J.D. Ivers

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 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...

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/cm/sup 2/. 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.

Electron emission from ferroelectric ceramics

The results of experiments on the emission of electrons from ferroelectrics are reported. A sample of PZT is pulsed to reverse the direction of the spontaneous polarization within the material. This releases the surface charge accumulated on the ferroelectric, which is needed to reduce the external electric field to a low value. Surface charge densities may amount to about 0.5 C/m/sup 2/. In these experiments, the emitted electron current density reaches a peak value of up to 70 A/cm/sup 2/ for a diode gap of 4 mm and an extraction voltage of 500 V. Beam current densities exceed the Child-Langmuir law for space-charge-limited emission by factors of up to 400. Measurements showing the dependence of the emission on the diode voltage and spacing are presented. The authors understanding of the emission process and its limitations are summarized.<<ETX>>

Efficient operation of a high-power X-band traveling wave tube amplifier

We report experimental results demonstrating 54% power conversion efficiency (43% energy conversion efficiency), from a two-stage X-band traveling wave tube amplifier designed for high-power operation. The first stage of the amplifier is a 12-cm-long Boron Nitride dielectric section used to modulate the electron beam. The second stage consists of a long high-phase-velocity bunching section followed by a short low-phase-velocity output section. Output powers of up to 78 MW with narrow spectrum width were obtained with ∼700 kV, ∼200 A beam.

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. >

Proton Injection into a Large Amplitude Space Charge Wave

An account is presented of some aspects of recent progress in the Collective Ion Space Charge Accelerator (CISCA) program at Cornell University. The object of this program is to explore the potential of the slow space charge wave on an electron beam for use in an ion accelerator. We describe in this paper the results of a study of a Luce diode as an ion source and outline initial results obtained when the proton beam is injected into a space charge wave growth section. We find that it is possible to inject a beam of protons through a vacuum diode, used to generate the beam for wave growth, and for the conditions achieved to date to maintain the growth of a coherent wave.

Neutralization and transport of high‐current proton beams in a two‐stage linear induction accelerator

Experimental results on the propagation and transport efficiency of a 1 MV, 5 kA, 50 ns annular proton beam through a two‐stage linear induction accelerator are presented. The beam is generated in a magnetically insulated diode and propagates with high‐efficiency along a 0.6 T axial magnetic field to a second accelerating gap located 30 cm downstream. The second accelerating gap increases the beam energy to 1.3 MeV. A full‐cusp geometry provides the magnetic insulation in both the diode and the second gap. We report in this paper an 86% (±5%) transport efficiency and an increase of 1.6° in the beam divergence for propagation through the post acceleration gap.

Space‐charge‐neutralized proton beam transport in an axial magnetic field

The generation and transport of a 1‐MV, 5‐kA, 50‐ns annular proton beam has been studied. The diode is magnetically insulated with a full‐cusp field geometry. Beam transport in a 6‐kG axial magnetic field is measured for injection into vacuum and neutral gas. The neutral gas background is supplied by a fast puff valve system. The gas background is found to enhance the space‐charge neutralization of the beam. Under these conditions the beam divergence is <1° and the annular profile is maintained throughout the 30‐cm transport region. We discuss a simplified model of the beam injection process to explain the experimental results.

A ferrite‐loaded autoaccelerator

An account is presented of the performance of an autoaccelerator in which energy is extracted from the front portion of a relativistic electron beam, via a ferrite‐loaded cavity, and stored in an oil‐filled transmission line. The extracted energy is returned to augment the energy of the electrons in the latter half of the beam. An increase of up to 33% in the electron beam energy is reported. The device was also used to investigate beam energy recovery. In this application the line was terminated in a matched load and up to 60% of the beam energy recovered. The autoaccelerator performance was limited by current pulse rise time degradation in the ferrite and by the formation of virtual cathodes during the deceleration phase. The latter process also limits the energy recovery.