This paper describes current projects in High Power Microwave (HPM) source development within the Air Force's basic research program managed by the Air Force Office of Scientific Research and conducted at the Air Force Research Laboratory's Directed Energy Directorate and at universities throughout the U.S. and the world. Studies are proceedings along two separate paths: narrow band high power microwave tubes and ultra-wide band sources. While these two paths invoke drastically different physical mechanisms that produce microwave radiation qualitatively distinct from each other, there are many common technical challenges. The focus of this report will be the active research in the Air Force ultra-wide band program, but many of the technical improvements being sought for improved materials, high voltage breakdown mitigation, and advanced design techniques will help the entire high power microwave program. The paper concludes with a brief discussion of the future of HPM research.

Recent work at the University of New Mexico has studied the use of a circular disk cathode as the electron source in a long-pulse Backward Wave Oscillator (BWO) experiment. The use of this cathode was motivated by recent studies by Loza and Strelkov of the General Physics Institute in Russia that demonstrated that a relativistic electron beam with stable cross section could be sustained for over one microsecond. In our first investigations using this new cathode configuration we found that the microwave pulse length generated from a long pulse BWO increased somewhat compared to the case when a traditional annular `cookie-cutter' cathode was used. We attribute this pulse lengthening to the hypothesis that the disk cathode generates a relativistic electron beam that is less likely to radially expand, thereby minimizing wall interception and the generation of unwanted plasma. In this paper we describe details of work- in-progress relating to a comparison of microwave generation from a disk cathode and annular cathode in a long-pulse BWO.

A stability analysis is made of an electron beam, propagating along and gyrating about a uniform magnetic field, for the case of a spatiotemporal equilibrium distribution in the phase angle of the transverse electron momentum component. A Lorentzian distribution exists in the axial momentum, whereas the magnitude of the transverse momentum is definite. The analysis is carried out by applying Lorentz transformations to previously derived integral equations (relating field amplitudes) for nongyrotropic equilibrium distributions that include a distribution in momentum. For small axial momentum spreads, these integral equations are approximated by algebraic equations. These equations relate field amplitudes for frequencies and wavenumbers that different by complex values. A dispersion relation is obtained. The range of validity of this dispersion relation is the subject of current research.

We present preliminary results for a model of a relativistic planar magnetron. Similar to our treatment of the non- relativistic case, we start with the cold-fluid relativistic equations and Fourier decomposition the equations, separating the physical quantities into a background mode and a pump mode. We use our previous techniques to determine the background mode and the RF pump mode.

The azimuthally invariant fluid equilibrium is obtained for a periodic strongly bunched charged annular beam with arbitrary radial density profile inside of a perfectly conducting cylinder and an external constant magnetic field. The electric and magnetic fields, which are utilized in the equilibrium solution, are computed self-consistently using an electrostatic Green's function technique in the longitudinal rest frame of the beam. An upper bound on the maximum self-field parameter, which allows beam equilibrium is obtained. As an application of the model, we find annular beam equilibrium for the Relativistic Klystron Oscillator experiment at Phillips Laboratory and the Backward Wave Oscillator experiment at the University of New Mexico. In addition, we compare the self-field parameters of these with the maximum theoretical values.

The Microwave-to-Optical Transformation (MOT) method is based on the Optical Admittance Diagram an optical thin film filter design tool that uses the characteristic matrix, and the quarter wave rule for the design and analysis of microwave and optical computing circuits/components. As previously reported, this technique has also been extended for characterization of the electric field strength of certain microwave devices. This paper discusses a MOT designed co-planar waveguide transition component or network for use in launching power to a new generation microwave source known as a Field Emitter Array (FEA). This paper will give a brief description of an FEA and discuss the feasibility of designing a co-planar waveguide for this particular application.

For a nonequilibrium relativistic beam-plasma system, an expression is derived for the time-average force acting on an unpolarized relativistic test particle, calculated to second order in the total electromagnetic field. This relation is useful in calculations of the conditions for the possible occurrence of a collective bremsstrahlung radiative instability.

We provide here an update on the status of ferratron development. The ferratron is a high-voltage, fast risetime, gas switch with low-jitter and high repetition rate. It is triggered by the emission of electrons from a ferroelectric device. It may be suitable for phased arrays due to it's low jitter. We have designed and built a test chamber for measuring the output of the ferratron. The test chamber is designed to launch a fast-risetime wave into an electrically large coaxial structure.

An aircraft may be subjected to icing for a variety of meteorological reasons during the flight. Ice formation on the plane and in particular on the aerodynamically carrying structures adversely affects the flight behaviour. Conventional de-icing methods for aluminum wings are characterised by a high energy consumption during the flight and slow ice melting due to thermal diffusion of the heat in the wing material. In addition to advanced turbines, novel materials and composites have to be used in order to reduce the weight and, hence, the fuel consumption. These composite materials have a far worse thermal conductivity than metals and undergo delamination when hot air systems, resistance or ohmic heating mats are used. In the paper, the unique advantages of a novel High Frequency Microwave Anti-/De-icing System for large future aircraft with carbon reinforced leading edge structures are presented.

