M.V. Fazio

Ultracompact pulsed power

Now more than ever, the pulsed power field is driven by size, weight, and volume constraints. In both the military and commercial arenas, there is an overwhelming need to provide more and more capability in ever smaller and lighter packages. The need for higher energy density, power density, reliability, and efficiency is driving progress in the field. This paper provides a review of the state of the art in various types of pulsed power components such as solid-state switches, capacitors, and power sources. In some cases, familiar components such as switch tubes are being replaced with whole new classes of devices. Batteries are being replaced by hydrocarbon or hydrogen-fueled mechanical systems that alter our paradigm for prime power sources. Trends over the past few years and future possibilities for ultracompact systems are discussed, including advanced techniques for heat removal and energy recovery. Finally, a few practical examples of ultracompact systems are given, emphasizing peak power and peak power density.

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.

Intense space-charge beam physics relevant to relativistic klystron amplifiers

In this paper, we examine intense space-charge beam physics that is relevant to beam bunching and extraction in a mildly relativistic klystron amplifier, and give numerical examples for a 5 kA, 500 keV electron beam in a 1.3 GHz structure. Much of the peculiar beam physics in these types of devices results from the partitioning of beam energy into kinetic and potential parts. Both tenuous-nonrelativistic and intense-relativistic beams produce effects different in nature from those produced by intense, mildly relativistic beams because the potential energy requirements are either negligible or fixed. In particular, we demonstrate anomalous beam bunching aided by the nonlinear potential requirements and we discuss maximum power extraction as a function of beam bunching. We show that although the space-charge effects can produce quite high harmonic current content, the maximum power extraction from the beam into RF typically occurs at relatively modest bunching. >

A 500 MW, 1 /spl mu/s pulse length, high current relativistic klystron

This paper describes the experimental development of a long pulse high current relativistic klystron amplifier (RKA). The desired performance parameters are 1 GW output power and 1 /spl mu/s pulse length with an operating frequency of 1.3 GHz. Peak powers approaching 500 MW have been achieved in pulses of 1 /spl mu/s nominal baseline-to-baseline duration. The half power pulse width is 0.5 /spl mu/s. These pulses contain an energy of about 160 J. RF output rises linearly in concert with the beam current pulse, and terminates abruptly just before the highest part of the pulsed voltage curve is reached. A possible explanation, not yet experimentally confirmed, for the premature termination of the RF pulse is an output cavity gap voltage that is too high, causing electron reflection at the gap and RF breakdown across the gap. A new output cavity has been designed with a much lower shunt impedance and a loaded Q of 4. >

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. >

Beam-cavity interaction physics for mildly relativistic, intense-beam klystron amplifiers

We examine beam-cavity interaction physics relevant to mildly relativistic, intense-beam klystron amplifiers. This is an interesting but difficult regime of operation, because of the combination of high beam current and low voltage. The advantage of this regime is that it is relatively easy to access high beam powers (and potentially high microwave output powers) at relatively low beam energy. We calculate the effect of the extremely high beam loading in the input and idler cavities. The output cavitys shunt impedance must match the low beam impedance in order to prevent high output gap voltages that will reflect electrons back upstream. This leads to very low cavity Q factors ( >

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.

Development of a long-pulse 1.3 GHz relativistic klystron amplifier

A research approach for obtaining kilojoule microwave pulses of microsecond duration at 1.3 GHz from the relativistic klystron amplifier is described. Achieving kilojoule microwave pulses requires extending electron beam pulse durations and maximizing the microwave extraction efficiency at the fundamental frequency. An electron beam diode has been constructed that delivers peak currents in excess of 5 kA with a monotonically increasing current pulse exceeding durations of 1 mu s at beam kinetic energies above 400 keV. Close attention has been given to minimizing the current losses from the diode. Maximum microwave extraction efficiency at the fundamental frequency has been related to the beam bunching amplitude and output cavity shunt impedance in terms of a simple circuit theory. The circuit theory predictions have been tested by particle-in-cell code calculations of the electron beam interactions with the proposed cavity structures. The successful cavity structures have been constructed and are awaiting testing. >

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. >

Experimental results from a J-band relativistic klystron amplifier developmental effort

Experimental results to-date will be presented from a developmental effort to a produce a J- band (5.85 - 8.2 GHz) relativistic klystron amplifier (RKA) of the high current Naval Research Laboratory (NRL) genealogy. The nominal experimental parameters of this RKA are: V0 approximately equals 600 kV; I0 approximately equals 2 - 4 kA; Bz approximately equals 1.5 T; (tau) beam approximately equals 300 ns; vin approximately equals 6.6 GHz; Pin <EQ 500 kW. Because of the smaller component sizes which accompany this frequency ((lambda) approximately equals 4.5 cm as compared with (lambda) approximately equals 30 cm for the bulk of other RKA research efforts), much of the discussion will concentrate on the physical principles, fabrication issues, and experimental pitfalls associated with scaling the RKA design.