The achievements resulting from the application of advanced pulsed power to the generation of high power microwaves (HPM) have included the generation of multi-gigawatt pulses of RF energy. The power achievable is orders of magnitude greater than conventional microwave sources can generate. However, the introduction of the HPM technology into logical applications has been limited to date due to the phenomenon of pulse shortening in which the RF pulse terminates before the pulse power source used to produce it. Conventional microwave tubes can generate a few to 10 MW of power with pulsewidths of many microseconds when required. High power microwave sources can produce gigawatts of power, but only for relatively short pulsewidths, typically tens to hundreds of nanoseconds. An international effort during the past few years has generated important new discoveries toward the elimination of pulse shortening. Some of the new techniques have the potential for helping the conventional tube industry as well as being practical for high power microwave sources. This paper reviews the pulse shortening problem, its causes, and the worldwide scope and direction of research conducted to date to resolve it. The paper also discusses the potential remedies for the problem and recommends a course of research to further progress on the issue.

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Evolution of pulse shortening research in narrow band, high power microwave sources

An overview is provided for the novel relativistic klystron amplifiers which are under active study at the Naval Research Laboratory. These amplifiers are driven by an annular intense relativistic electron beam (500-kV, 10-kA range), which is modulated by an external RF source (1.3-GHz, 100-kW range). Experiments, theory, simulation, and simple models are presented to illustrate the unusual properties of such devices which result from the intense space charge of the beam. Chief among them are electrostatic insulation against vacuum breakdown at high power levels, efficient current modulation, short bunching length, and amplitude and phase stability of the output signal. Many of these unexpected features were revealed in two separate experiments: one with a lower current beam (5 kA, 2-cm beam radius), and the other one with a higher current beam (16 kA, 6.6-cm beam radius). Three gigawatts of RF power at 1.3 GHz were generated with the large diameter beam at an efficiency of 35% with 37-dB gain. These experiments are reviewed, along with a combination of particle simulation results and analytic models which facilitate the interpretation. Special attention is paid to the unfamiliar features of these amplifiers, and the critical problems which must be solved before such amplifiers can fulfil their potential in a wide range of applications are addressed. >

Relativistic klystron amplifiers driven by modulated intense relativistic electron beams

Improved concurrent electromagnetic particle-in-cell (ICEPIC), developed at the US Air Force Research Laboratory, is a three-dimensional (3-D) particle-in-cell (PIC) code specifically designed for parallel high-performance computing resources. ICEPIC simulates collisionless plasma physics phenomena on a Cartesian grid. ICEPIC has several novel features that allow efficient use of parallel architectures, including automated partitioning, dynamic load balancing, and an advanced parallel PIC algorithm. Even though ICEPIC is still in development, it has successfully simulated real-world high-power microwave (HPM) experimental devices, such as the magnetically insulated line oscillator (MILO) and the relativistic klystron oscillator (RKO), and the results from recent RKO simulations will be presented.

3-D ICEPIC simulations of the relativistic klystron oscillator

In its classical embodiment-a circular cylindrical cavity driven by a rectilinear electron beam-the simplest of the transit-time microwave tubes, the monotron, has its conversion efficiency limited to 20.0%. However, we demonstrate here that on considering a stepped electric-field axial profile described by a tanh(z) function, the monotron efficiency can attain the theoretical maximum value of 57.0%, almost three times as high as that obtained from the conventional uniform field distribution. This is accomplished by using two cavities electromagnetically coupled by an annular slit crossed by an electron stream. From a one-dimensional analysis, a two-cavity monotron is developed to operate in the TM/sub 010/ mode at 4.0 GHz when self-excited by a 20-keV, 10-A hollow electron beam. The device operation is examined through a 2.5-dimensional particle-in-cell (PIC) simulation giving a 47.5% conversion efficiency. Departure from the theoretically predicted efficiency is explained by beam-thickness effects.

Stepped electric-field profiles in transit-time tubes

The Air Force Research Laboratory RKO source has recently demonstrated the ability to convert electron beam power to RF power until the termination of the electron beam pulse, achieving a power of 1.5 GW at an energy of 170 J. These results represent an increase in power of 25-30% in power and energy extracted from this source. This paper discusses the principal research areas encountered in lengthening the RF pulse (FWHM) from 50 ns to the present 120 ns and the associated increase in the RF energy.

Increasing the RF energy per pulse of an RKO

By the use of a simple model, we explicitly incorporate the coupling between the driver cavity and the booster cavity in a relativistic klystron amplifier (RKA). We show that this RKA configuration may turn into an injection-locked oscillator only when the beam current is sufficiently high. Other features revealed by this model include: the downshifted frequency mode ("0" mode) is unstable whereas the upshifted frequency mode ("/spl pi/" mode) is stable; the growth rate of the "0" mode is relatively mild so that the oscillation can start only in an injection-locked mode; the oscillation does not require the presence of reflected electrons; and the separation of the cavities must be sufficiently short. These, and other features, are found to be in qualitative agreement with the recent experiments on the injection-locked relativistic klystron oscillator (RKO) that were conducted at Phillips Laboratory.

http://ieeexplore.ieee.org/iel1/27/11234/00533098.pdf

A model of injection-locked relativistic klystron oscillator

Summary form only given. This work provides an analytic study of the effects of cavity coupling on the current modulation of an intense relativistic electron beam (IREB). This problem is of considerable interest to many two-beam accelerator concepts where a modulated IREB is used as the driver, as cavity coupling may enhance the current modulation on the primary beam. It is also an important issue to the relativistic klystron oscillator and relativistic klystron amplifier, or a hybrid of the two. Coupling between the input cavity and the booster cavity affects the gap voltages, and to a large extent, determines whether a virtual cathode would be formed. We adopt a simple model where the cavity coupling is incorporated explicitly, including the effects of finite transit time. Steady state solutions are constructed these solutions are found to be in qualitative agreement with experimental results from Phillips Laboratory.

http://ieeexplore.ieee.org/iel3/3817/11161/00551635.pdf

K.J. Hendricks

Papers

A model of injection-locked relativistic klystron oscillator

A model of injection locked relativistic klystron oscillator