by varying the triggering conditions. The beam current pulse reveals a rising and a steady phase. In the rising phase, the time variation of the beam current is found to be linearly dependent on both the trigger voltage and the diode voltage at the time when the current starts. In the steady phase, field emission characteristics are observed. The results show that the diode voltage is not only accelerating the emitted electrons but also assisting the electron emission from the ferroelectric cathode. An empirical model is proposed and is found to yield a reasonable beam current pulse when the electric field on the surface of the cathode is uniformly distributed. It also provides us with a new possibility to diagnose the emission process of a ferroelectric electron gun.

Design of Ferroelectric Diode for High-Current Matched Electron-Beam Generation

An electron gun using a ferroelectric cathode with a two-stage compression system has been designed. The performance of the beam generated by the gun has been investigated by simulation and experiment. When the applied diode voltage is 440 kV, a beam current of 230 A is obtained. The waveform of the beam current follows the diode voltage reasonably well, and their flat top overlaps for about 200 ns. Faraday cup measurement shows that the beam radius is 4.1 mm after second-stage compression, and the axial variation of the beam radius has been minimized.

TWT amplifier using a ferroelectric cathode for electron beam generation

We report first experiments on high-power traveling wave amplifiers using an electron beam generated with a ferroelectric cathode. The electron beam, which is driven by a ferrite core transformer-pulse line system, has energy of 440 keV and a beam current of 40-50 Amperes. The beam pulse duration is about 250 ns and the system is operated at 0.07 Hz. The beam is generated from a command switched ferroelectric cathode located in the fringing field of a solenoid. A single stage disk loaded TWT structure is used as the /spl sim/9 GHz amplifier. A gain of 10 dB is observed over the frequency range from 8.9 to 9.4 GHz. The main purpose of the experiment is to demonstrate that the ferroelectric generated beam is of good enough quality for microwave amplification in X band, and to pave the way for use of this cathode in future high power microwave source experiments. We compare the results obtained using this system with the output from a PIC code simulation.

New results on electron emission from PZT ferroelectric cathodes

Ferroelectric cathodes may offer a source of high current density electron beams for applications where the use of conventional cathodes is limited by the required current density, cathode poisoning, or lifetime. In a ferroelectric cathode, electrons are emitted when the spontaneous polarization is rapidly changed by a pulsed electric field applied across the ferroelectric. When no additional voltage is applied to a planar diode gap, emission current densities are on the order of /spl sim/1 A/cm/sup 2/. When an additional field is applied to the gap, we have measured current densities of up to 100 A/cm/sup 2/. In this paper we report on two topics: (1) beam extraction into a drift tube at low (10-20 kV) voltage and, (2) electron emission, transverse to an applied magnetic field, from a cylindrical ferroelectric cathode.

Ferroelectric cathodes as electron beam sources

In the past decade a number of research groups have studied electron emission from ferroelectric ceramics. These materials have saturation polarization P/sub s/, of up to 100 /spl mu/C/cm/sup 2/. The emission occurs when the polarization state of the ferroelectric is changed rapidly by an applied electric field, and a fraction of the surface screening charge is released. We report experimental results obtained using Lead-Zirconate-Titanate (PZT) ceramic as the electron source in a planar diode geometry. Experimental measurements of time-dependent variations in the emission are presented and results from a theoretical model are compared to these measurements. We also present new data on the scaling of the emission current density for anode voltages of up to 50 kV. The new data will be used in the design of an electron gun using a ferroelectric cathode.

Electron beam generation using a ferroelectric cathode

Data is presented on the production of electron beams from a ferroelectric cathode at voltages of order 0.5 MV and current densities of order 100 A/cm2. In comparison with data at lower voltages the beam current scales as the three halves power of the voltage. An interpretation of the voltage dependent scaling, based on the coupling of electrostatic energy from the ferroelectric to the gun, is presented.

Studies of high efficiency interaction in traveling wave structures

We review the main experimental results on high power traveling wave amplifiers performed at Cornell; the highest efficiency achieved was 40% in a uniform periodic structure. For a higher efficiency, it is necessary to taper the output section. An analytical method to analyze and design quasi‐periodic output structure, is presented. It relies on the concept of interaction impedance matrix which is a generalization of the scalar interaction impedance in periodic structure. The efficiency is directly related to the largest eigen‐value of this matrix. We conclude with a brief discussion of the design of a new electron beam source for rf generation studies at low repetition rates.

A ferroelectric cathode electron gun for use in high power microwave sources

A two-stage 500 kV, 200 A, ferroelectric electron gun has been designed, fabricated and tested. We report on the operational characteristics of the gun including measurements of the beam dynamics. The optimum conditions for application of the trigger and its timing are also reported. Faraday cup measurement shows that the beam radius is 4.5 mm in good agreement with simulation. The gun is designed for use in traveling wave tube amplifiers (TWT) and testing of an X-band amplifier driven by the gun is reported. A peak output power of 5.9 MW has been observed from a single stage amplifier driven by a 100 A, 450 kV beam. This corresponds to energy converging efficiency of 13.1% and is the first observation of high power (∼MW) microwave generation using the beam generated from a ferroelectric cathode.

Current Pulse Shaping in Electron Gun Using Ferroelectric Cathode

We have operated an electron gun using a ferroelectric cathode at a diode voltage of 450 kV. The beam current and its waveform have been studied under various triggering conditions. Adjusting the amplitude and applied timing of the trigger pulse has led to a unique method of current pulse shaping for the ferroelectric diode. The method is convenient and the beam current with a reasonable pulse shape is achievable even with a relatively poor condition of the voltage pulse. Optimum conditions for applying the trigger pulse have been determined and the beam current with the required shape is obtained.

50 Hz electron emission from PZT ferro-electric cathodes

Summary form only given, as follows. Ferro-electric cathodes may offer a source of high current density electron beams for applications where the use of conventional field emitters is limited by repetition rate and lifetime. In a ferro-electric cathode, electrons are emitted when the spontaneous polarization is rapidly changed by a pulsed electric field applied across the ferroelectric. When no additional voltage is applied to a planar diode gap, emission current densities are on the order of 1 A/cm/sup 2/. When an additional field is applied to the gap, we have measured current densities up to 100 A/cm/sup 2/. In a new configuration that permits beam extraction into a drift tube, the cathode is pulsed 10-20 kV negative and electron current densities of /spl sim/20 A/cm/sup 2/ at repetition rates up to /spl sim/50 Hz (power supply limited) have been measured. The one inch diameter ferro-electric cathode is located in the fringing region of a 1.5 kG solenoid magnetic field /spl sim/2.8 cm from the entrance of a grounded drift tube. A Faraday cup is located several centimeters inside the drift tube and measurements show that repeatable beam current can be extracted from the ferroelectric cathode in this geometry.