Exceptionally pure epitaxial diamond layers have been grown by microwave plasma chemical vapour deposition, which have low boron doping, from 5 × 10 14 to 1 × 10 16 cm −3 , and the compensating n-type impurities are the lowest reported for any semiconducting diamond, <3 × 10 13 cm −3 . The hydrogen impurities that bind with the boron making them electrically inactive can be significantly reduced by baking the diamond to >700 • C for ∼1 s in air. Schottky diodes made on these epitaxial diamond films have breakdown voltages >6 kV, twelve times the highest breakdown voltage reported for any diamond diode and higher than any other semiconductor Schottky diode.

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/cm 2 are measured across an A-K gap of 5ϫ10 Ϫ2 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 ϳ1 s 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.

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A two-stage 500 kV 200-A ferroelectric electron gun has been designed, fabricated, tested, and used in a high power microwave amplifier experiment. 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 4.1 mm in good agreement with simulation. The gun is designed for use in traveling-wave tube amplifiers, 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.

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.

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Epitaxial diamond with remarkably low p-type doping (1×1014–1×1017 cm−3) and exceptionally low compensation ∼1×1013 cm−3, has enabled the demonstration of a optically-switched conduction modulation of the epitaxial layer. Charge exchange between the diamond substrate and the epitaxial layer makes it possible to modulate the conductivity of the epitaxial layer. Incandescent light will make the lightly p-doped epitaxial layer insulating and ultraviolet radiation will make the layer conductive again. Once the layer conductivity has been established it will remain in the same electrical state for days, if kept in the dark.

Our previous work on higli-powcr c f k i c n t X-Band TWT amplifiers has used a two stage device with bunching prnduced in a grcalcr tliaii light phase velocity region, immediately followed by a slioft low phase velocity output stnictore. The device is driven by a 7inm diainclcr 750 kV, 450.4 pencil clcctrnn beam. The structure, which has a 4 GHz bandwidth in the bunching scction, produces an ainplilicd notput with a power io the range 20-60 MW. At higher output powers pulsc shortening dcvclops. A serious candidate lor the pulse shortening is cxcitation of thc HEM,, mode in the structure. This mode overlaps tlic frequency domain o l the desired l M o 5 motlc. We have tlcsigncd and tcslcd new amplifier structures in which the scparatinn nf these rnodcs is substantially iocmiscd. Thc perfurmancc 0 1 tlic new amplificr(s) will he compared with that of the oltlcr device, and the rclcvancc of the hybrid modes tu pulsc shortening assessed.