J. D. Ivers

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.

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.

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.

High current ion beam generation and transport system

The efficient transport of a 6.0‐kA, 1.1‐MV proton beam generated in a magnetically insulated diode and propagated along an axial magnetic field has been studied. The beam is better than 98% charge neutral. Focusing of the beam is observed as a result of beam transport through the radial magnetic insulation field. The application of this type of beam generation and transport system to a high current linear induction ion accelerator is discussed.

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.

A ferroelectric cathode, electron gun for high power microwave research

A pulse modulator, previously described at the 1995 PAC meeting, has been reconfigured to improve the pulse shape at a slightly lower beam energy, but with a higher current (500 kV, 1kA). The device has been run at rated voltage and current into a resistive load for pulse durations in excess of 250 ns and at /spl sim/0.1 Hz repetition rate. The modulator, which is designed for use in our high power microwave research program, has been coupled to an electron gun which uses a ferroelectric cathode and has been operated in this mode producing a 500 kV, 200 A electron beam. We report in this paper on the revised design and performance of the modulator and present preliminary data on the electron gun design and characteristics.

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.

Faraday cup to measure ion current in a strong magnetic field

A Faraday cup has been designed and tested to measure the ion current density in an intense ion beam immersed in a strong magnetic field. The ion flux is charge neutralized by a low‐energy codrifting electron beam. The codrifting electrons are deflected from the cup by use of an apertured magnetic plate at the entrance to the cup. Comparison of the integrated Faraday cup traces with the proton flux found from carbon activation measurements show that the ion Faraday cup consistently measured 59%±8% of the protons.

Proton Acceleration in an Induction Linac

An account will be given of the operational characteristics of the first module of a proton induction linac. The system, which has a design output of 1.0 MeV. at 1000 A. in a 50 nsec. pulse, has a 350 keV. electrostatic accelerator for proton beam generation, followed by a 700 keV. inductively driven accelerator. The induction gap is magnetically insulated. We describe the design and performance of the accelerator and show measurements of the beam characteristics. Techniques for proton beam transport, and preliminary measurements of the beam propagation, will also be presented.