R. F. Hoeberling

Narrow‐band microwave generation from an oscillating virtual cathode in a resonant cavity

An experimental approach using a high Q, resonant cavity surrounding an oscillating virtual cathode has achieved frequency stabilization, and repeatable narrow‐band operation of the virtual cathode microwave source. A cylindrical cavity resonator is used with the microwave power being extracted radially through circumferential slot apertures into dominant‐mode L‐band waveguide. The electron‐beam/cavity interaction produces strong feedback between the induced cavity field and the oscillating virtual cathode, forcing it to lock to the resonant frequency of a cavity mode over a large variation in electron beam current. The 3‐dB frequency bandwidth observed during single 100‐ns pulses is less than 1%. The 3‐dB bandwidth appears to be limited by the finite temporal width of the microwave pulse.

Advances in virtual cathode microwave sources

The evolution of the virtual cathode microwave source and its current performance are described. Explosive generator driven virtual cathode oscillators, resonant cavity sources, and phase-locking and amplifier sources are covered. Areas for future development are discussed. >

The virtual cathode microwave amplifier experiment

This paper describes the results of an experiment that tests a new high‐power microwave amplifier concept that uses the virtual cathode phenomenon as an amplifier rather than as a free‐running oscillator. The virtual cathode is surrounded by a cavity resonator that is driven by an input signal. The output from this virtual‐cathode amplifier is frequency locked to the input signal, and exhibits amplification over at least a 10‐dB dynamic range.

A microsecond-pulse-length, frequency-stabilized virtual-cathode oscillator using a resonant cavity

Experiments at the microsecond pulse length have demonstrated that the virtual-cathode oscillator generates narrowband (0.3% bandwidth) microwave pulses when the virtual cathode is surrounded by a resonant cavity and is driven by an appropriate electron beam. This result is a significant departure from the behavior of a free-running virtual-cathode oscillator where the frequency depends on the (current density)/sup 1/2/. The long-pulse experimental results are described. >

Long-Pulse Applications of Pulse-Forming Lines for High-Power Linac Application

The ever present demands for high efficiency in the RF power stations for particle accelerators has caused increased interest in longer RF pulses (tens of microseconds) for linacs such as the Pion Generator for Medical Irradiation (PIGMI) and Free Electron Laser (FEL). For either RF power station, a fundamental decision is whether to use a modulating anode/hard-tube driver or pulsed cathode/line-type pulser configuration. The choices in the extremes of low power for very long pulses or for very-high-power, short pulses are, respectively, a modulated anode/hard tube modulator and pulsed cathode/pulse forming line. However, the demarcation between these two extremes is not clearcut. The criteria (cost, flexibility performance, reliability, efficiency) that resulted in the RF station definition of these two specific systems will be described.

Los Alamos High-Power Microwave Source Development: The Resonant-Cavity Virtual Cathode And The Large-Orbit Gyrotron

The need for new, very intense microwave sources has been stimulated by the requirements for high-power phased arrays, vulnerability and effects tests sources, and the next generation of particle accelerators including the proposed linear colliders. All of these applications place stringent demands on the microwave source in addition to high power, including frequency and phase stability, efficiency, and long pulse ( > 1 μs) repetitive operation. Los Alamos is working on the resonant-cavity virtual cathode source and the large-orbit gyrotron in an attempt to experimentally address some of these issues. The resonant-cavity virtual cathode research has succeeded in demonstrating single-mode, narrow-band operation by surrounding the oscillating virtual cathode with a resonant-cavity structure. An experimental effort to injection lock the oscillating virtual cathode to a low-power injected signal is currently underway. The large-orbit gyrotron research is directed toward developing a technique for efficiently coupling the microwave output power into rectangular wave-guide. A 1-MV, 20-kA modulator with a 1-μs pulse length and a 5-Hz pulse repetition frequency (prf) has been designed and is currently in development; it will be used to drive a repetitively pulsed large-orbit gyrotron. The overall experimental effort is described and current experimental results are discussed.