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Antennas operating at the high-frequency (HF) band (3–30 MHz) are frequently electrically small due to the large wavelength of electromagnetic waves (10–100 m). However, the bandwidth-efficiency products of passively matched electrically small antennas (ESAs) are fundamentally limited. Wideband HF waveforms using bandwidths of 24 kHz or more have recently received significant attention in military communications applications. Efficiently radiating such signals from conventional passive ESAs is very challenging due to fundamental physical limits on bandwidth-efficiency products of ESAs. However, active antennas are not subject to the same constraints. In this work, we present the design and experimental characterization of a high-power, class-E active ESA with enhanced bandwidth-efficiency product compared to that of passively matched ESAs. Specifically, the proposed class-E active ESA can radiate wideband HF signals with bandwidths of 24 kHz or more at 3 MHz (fractional bandwidths of ≈ 1 % – 4 %), with total efficiencies ranging from 40 % up to 80 % depending on the modulation type and radiated power levels approaching 100 W. Our approach uses a highly efficient, integrated class-E switching circuit specifically designed to drive an electrically small, high-Q HF antenna over a bandwidth exceeding 24 kHz at 3 MHz. Using a high-Q RLC antenna model, we have successfully demonstrated wideband binary ASK, PSK, and FSK modulations with the proposed class-E active ESA. Experimental results indicate that the bandwidth-efficiency product of this class-E active ESA is 5.4–9.8 dB higher than that of an equivalent passive design with the same data rate and bit-error-rate. [ABSTRACT FROM AUTHOR]

We present the results of an experimental investigation of polystyrene's suitability as a dielectric material for designing narrowband, infrared metasurface-based absorbers. Our study included characterization of processing parameters of polystyrene films, such as surface roughness and film thickness, which are critical for device fabrication, and the dielectric properties, which we measured, using spectroscopic ellipsometry. Our results confirm that thin film polystyrene is a relatively low-loss dielectric material at long-wave infrared (LWIR) frequencies. Subsequently, this material was used as the dielectric substrate of a thin, narrowband, metasurface-based absorber at LWIR frequencies. The fabricated narrowband absorber demonstrates excellent performance in the LWIR band with absorptivity as high as 96% and a full-width-at-half-maximum bandwidth of 0.502 μ m or 4.87%. [ABSTRACT FROM AUTHOR]

We report experiments and analysis of field emission from metallic knife-edge cathodes, which are sputter-coated with thin films of lanthanum hexaboride (LaB6), a low-work function material. The emission current is found to depend sensitively on the thickness of the LaB6 layer. We find that films thinner than 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode. This result is unexpected due to the higher work function of the bare metal cathode. We show, based on numerical calculation of the electrostatic potential throughout the structure, that the external (LaB6/vacuum) barrier is reduced with respect to uncoated samples for both thin and thick coatings. However, this behavior is not exhibited at the internal (metal/LaB6) barrier. In thinly coated samples, electrons tunnel efficiently through both the internal and external barrier, resulting in current enhancement with respect to the uncoated case. In contrast, the thick internal barrier in thickly coated samples suppresses current below the value for uncoated samples in spite of the lowered external barrier. We argue that this coating thickness variation stems from a relatively low (no higher than 1018 cm-3) free carrier density in the sputtered polycrystalline LaB6. [ABSTRACT FROM AUTHOR]

Reports that optical absorption spectroscopy has been used to measure absolute, average gas phase densities of neutral copper, ground and metastable states, and neutral argon, metastable and resonance states in an ionized physical vapor deposition plasma. Calculation of the absolute line average densities from fractional absorption data.

Presents a study which reduced the free-electron-laser (FEL) equations to a set of one-dimensional, normalized equations that allow a universal dimensional analysis. Performance characteristics of specific high-gain FEL; Estimates for the efficiency enhancement achieved with a tapered section; Design of high-gain millimeter-wave FEL amplifier using a superconducting short-period wiggler and sheet electron beam.

Presents procedures and quantitative analyses for obtaining and evaluating experimental short-period-wiggler free-electron lasers (FEL) oscillators that have been optimized with respect to efficiency. Applications for candidate sources of high-power, tunable, coherent radiation; Equilibrium operating point for the FEL oscillator; Effects of beam axial energy spread on the FEL interaction efficiency.

Presents a study that described a model for predicting equilibrium ion charge state distributions and soft x-ray emissions for highly charged ion plasmas confined in magnetic mirrors. Description of the model and simulation code; Investigation of typical code inputs and outputs and parameter space; Simulation results and discussion.

Studies the propagation of wiggler focused relativistic sheet electron beams. Details on the theory; Information on the experimental configuration; Results of the study.

Presents information on a study which investigated the maximization of the ion temperature of the plasma generated by a gas-injected washer gun. Experimental setup; Summary of relevant diagnostics; Experimental results.