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We present the design and proof-of-concept demonstration of a dual-band reflectarray antenna capable of generating two beams at $X$ - and $Ku$ -bands, whose directions of maximum radiation can be set independently of each other. The reflectarray uses dual-band polarization-rotating (PR) unit cells with 1 bit phase quantization. The proposed unit cell provides independent phase shifts at two frequency bands by controlling a cluster of two orthogonal dipoles to operate as either +90° or −90° reflective polarization rotators. Two dipole clusters are interleaved in a unit cell and used to achieve dual-band operation. Simulation results of a unit cell designed to operate at $X$ - and $Ku$ -bands demonstrate independently controlled, dual-band PR operation over 16.1% and 20% bandwidths for the lower and higher bands, respectively. The dual-band unit cells were used to construct three different static reflectarray prototypes demonstrating the different ways where such independent phase control at the two bands of a dual-band reflectarray antenna may be advantageous. The three static reflectarray prototypes were simulated, fabricated, and experimentally characterized. Simulation and measurement results of these prototypes agree well and demonstrate independent phase shift control at the $X$ - and $Ku$ -bands over a relatively wide field of view without exciting grating lobes.

We present a new method for achieving 2-bit phase quantization for beam-steerable reflectarrays by exploiting four distinct reflection modes of a unit cell. Two of the four modes perform polarization-rotation (PR) operation while the other two are non-PR modes when the unit cell is illuminated by an $x$ - or $y$ -polarized incident wave. With a specific phase relation between the four reflection modes, the unit cell acts as a 2-bit phase shifter under illumination with an incident wave polarized along a diagonal direction of the square-shaped unit cell. The proposed operating principle was demonstrated with an illustrative unit cell consisting of three metallic layers and using four single-junction connections that can be replaced with single-pole single-throw (SPST) switches for reconfiguration. Simulation results predict an operating bandwidth of about 20% for the unit cell and a good beam scanning performance over a scan range of ±45° for a beam-steerable reflectarray constructed using this unit cell. Two static prototypes of the beam-steerable reflectarray antenna providing beam collimation at two different directions at 10 GHz were fabricated and experimentally characterized. Measurement results show good agreement with the simulation results, validating the proposed design approach for 2-bit reflective unit cells.

With the focus on increasing instantaneous bandwidth capabilities of high-power microwave (HPM) devices, the Metamaterial Enhanced Resistive Wall Amplifier (MERWA) is being investigated for potential use as an HPM amplifier 1 , 2 , 3 . Rather than growth of a circuit wave, the MERWA operates via growth of the beam’s slow space charge wave. MERWA theory uses a 2D impedance matching technique at the boundary of the beam to show that growing wave solutions exist if a nearby metamaterial wall presents a resistive-inductive admittance to the beam 1 . Although the theory never expects or requires that an axially propagating cold circuit wave would exist, a low power experimental prototype was found to support a lossy backward wave in the vicinity of growth 2 , 3 . Using a two-wave coupling theory, we show that the space charge wave growth (of this specific prototype) can alternatively be explained in terms of coupling between the beam’s slow space charge wave and the lossy backward circuit wave. In contrast to the coupling with a lossless backward wave, which is known to produce a pair of non-growing waves, the interaction with the lossy backward wave produces a region of stable exponential space charge wave growth below an oscillation threshold. This two-wave coupling perspective provides insight into performance capabilities, limitations, and tradeoffs. Among these insights, we discuss how the amount of cold circuit loss affects the maximum stable growth rate, oscillation threshold, and gain-bandwidth product. We will compare analytical predictions against simulation results for validation.

Reconfigurable reflectarrays (RAs) have attracted significant interest as affordable and relatively low-complexity beam scanning antennas for radar and satellite communication applications. We present a dual-band, beam-steerable RA capable of independent beam control at X and Ku bands based on a 1-bit polarization-rotating unit cell (PRUC). Two static versions of the RAs with different beam scanning scenarios are simulated, fabricated and measured. Good beam steering performance with independent beam control at each band is achieved.

We present a new technique for designing wideband, single-layer reflectarray antennas. The proposed reflectarray exploits a wideband, polarization rotating unit cell that rotates the polarization of the reflected wave with respect to that of the incident wave by 90°. This unit cell and its mirror image were used as 1 bit spatial phase shifters capable of providing a 0°/180° phase shift over a wide bandwidth of 5.4–12.7 GHz. Using this, a reflectarray antenna with 1 bit phase quantization was designed to operate at ${X}$ -band with aperture dimensions of $28.8\,\,\text {cm}\times 28.8$ cm and a focal length of 30 cm. A prototype of the proposed reflectarray was fabricated and characterized using a spherical near-field measurement system. We determined the antenna’s 3 dB gain bandwidth to be approximately 37.4% with a maximum realized gain of 24.2 dBi. Over its entire band of operation, the polarization purity of the antenna remains better than 13 dB and sidelobe levels of better than 12 dB were achieved.