The use of carbon fiber reinforced composite materials (CFRP) in aerospace industries is increasing due to their unique combination of characteristic features such as light weight, high specific firmness etc. For the time being, the main obstacle for widespread industrial applications are high manufacturing costs and one cause for them are the high curing temperatures between 100 and 200 degree(s)C. Heating the CFRP materials to these temperatures in a conventional furnace is an energy consuming and therefore costly procedure. This paper presents a heating procedure by the means of millimeter-waves. The advantages of this new method are presented along with theoretical considerations and numerical simulations of the heating process. The first experimental results in the form of millimeter-wave cured CFRP slabs are demonstrated.

A cylinder-cone disk-loaded waveguide oscillator is presented. It is a high power microwave source without focusing structure. The device allows a larger tunable range of frequency and power. The mechanism of microwave generation in the device is analyzed, and the results of the numerical simulation for the device of a kind of size are given.

The evolution of initial perturbations in a plasma-filled system with a high current relativistic electron beam is considered. The beam current is assumed to be comparable or higher than the limiting vacuum current. The approach is based on equation for slowly varying amplitude of the induced waveform, which enable us to separate out the most intrinsic peculiarities of the overlimiting electron beam instabilities. An equation for slowly varying amplitude is derived and solved. An analytical expression describing space structure and development dynamics of the fields is obtained and analyzed. The well-known maximal growth rate of beam instability is actually the growth rate in the peak of induced wave train. Results are valid both for instability due to aperiodic modulation of the beam density in medium with negative dielectric constant and for instability due to excitation of beam wave with negative energy. Comparison with the case of a sublimiting beam is carried out.

The paper is devoted to the high power overmoded microwave devices' design problem. Reasonable estimation of electron beam limit power level is given. The problems connected with degeneration of electromagnetic modes with different azimuthal indexes near the p-cutoff of the operating E01 mode are considered and illustrated on instance of an experimental device. Dispersive curves for the case of electromagnetic waves' cyclotron absorption process are studied. The results of investigation of limit power level transported in open resonators of one-mode and overmoded devices by operating and parasitic modes are displayed by series of animation frames. All estimations and calculations are fulfilled by help the 3D linear mode computer code 'MULTIWAVES-2000'.

To evaluate power losses of microwave generators, the transmission of radiation of the open end of a circle waveguide with the TM_0n-type waves through a dielectric disc is considered. The field inside the disc presents a set of modes of a dielectric waveguide. Some part of modes is shown to participate in the energy transfer through a window, another part causes energy scattering at large angles to the generator axis. The part of energy spent for scattering depends on the radiated mode type, transverse disc dimensions, and distance to the radiator. Energy losses due to reflection depend as well on the window thickness.

A nonlinear time-domain hybrid code for Cerenkov generators with overmoded slow-wave structures has been developed in which the analytical representation of an axially symmetric electromagnetic field and particle-in-cell method for an electron beam was used. To present a summary electromagnetic field of the open resonator, the supplementary functions are used along with the high-Q functions corresponding to longitudinal modes. The code was used for numerical simulation of different regimes of the beam and field interaction near the (pi) -type oscillations of TM₀₁-mode in a one-sectional slow-wave structure. In the output radiation the main contribution is made by the modes TM₀₁ and TM₀₂. Calculation results are compared with the experimental data in the wavelength and generation efficiency.

We have developed a self-consistent theory for a relativistic backward wave oscillator (BWO) taking into account the interaction of fast cyclotron waves of the electron beam with both the operating contrary wave and the forward wave. The theory describes the extraction of electromagnetic energy from the interaction space, providing a correct description of the operation of a relativistic BWO in which the field structure is a product of the self- consistent solution. The theory suggests ways in which the requirement of a guide magnetic field in such devices can be reduced.

This research program investigates high power microwave generation utilizing a microsecond electron beam accelerator to drive a relativistic magnetron. Peak microwave power levels have been achieved exceeding 200 MW total (100 MW per-cavity for two-cavity extraction) from a six-vane structure. Time-frequency analysis shows that microwave emission is primarily single-mode with a total pulse duration in the range of 50 - 100 ns. Relativistic magnetron end-loss current measurements have been performed. Preliminary total efficiency estimates for the relativistic magnetron are in the range of 13%, including endloss current. If endloss current is subtracted, the magnetron electronic efficiency nearly doubles to 25%. The goal of future research is to explore techniques for increasing the microwave power, efficiency and pulselength of relativistic magnetrons.

Recent results are reported that improve the prompt radiated response from reflector and lens impulse radiating antennas (IRAs) by improved control of the aperture fields of the TEM mode distribution. Both reflector and lens IRAs benefit from trimming the aperture to eliminate portions where the vertical component of the electric field is oriented in the wrong direction. Reflector IRAs can further benefit from reorientation of the TEM feed arm angles as well as the relative size of the feed arms to the maximum aperture radius. Significant improvements over 45-degree feed arms can be realized by making 200-Ohm reflector IRAs with feed arms at 30 degrees from the vertical with the circle of symmetry slightly inside the aperture outer boundary and small amounts of aperture trimming.