John Swegle

Scaling studies and time-resolved microwave measurements on a relativistic backward-wave oscillator

A relativistic backward-wave oscillator (BWO) operating at a frequency near 8 GHz has been built. The parameters of the 60-ns electron beam driving this microwave source are varied over the ranges 0.8-1.5 MV and 2-10 kA. Several different annular cathodes for launching the electron beam are tried, varying the outer radius and shape. The axial magnetic field guiding the beam through the BWO is varied between 0.6 and 3 T. The power transfer downstream to an output waveguide is investigated as a function of the shape of the transition from the BWO to the waveguide. The scaling of the output power and frequency with these variations is discussed. Time-resolved measurements of 2-ns-long segments of the microwave output are shown. In observations of the microwave signal, it is found that the frequency shifts as the output power envelope passes through a sharp dip. It is proposed that this shift corresponds to a change in the longitudinal operating mode of the BWO. >

High-power microwaves at 25 years: the current state of development

The field of high-power microwaves (HPM) has matured considerably in the 25 years since the initial development of relativistic backward wave oscillators (BWOs) by researchers from the Institute of Applied Physics and the Lebedev Institute in Russia and from Cornell University in the US. In this paper, we review some of the signs of the maturity of the field, including changes such as an observed narrowing in the number of source types under development, an increase in commercial suppliers, and a growing internationalization of the research field. In addition, within the context of historical developments in the field, we discuss the development of high peak power systems and the apparent abandonment of the pursuit of ever-higher power in favor of the development of gigawatt-level systems with manageable weight and volume, repetitive operation, and tunability.

Applications of high power microwaves

We address a number of applications for HPM technology. There is a strong symbiotic relationship between a developing technology and its emerging applications. New technologies can generate new applications. Conversely, applications can demand development of new technological capability. High-power microwave generating systems come with size and weight penalties and problems associated with the x-radiation and collection of the electron beam. Acceptance of these difficulties requires the identification of a set of applications for which high-power operation is either demanded or results in significant improvements in performance. We identify the following applications, and discuss their requirements and operational issues: (1) High-energy RF acceleration; (2) Atmospheric modification (both to produce artificial ionospheric mirrors for radio waves and to save the ozone layer); (3) Radar; (4) Electronic warfare; and (5) Laser pumping. In addition, we discuss several applications requiring high average power that border on HPM, power beaming and plasma heating.

Nonrelativistic planar double-stream equilibria in magnetically insulated diodes

The study of magnetically insulated electron flow is a topic of relevance to the startup state of magnetrons, cross-field amplifiers, and magnetically insulated line oscillators, as well as to the operation of magnetically insulated transmission lines and ion diodes. In a recent paper, Christenson and Lau explored double-stream equilibria in magnetically insulated diodes analytically for the case of vanishing injection electron velocities at the cathode and numerically for the case of nonzero injection velocities. Here, the analytical results are generalized to the case of nonzero injection velocities to allow the calculation of the equilibrium current density and electron sheath thickness as functions of the voltage, magnetic field, and gap width. An interesting outcome of the analysis is that it is shown that space-charge limited electron emission from the cathode is no longer possible for equilibria in which the electrons are monoenergetic with vanishing canonical momenta. In addition, for the special case of zero injection energies, but not necessarily zero electric field at the cathode, closed-form expressions are derived for the orbital quantities of the electrons-positions and velocities-as a function of time measured from the moment of emission from the cathode. Unfortunately, a spatial variation is not apparently available in closed form; however, one can express these quantities alternatively in terms of a generalized potential with an implicit spatial variation.

Applications of high-power microwaves

We address a number of applications for HPM technology. There is a strong symbiotic relationship between a developing technology and its emerging applications. New technologies can generate new applications. Conversely, applications can demand development of new technological capability. High-power microwave generating systems come with size and weight penalties and problems associated with the x-radiation and collection of the electron beam. Acceptance of these difficulties requires the identification of a set of applications for which high-power operation is either demanded or results in significant improvements in performance. We identify the following applications, and discuss their requirements and operational issues: (1) High-energy RF acceleration; (2) Atmospheric modification (both to produce artificial ionospheric mirrors for radio waves and to save the ozone layer); (3) Radar; (4) Electronic warfare; and (5) Laser pumping. In addition, we discuss several applications requiring high average power that border on HPM, power beaming and plasma heating.

End-to-End Modeling to Scope High-Power Microwave Systems

Summary form only given. High-power microwave systems are complex assemblies of subsystems that include the prime and pulsed power, the microwave sources themselves, mode converters, and antennas. Further, to fully scope a system, one must include the platform, cooling subsystems, and the propagation of the microwaves from the antenna. We will describe a modeling code built of rather simple phenomenological models for individual subsystems that taken in total can be used to scope out the primary operational parameters of these complex systems: the output power, the volume, mass, and linear dimensions of the system, and main internal parameters, such as the prime and pulsed power requirements. We will show results for two different design concepts for a mobile microwave system with power at the gigawatt level.

Nonrelativistic, planar double-stream equilibrium in magnetically insulated diodes

The study of magnetically insulated electron flow is relevant to the operation of magnetically insulated transmission lines and ion diodes as well as to M-type microwave sources. Recently, double-stream equilibria in magnetically insulated diodes have been explored analytically for the case of vanishing injection electron velocities at the cathode and numerically for the case of nonzero injection velocities. Here, analytical results derived for the case of nonzero injection velocities indicate that space-charge limited emission from the cathode is no longer possible for equilibria in which the electrons are monoenergetic with vanishing canonical momentum.