We describe the results of a wideband, reconfigurable reflectarray concept based on polarization-rotating unit cells (PRUCs) with a minimum-switch topology design. To enhance the simplicity and decrease the cost, the approach focuses exclusively on a one-bit phase shifting architecture. We investigated PRUCs having four conducting, single-headed arrow structures on their top surfaces and crossed, conducting strips acting as hard-wired switches on their bottom surfaces. The top-surface arrows and the bottom-surface strips are connected with four vertical vias. The switch states of the bottom conducting strips determine the two operating states of the unit cell for rotating the polarization of the reflected waves by +90° and -90° with respect to that of the incident, linearly polarized waves. The PRUC provides a co-polarization reflection coefficient of better than -1 dB over the wide frequency range of 8-12 GHz, as demonstrated by the simulation results. The PRUCs were incorporated into an illustrative reflectarray operating in the X-band. The full-wave simulation results show good beam scanning performance over the range of ±45° with a maximum scan loss of 3 dB. Two static realizations of the reflectarray for beam collimation at θ = 0° and θ = 45° in the xz plane were fabricated and experimentally characterized. The measurement results show good agreement with the simulation results and demonstrate the beam steering operation of the proposed reflectarray.

The metamaterial-enhanced resistive wall amplifier (ME-RWA) has been shown to be capable of producing large gains over wide bandwidths. Previous theoretical investigation of the ME-RWA focused on an idealized case using a homogenous dielectric with Drude dispersion to provide the negative permittivity required to achieve high gains. This paper reports the results of investigating practical metamaterial implementations with approximate Drude-type responses that enable the realization of a ME-RWA. Using these metamaterials, an architecture for a ME-RWA using periodically spaced inductive meandered lines is proposed. Our theoretical analyses and particle in cell simulations of this device predict that the proposed technique provides a practical method of implementing ME-RWAs, capable of providing a large gain over a substantial bandwidth.

We propose a semi-analytic approach to investigate the multipactor in parallel plate structures with improved accuracy. The effects of emission velocity and transit time of electrons are studied and their corresponding susceptibility charts are calculated. As a result, the proposed model covers more dynamics of multipactor. In calculating the susceptibility chart, we propose a decision-making process taking into account impact velocity, required voltage, and trajectory of electrons. Therefore, we obtain a more accurate susceptibility chart that excludes non-physical scenarios. Our investigation reveals a different mechanism of multipactor, where the transit time of an electron across the gap can be either shorter or longer than the conventional half-cycle of the RF signal. The extent of this deviation depends on the maximum emission velocity of the secondaries. With this deviation, we demonstrate that the multipactor evolves over broader practical situations occurring not necessarily within a half-cycle of the RF signal. Finally, we study the higher-order multipactor resonances and also calculate their susceptibility charts. We demonstrate an overlap between the susceptibility charts of different orders depending on the emission velocity of primary electrons. The discussions herein provide an insight into the underlying dynamics of the multipactor, which can be used for practical multipactor suppression methods.

We demonstrate the use of polarization rotating (PR) unit cells in designing wideband reflectarray (RA) and transmitarray (TA) antennas. A reflective/transmissive PR unit cell and its mirror image can provide 1-bit phase quantization over a very wide frequency range. The PR unit cells were used to design a RA and TA antennas, both with aperture dimensions of $30 \times 30$ cm$^{2}$ and focal lengths of 30 cm, for operation at X-band. The fabricated RA and TA antenna prototypes provided a maximum realized gain of 24.5 dBi, 3-dB gain bandwidths of up to 40%, and polarization purity and side lobe levels of better than 11.5 dB.

Frequencies in the range of super-low frequency (SLF) to very-low frequency (VLF) are ideal for communication in underwater and underground environments. However, large antenna structures are often needed for high efficiency and bandwidth. We experimentally demonstrate an efficient and compact SLF transmitter based on the concept of a spinning permanent magnet. We propose and experimentally demonstrate a modulation technique that does not require change in the rotational speed of the magnet. Findings suggest that our electronic modulation technique can give greater efficiency and bandwidth compared to a mechanically based modulation approach.

