G. S. Kerslick

Electron‐beam diodes using ferroelectric cathodes

A new high current density electron source is investigated. The source consists of a polarized ceramic disk with aluminium electrodes coated on both faces. The front electrode is etched in a periodic grid to expose the ceramic beneath. A rapid change in the polarization state of the ceramic results in the emission of a high density electron cloud into a 1 to 10mm diode gap. The anode potential is maintained by a charged transmission line. Some of the emitted electrons traverse the gap and an electron current flows. The emitted electron current has been measured as a function of the gap spacing and the anode potential. Current densities in excess of 70 A/cm2 have been measured. The current is found to vary linearly with the anode voltage for gaps <; 10 mm, and exceeds the Child-Langmuir current by at least two orders of magnitude. The experimental data will be compared with predictions from a model based on the emission of a cloud of electrons from the ferroelectric which in turn reflex in the diode gap.

Analysis of a diode with a ferroelectric cathode

It has been shown experimentally that electron current densities of more than 30 A/cm2 can be achieved from a cathode made of ferroelectric ceramic, when applying a field of order 0.1 MV/m. This current exceeds the Child–Langmuir current by two orders of magnitude. The current in the diode varies linearly with the applied voltage, provided that the latter is positive. In this theoretical study we show that the ferroelectric material plays a crucial role in the emission process. When a voltage is applied to the ferroelectric, the internal polarization field varies and the amount of screening charge required decreases. As a result, the electrons distribution near the cathode changes, forming a cloud which fills part of the diode gap. If now a positive voltage is applied to the anode, electrons are readily transferred through the diode gap. The qualitative and quantitative results of the theory are in good accordance with the experiment.

A high-power, traveling wave tube amplifier

High-power X-band traveling-wave tube amplifiers (TWTs) have been fabricated and tested. The tubes have gains ranging from 13 to 35 dB at 8.76 GHz and output powers ranging from 3 to 100 MW. The amplifiers are driven by the interaction of a slow space-charge wave, propagating on an electron beam, with an electromagnetic wave supported by the structure. The electron beam, which is produced from a magnetic-field-immersed field-emission cathode, has an energy of 850 keV, a current in the 1-kA range, and a pulse duration of 100 ns. The amplifiers are designed to operate as narrow-band devices in the TM/sub 01/ mode. A report is presented on the amplifier characteristics, and their performance is compared with calculated performance using conventional TWT theory. The scaling of the gain and bandwidth with the beam current are approximately as expected from theory, but the absolute magnitude of the gain is somewhat greater than expected. >

Electron emission from lead–zirconate–titanate ceramics

We report extensive experimental data on electron emission from lead–zirconate–titanate ferroelectric ceramics. A 1–2 MV/m pulse is applied to a gridded ferroelectric cathode and diode currents of up to 120 A/cm2 are measured across an A–K gap of 5×10−2 m, with the anode at 35 kV. Both the current and the anode voltage pulse duration may extend to several microseconds. The measurements extend previously reported data by nearly two orders of magnitude in the diode voltage and by a factor of more than 3 in the diode spacing. Two major regimes of operation were identified. In the first ∼1 μs the ferroelectric cathode controls the electron flow through the diode. Beyond this time plasma effects dominate the current flow. The results are of importance to the development of novel cathodes for high current electron beam generation.

A high-power two stage traveling-wave tube amplifier

Results are presented on the development of a two stage high‐efficiency, high‐power 8.76‐GHz traveling‐wave tube amplifier. The work presented augments previously reported data on a single stage amplifier and presents new data on the operational characteristics of two identical amplifiers operated in series and separated from each other by a sever. Peak powers of 410 MW have been obtained over the complete pulse duration of the device, with a conversion efficiency from the electron beam to microwave energy of 45%. In all operating conditions the severed amplifier showed a ‘‘sideband’’‐like structure in the frequency spectrum of the microwave radiation. A similar structure was apparent at output powers in excess of 70 MW in the single stage device. The frequencies of the ‘‘sidebands’’ are not symmetric with respect to the center frequency. The maximum, single frequency, average output power was 210 MW corresponding to an amplifier efficiency of 24%. Simulation data is also presented that indicates that the...

Axial extraction of high‐power microwaves from relativistic traveling wave amplifiers

We report theoretical and experimental results from research into coaxial extraction of high‐power microwaves from X‐band traveling wave tube amplifiers. Power levels exceeding 60 MW have been measured at 9.1 GHz. The output level is relatively constant for the full 70 ns duration of the 700 kV, 500 A electron beam pulse. Results indicate that this coaxial geometry is broadband when compared to traditional, highly tuned radial extraction and may thus have applications in a range of high‐power microwave devices.

Sideband development in a high-power traveling-wave tube microwave amplifier

The work presented describes the characteristics of single stage and severed high‐efficiency, high‐power traveling‐wave tube amplifiers operating in X band at 8.76 GHz. Average amplified output powers of 210 MW have been achieved at 24% efficiency. At high output power levels (≳100 MW) sidebands develop increasing the average radiated power to over 400 MW with a microwave conversion efficiency of over 45%. In single frequency operation phase stability to within ±8° has been demonstrated.

On the bandwidth of a short traveling wave tube

We analyze the bandwidth of a short traveling wave tube. When no beam is present the bandwidth is proportional to the group velocity and inversely proportional to the total length of the system. The bandwidth of the gain factor, the imaginary part of the wave vector, is mainly determined by the beam current. However, regardless of the current intensity, the gain factor is identically zero for frequencies which, in the empty structure, correspond to phase velocities larger than the speed of light. The bandwidth of a short amplifier, is narrowed relative to the ‘‘cold’’ bandwidth by the same amount the amplitude of the growing wave increases, provided that the frequency and the current allow the growing wave to be dominant. This relation is verified with experimental data and good agreement is found.

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.

Low group velocity traveling wave tube amplifiers

We report experimental and theoretical results from research into high power X-band traveling wave tube amplifiers designed to eliminate sidebands caused by reflections from the output of such structures. These amplifiers have a low energy velocity, such that the time it takes a wave to be reflected from the output to the input is of the order of, or greater than, the electron beam pulse duration. The elimination of sidebands and the effects of reflections is achieved by this transit time isolation, The bandwidth of the output spectrum is limited by the low pass-band of the periodic structures. Such amplifiers have been operated at power levels of up to 160 MW at 9 GHz for 50 ns pulse durations. >