A challenge for high-power millimeter-wave (mmw) traveling-wave tube (TWT) amplifiers is to realize high-power operation without incurring oscillation from backward-wave (BW) interaction. Conventional wisdom purports that contrawound (including ring bar) helix TWTs are superior to monofilar helix TWTs for stability against BW oscillations due to a zero or at least greatly suppressed BW interaction impedance (K BW ) compared with the forward-wave (FW) interaction impedance (K FW ). We use 3-D electromagnetic (EM) field calculations of a ring-bar helix to compare the BW and FW interaction impedances. Using a sine/cosine basis set and a comparison at a fixed phase velocity (rather than fixed frequency assumed in previous analyses), we show that the BW interaction impedance is not always significantly suppressed in the ring-bar/contrawound helix. In spite of the lack of ubiquitous BW stability, we use EM simulations to illustrate that there remain specific, efficient, high-gain mmw ring-bar designs with K BW

Electromagnetic metamaterials offer a significant potential to enable new capabilities in many applications. Under high-power illumination, metamaterials and periodic structures experience internal breakdown, altering frequency response, and/or yielding thermal damage. Our prior research observed simultaneous breakdown discharges at two separate sites within a multiresonator metamaterial unit cell, even though the electric field intensities at one of the resonator sites should have been well below the threshold intensity required for breakdown. Here, we investigate three candidate mechanisms for the simultaneous breakdown discharges: energetic electrons, ultraviolet (UV) radiation, and vacuum UV (VUV) radiation. Experiments inserting different dielectric barriers between the two resonators of a multiresonator unit cell were able to selectively isolate the coupling influence of the candidate mechanisms. It was established that, VUV radiation from the discharge at the resonator with a lower electric field breakdown threshold causes simultaneous breakdown at the other resonator where the field intensities are otherwise too low to induce breakdown.

The 1-D small-signal theory for the fundamental spatial harmonic mode developed by Pengvanich et al. is adapted to include the effect of space charge forces in the electron beam. This model allows us to look at how traveling-wave tube (TWT) performance is affected by random fabrication errors, which are modeled as random perturbations of the phase velocity, interaction impedance, and loss along the TWT's length. In particular, we examine the effect on TWT gain and instantaneous 1-dB bandwidth. Random variation of the phase velocity is found to have the largest effect on both the gain and bandwidth, but the impact is reduced as the amount of space charge in the beam is increased.

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz-30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies. OA hybrid

The metamaterial-enhanced resistive wall amplifier (ME-RWA) is a wideband, high power, vacuum electron device1. In the ME-RWA, the resistive liner of the traditional resistive wall amplifier has been replaced with a lossy epsilon negative (ENG) metamaterial. This work investigated a relativistic, high power microwave (HPM) ME-RWA. Specifically, we examined the feasibility of a 50 MW, L-Band ME-RWA. The investigation of the relativistic HPM ME-RWA includes a theoretical investigation and particle-in-cell (PIC) simulations. In our theoretical analysis, we have modified the traditional resistive wall theory, presented by Birdsall and Whinnery2, to include relativistic effects. To simulate the ME-RWA's performance, we have used the CST Particle Studio software and used homogenous dielectrics with a Drude dispersion response, with the plasma frequency higher than the operating frequency, to emulate materials with negative permittivity. The results of our relativistic ME-RWA theory as well as the results of our PIC simulation will be presented to demonstrate the superior performance of the ME-RWA.

Non-foster impedance matching circuits are one method to achieve ultra-wideband matching to electrically small antennas (ESAs). In this work, we examine the requirements of an active element that can accommodate the high voltage and current swings required to drive an ESA. We have examined the requirements necessary to transmit 5 W of power from an electrically small dipole and loop antenna, each with a maximum linear dimension of 1 m, across the entire high frequency (HF) band. In addition, we evaluated the use of gridded vacuum tubes as the active elements of the non-foster matching circuit.

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from ; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Magneto-inductive (MI) communication is a viable technology for wireless communications in underwater and underground environments. In this paper, a new design for an MI transmitter is presented. Unlike conventional MI transmitters that utilize coiled loops or solenoids to generate magnetic fields, we demonstrate the feasibility and advantages of using a rotating permanent magnet. We also present and experimentally verify a modulation technique that does not involve changing the rotational speed of the magnet. By electrically changing the permeability of a surrounding shield, the fields from the rotating magnet are amplitude modulated. Our findings suggest that increased efficiency and bandwidth can be realized compared to conventional MI transmitters.

A lack of compact, powerful, and efficient sources in the submillimeter-wave or terahertz (THz) frequency regime (~ 300-3000 GHz) has motivated the research on microfabricated vacuum electronic devices. Regenerative feedback traveling-wave tube (TWT) oscillators have been proposed and validated as good candidate sources of THz-regime radiation. Recently, a microfabricated folded-waveguide (FWG) TWT oscillator has demonstrated a peak output power of 50 mW at 656 GHz with an intrinsic circuit efficiency of 0.45%. By continuous variation of the beam voltage, the operating frequency was varied between 607 and 675 GHz. However, the oscillation frequency exhibited a step-tuning behavior, rather than a smooth variation with beam voltage. It is hypothesized that this step tuning results from a combination of the TWT gain characteristics and a phase self-consistency condition. To confirm this hypothesis, we theoretically model a 40-GHz FWG-TWT oscillator and experimentally study a 5-GHz helix-TWT oscillator. The FWG-TWT results exhibit an inclined step-tuning characteristic, similar to previously unreported observations on a 650-GHz THz-regime oscillator. Similar characteristics are also observed, along with multifrequency oscillations, in the helix-TWT experiments.

Effective terahertz (THz) component and systems design requires accurate predictive models for the frequency-dependent conductivity of materials. We have measured the THz conductivity of metals (including rough surface effects) and doped silicon at 0.4 – 0.85 THz. After a comprehensive analysis, we conclude that the THz-regime conductivity of highly-doped Si is accurately described by Drude theory, while metal conductivity is accurately described by a combination of measured DC conductivity, Drude theory, and the Hammerstad-Bekkadal formula.

In this paper, results of different models are compared for calculating effective, cold-circuit (beam-free) phase velocities and interaction impedances of folded waveguide (FW) slow wave circuits for use in millimeter-wave traveling wave tubes (TWT). These parameters are needed for one-dimensional (1-D) parametric model simulations of FW traveling wave tubes (FWTWTs). The models investigated include approximate analytic expressions, equivalent circuit, three-dimensional (3-D) finite difference, and 3-D finite element. The phase velocity predictions are compared with experimental measurements of a representative FW circuit. The various model results are incorporated into the CHRISTINE1D code to obtain predictions of small signal gain in a 40-55 GHz FWTWT. Comparing simulated and measured frequency-dependent gain provides a sensitive, confirming assessment of the accuracy of the simulation tools. It is determined that the use of parametric 1-D TWT models for accurate, full band predictions of small signal gain in FWTWTs requires knowledge of phase velocity and impedance functions that are accurate to

We present a new view of nonlinear distortion in a klystron based on an analytically solvable nonlinear Eulerian model. The nonlinear contributions to the analytic solutions for the beam modulations are "nonlinear space charge waves" in the sense that they are produced by the nonlinear mixing of the linear space charge waves. For a single-frequency input, amplitude and phase distortion are shown to be results of "self-intermodulation" at the drive frequency, or mixing of harmonic distortions with the fundamental. The self-intermodulation contributions add out of phase with the linear space charge waves to produce gain compression and phase distortion. By comparing the results to a conventional large-signal Lagrangian model we find this physical picture is accurate for drive levels up to 1.2 dB of gain compression. For a two-frequency input we predict the third-order intermodulation distortion generation and suppression with the nonlinear space charge wave theory.

The nonlinearity inherent in the traveling wave tube (TWT) amplifier distorts amplified signals and results in reduced efficiency and bandwidth, limiting its use in communication and electronic countermeasure applications. Signal predistortion is an effective technique for suppressing nonlinear distortion in TWTs that provides high suppression and requires simple circuits for implementation. While conventional predistortion linearizers are based on third-order intermodulation (3IM) injection, this paper proposes using second-order (second-harmonic) signal injection in predistortion circuits. A detailed experimental investigation and comparison of second-order versus third-order signal predistortion is presented. It is observed that both schemes result in suppression of up to 30 dB (55 dBc) for the 3IM distortion products. However, experimental results indicate that second-harmonic signal injection performs better than 3IM in suppressing higher order products. The paper also investigates spatial evolution of the wave spectrum along the TWT axis with and without injection, and sensitivity of the suppression to injected signals amplitude, phase and the fundamental frequency.

The physics of signal injection to shape the output spectrum in a traveling wave tube (TWT) is studied using an analytic solution to the approximate nonlinear S-MUSE model and with the large signal code LATTE. The results verify the long-standing conjecture that a frequency canceled by signal injection is composed of a component due to the injected signal and a component due to the nonlinearity of the TWT. Furthermore, the structures of the solutions are exploited to explain and predict behavior for various signal injection schemes. The scenarios studied include second harmonic injection to reduce the second harmonic and enhance the fundamental, multiple harmonic injection to eliminate the sensitive output power dependence on injected phase, second harmonic injection to reduce intermodulation distortion, and multiple signal injection to reduce the intermodulation spectrum. Insights are given regarding the effectiveness of an injection depending on whether the injected signal is within or outside the linear gain bandwidth of the TWT.

The helix traveling-wave tube (TWT) is a vacuum electronic device which utilizes an electron beam to amplify electromagnetic waves. Although having a broad frequency gain bandwidth compared to other vacuum electron devices, the helix TWT has been used in relatively narrowband ways in its applications in communications and electronic countermeasures. Recent interest in ultrawideband (UWB) radio and radar, combined with the recent development of UWB helix TWTs, has stimulated interest in the amplification of very broadband multifrequency signals and impulses. A number of studies have been conducted on the harmonic and intermodulation effects of multiple signals in a TWT, but there has not been much investigation into impulse amplification. Utilizing a nonlinear time-domain model developed for this purpose, the response of a wideband (helix) TWT to an input Gaussian pulse is examined. Differences between the large signal response to positive and negative Gaussian inputs are examined, and it is shown that a positive impulse input yields better pulse shape preservation, higher saturation level, and possibly greater efficiency. The growth and saturation mechanisms of a positive input Gaussian are then investigated, showing distinct differences from steady-state growth and saturation mechanisms. This study indicates that the possibility of amplification of impulses in a TWT is very promising. The results and conclusions of this study generally apply to all types of TWTs with appreciable bandwidth (e.g., couple-cavity, folded waveguide). However, as the helix TWT has the largest bandwidth, it is the most interesting and is the specific TWT studied here.

We derive from Eulerian electron beam equations the multifrequency spectral Eulerian (MUSE) model, a new one-dimensional (1-D) nonlinear multifrequency model of a traveling wave tube (TWT). We also derive from the same equations a Lagrangian "disk" model, LATTE, so that MUSE may be directly compared to a Lagrangian approach. The models are compared to the large signal code Christine 1-D on a set of TWT parameters which are based on a single section of the Hughes 8537H L-band TWT. Aspects of the physics, nonlinearities, and simulation dimensions of the MUSE model are discussed, as well as its relation to the method of collective variables. A simplified MUSE model S-MUSE useful for analysis is also presented and its applications are discussed.

Under high power illumination, electromagnetic metamaterials and periodic structures experience internal breakdown that alters their frequency responses. It was recently demonstrated that a breakdown discharge created at a given location within the unit cell of a metamaterial mediates the breakdown at other locations within that unit cell or at the neighboring unit cells in spite of the fact that the intensity of the incident field is not high enough to cause breakdown at these other locations under normal situations. In this paper, we investigate the physics behind this phenomenon and examine three candidate mechanisms for these simultaneous breakdown discharges. These include energetic electron transport, ultraviolet (UV) radiation, and vacuum ultraviolet (VUV) radiation. By experimentally examining the unit cell of a metamaterial composed of multiple resonators, we were able to selectively isolate the coupling influence of the candidate mechanisms. It was established that VUV radiation from the initial discharge yields simultaneous breakdown at the other locations of interest.

Numerical and theoretical analyses show that stable, two-plane focusing of finite width, elliptical cross section, sheet electron beams with high space charge (low voltage, high current density) can be accomplished using periodically cusped-magnetic (PCM) fields. Magnetic field strength requirements for focusing high-space-charge sheet beams are within technological capabilities of modern permanent magnet technology. Both an offset-pole PCM stack and a PCM stack combined with a periodic quadrupole magnet (PQM) edge array are shown to be effective for two-plane sheet beam confinement. The PCM-PQM hybrid configuration offers inherent advantages for independent adjustment of confinement fields to achieve beam matching (minimum ripple) in both transverse dimensions. The offset-pole configuration offers the advantage of open-side access for applications such as vacuum electronic microwave devices. It is also shown that PCM-focused sheet beam envelope stability obeys criteria equivalent to that previously identified for round-cross-section electron beams in periodic permanent magnet focusing.

The shape of the gain curve and physics of interaction between microwave fields and the electron beam for a periodic rectangular grating slow-wave Traveling Wave Tube (TWT) amplifier are investigated. This research focuses on the collective (ωpτ≫1) low-voltage (⩽10 kV) interaction in the backward wave regime for the Ku-band (12.5–17.5 GHz) range of frequencies. For amplifier experiments, a round “probe” beam (10 kV, 0.25 A, 1 mm diameter) confined by a 1 kG solenoidal focusing magnetic field was utilized. Simultaneous single-pass collective gain and electron-beam velocity spread measurements are performed by means of a Faraday cup repeller energy analyzer configured as the beam collector. Experiments near 13.0 GHz show the maximum normalized single-pass gain to be in agreement with the theoretical prediction for several axial velocity spreads (δv‖/v‖). Collective single-pass experimental results are presented and compared with theory. The effect of an increased interaction length which is accomplished by ...

Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating of a variety of ceramic, glass, and polymer materials. An explanation for these controversial observations has eluded researchers for over a decade. This paper describes a series of recent experimental and theoretical investigations that provide an explanation for these intriguing observations in terms of ponderomotive forces acting on mobile ionic species. The ponderomotive phenomenon, like its conventional-plasma analog, can be described in the continuum model limit by combining the continuity, Poisson’s, and transport equations. However, the solid-state plasma version typically manifests as a result of gradients in mobile charge mobility (e.g., near physical surfaces or interfaces), whereas the conventional plasma ponderomotive transport is typically a consequence of gradients in the radiation field intensity. Both cases can be captured in a single, general, mathematical articulation developed in terms of the mobile particle fluxes.

Background The use of pulsed electric fields (PEFs) to irreversibly electroporate cells is a promising approach for destroying undesirable cells. This approach may gain enhanced applicability if the intensity of the PEF required to electrically disrupt cell membranes can be reduced via exposure to a molecular deliverable. This will be particularly impactful if that reduced PEF minimally influences cells that are not exposed to the deliverable. We hypothesized that the introduction of charged molecules to the cell surfaces would create regions of enhanced transmembrane electric potential in the vicinity of each charged molecule, thereby lowering the PEF intensity required to disrupt the plasma membranes. This study will therefore examine if exposure to cationic peptides can enhance a PEF’s ability to disrupt plasma membranes. Methodology/Principal Findings We exposed leukemia cells to 40 μs PEFs in media containing varying concentrations of a cationic peptide, polyarginine. We observed the internalization of a membrane integrity indicator, propidium iodide (PI), in real time. Based on an individual cell’s PI fluorescence versus time signature, we were able to determine the relative degree of membrane disruption. When using 1–2 kV/cm, exposure to >50 μg/ml of polyarginine resulted in immediate and high levels of PI uptake, indicating severe membrane disruption, whereas in the absence of peptide, cells predominantly exhibited signatures indicative of no membrane disruption. Additionally, PI entered cells through the anode-facing membrane when exposed to cationic peptide, which was theoretically expected. Conclusions/Significance Exposure to cationic peptides reduced the PEF intensity required to induce rapid and irreversible membrane disruption. Critically, peptide exposure reduced the PEF intensities required to elicit irreversible membrane disruption at normally sub-electroporation intensities. We believe that these cationic peptides, when coupled with current advancements in cell targeting techniques will be useful tools in applications where targeted destruction of unwanted cell populations is desired.

A periodic, rectangular grating slow-wave-structure is considered for forward and backward wave low-voltage (⩽10 kV) Ku-band traveling wave tube (TWT) amplifiers. For forward wave operation, it is required that the ratio of groove depth, d, to grating period, p, be large (i.e. d/p⩾5) while small values of d/p allow backward wave operation. For large d/p, skin effect losses in the grating slots are large and can substantially reduce the growth rate produced by the beam-slow wave interaction. Phase and amplitude measurements of the grating structures utilizing a slotted line and a fast Fourier transform (FFT) analysis have been carried out. The results show that the measured dispersion relations for both shallow (d/p= 0.446) and deep groove gratings (d/p= 7.43) agree very well with the theoretical dispersion relations. For amplifier experiments, a round “probe” beam (10 kV, 0.25 A, 1 mm radius) from a Litton Pierce electron gun (model M707) is utilized. The beam is confined by means of a 1 kG focusing solen...

Plasma source ion implantation (PSII) is a technique for modifying stafaces that places the object to he modified directly into a plasma and then negatively pulse biases the object so as to implant positive ions. If the voltage is high enough, X-rays can he generated by electrons that are also accelerated by the pulse. This work describes techniques for imaging and characterizing the X-rays A pinhole camera was used to image the X-rays being emitted as electrons collided with surfaces in the chamber. The images show that X-rays are generated at the chamber walls and near the target. The time dependence of these X-rays during each pulse was examined using a PIN diode X-ray detector. Then, using another X-ray sensor and pulseheight analyzer, the spectra of the emitted X-rays was determined. The object is to relate the X-ray intensity and spectrum to the temporal and spatial values of the implantation dose so that it may he used as a process monitor and a control sensor.

We present a new approach to perform beam steering in reflecting type apertures such as reflectarray antennas. The proposed technique exploits macro-scale mechanical movements of parts of the structure to achieve two-dimensional microwave beam steering without using any solid-state devices or phase shifters integrated within the aperture of the antenna. The principles of operation of this microwave beam steering technique are demonstrated in an aperture occupied by ground-plane-backed, sub-wavelength capacitive patches with identical dimensions. We demonstrate that by tilting the ground plane underneath the entire patch array layer, a phase shift gradient can be created over the aperture of the reflectarray that determines the direction of the radiated beam. Changing the direction and slope of this phase shift gradient on the aperture allows for performing beam steering in two dimensions using only one control parameter (i.e., tilt vector of the ground plane). A proof-of-concept prototype of the structure...

Sheet electron beams focused by periodically cusped magnetic (PCM) fields are stable against low‐frequency velocity‐shear instabilities (such as the diocotron mode). This is in contrast to the more familiar unstable behavior in uniform solenoidal magnetic fields. A period‐averaged analytic model shows that a PCM‐focused beam is stabilized by ponderomotive forces for short PCM periods. Numerical particle simulations for a semi‐infinite sheet beam verify this prediction and also indicate diocotron stability for long PCM periods is less constraining than providing for space‐charge confinement and trajectory stability in the PCM focusing system. In this article the issue of beam matching and side focusing for sheet beams of finite width is also discussed. A review of past and present theoretical and experimental investigations of sheet‐beam transport is presented.

Sheet electron beams are attractive for high-power microwave sources due to their ability to transport high current, at reduced current density, through thin clearance apertures and in close proximity to walls or RF structures. This paper reports on the theoretical investigation of magnetic quadrupole formation of elliptical sheet electron beams for use in high-power microwave devices. The beam envelope equations for an initially round beam passing through a physical non-symmetric quadrupole pair in the presence of space-charge, finite beam emittance, and under the effects of third-order field components and longitudinal velocity variations are presented. The presence of space-charge compensates for over-focusing in the thin beam-dimension and allows for the formation of highly elliptic sheet electron beams. As an example, the results of our study were applied to an existing Pierce gun source with a beam radius of 0.6 cm, beam energy of 10 keV and current density of 2.0 A/cm/sup 2/. We find that an elliptical beam with major radius r/sub a/=3.61 cm, minor radius r/sub b/=0.16 cm and ellipticity (r/sub a//r/sub b/) of 22.5 can be produced with only modest quadrupole gradients of 64 G/cm and 18 G/cm. Quadrupole formation of elliptical sheet-beams may be particularly suited for experimental research applications since existing round-beam electron guns may be used and changes in beam ellipticity may be made without breaking the vacuum system. >

We present an experimental study of electromagnetic losses resulting from atmospheric absorption and scattering on terahertz (THz) radiation. A sealed, high-Q quasi-optical resonator system permits the determination of atmospheric attenuation, while also allowing for the measurement of water content within the system. The measured data is then compared with the predictive Millimeter-wave Propagation Model (MPM)

Sheet electron beams focused by periodically cusped magnetic (PCM) fields are stable against low‐frequency velocity‐shear instabilities (such as diocotron mode). This is in contrast to more familiar unstable behavior in uniform solenoidal magnetic fields. Two rectangular‐cross‐section magnetic configurations capable of focusing in both transverse dimensions are investigated: (i) a closed‐side two‐plane PCM configuration that is topologically equivalent to conventional round‐cross‐section PPM focusing; and (ii) an open‐side configuration that uses ponderomotive PCM focusing in the vertical plane and simple vzBy Lorentz force focusing in the horizontal plane. Both configurations are capable of stable sheet beam confinement. The open‐side configuration appears more practical both for focusing and for realizing matched (cold) beam conditions in which the beam envelope is free from oscillations. For realistic beams with finite emittance, the existence of a matched cold beam solution implies less emittance grow...

Traveling wave tube (TWT) regenerative feedback oscillators have shown promise as efficient, compact sources of terahertz (THz) regime radiation. In this paper, we present results of experimental studies of the spectral purity and stationarity of the oscillation, with particular emphasis on the transient phase following the initiation of oscillation.

The hybrid-mode dispersion relation and resonant growth rates are solved for a finite-thickness sheet electron beam propagating through a rectangular guide with a thin dielectric slab. Analytic results for the growth rates, bandwidth and mode competition for an infinite magnetic field in the limit of a dilute beam are then presented. To properly couple to the desired EH/sub 10/ hybrid amplifier slow wave mode, the coupled waveguide mode equations are solved for a dielectric taper to accomplish the TE/sub 10/ to EH/sub 10/ mode transition. A piecewide linear taper which suppresses the competing EH/sub 11/ mode and other hybrid modes is developed and the overall amplifier system is discussed. >

A lack of compact, powerful, and efficient sources in the submillimeter-wave or terahertz-regime of frequencies has motivated research of microfabricated vacuum electronic devices. Regenerative-feedback traveling wave tube (TWT) oscillators have been proposed as a candidate source [1]. Recently, a compact TWT oscillator was fabricated and demonstrated with an intrinsic efficiency of ∼ 0.34% at 638 GHz [2]. This device utilized a folded waveguide structure as the slow wave circuit. By continuous variation of the beam voltage, the operating frequency was varied between 607 – 675 GHz. However, the oscillation frequency exhibited a step-tuning behavior, rather than a smooth variation with beam voltage.

We consider u plunur grating and partially filled dielectric us slow wave struc1ures for a low-voltage elecrrori sheer beam, moderate power Cerenkov umplifer. The smull sigriul growth rate for boih slow wave .siruciures wiih partially filled sheet beams ure evuluated. The resulis (ire compured ut 35 mid 60 GHz. We find thui boih configurations offer attractive arid compuruble gain ai relatively low heam voltuges and run be developed us moderute power sheet beam sources.

Cavity Q's and reflection coefficients for cavities excited by single “ergodic” eigenmodes are quantitatively calculated based on the available expressions derived from the Hamiltonian formulation. The results are compared with those obtained for multimode excitation in large, “untuned” resonant cavities. Applications relevant to the microwave and millimeter-wave regime are discussed. In addition to absorption spectroscopy and materials processing, ergodic cavities may used as radiation-beam dumps for high-power testing of millimeter-wave sources.

A high-average-power FEL amplifier operating at submillimeter frequencies is under development at the University of Maryland. Program goals are to produce a cw, ∼1 MW, FEL amplifier source at frequencies between 280 and 560 GHz. To this end, a high-gain, high-efficiency, tapered FEL amplifier using a sheet electron beam and a short-period (superconducting) wiggler has been chosen. Development of this amplifier is progressing in three stages: 1) beam propagation through a long length (∼ 1 m) of a short-period ( λ w = 1 cm) wiggler, 2) demonstration of a proof-of-principle amplifier experiment at 98 GHz, and 3) designs of a superconducting tapered FEL amplifier meeting the ultimate design goal specifications.

The free‐electron‐laser (FEL) equations are reduced to a set of one‐dimensional, normalized equations that allow a universal (dimensional) analysis. In universal parameters, numerical integration of the FEL equations indicates a relatively constant saturated ponderomotive wave amplitude independent of both the normalized wiggler potential amplitude and the injected signal level. The constant ponderomotive wave amplitude and an empirical fit for the universal saturation length as a function of normalized wiggler potential amplitude and gain permits unnormalized design calculations for saturated power and saturated length over a wide parameter range. Tapering is considered by deriving analytical expressions for the intrinsic efficiency and taper length. Design values for a high‐gain, high‐efficiency, tapered amplifier at 280 and 560 GHz are presented.

Calabazas Creek Research, Inc. (CCR) and the University of Wisconsin, Madison (UW) are developing a 650 GHz traveling wave tube amplifier (TWTA). Simulations predict 360 mW peak output power with a 2-10% duty cycle. This paper summarizes the design and fabrication of the TWT. Testing is expected in November, 2008.

Results are reported for numerical modeling of concepts for non-circular beam formation for near-THz frequency TWTpsilas. Several methods for beam compression were examined, and one method appears promising for producing a 12 kV, 8 mA beam with a cross section 70 times 160 microns, while keeping the cathode loading to about 59 A/cm2. This would be suitable for a 650 GHz TWT. The beam loading is significantly lower than in similar frequency backward wave oscillators (BWO) and should result in much longer cathode lifetimes.

The design and feasibility of a 1-MW continuous-wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration includes a short-period (I/sub w/ approximately 1 cm) planar wiggler, a sheet electron beam, a 0.5-1.0-MV thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial DC power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low ( >

Calabazas Creek Research, Inc. (CCR) is funded to develop a wide variety of RF sources from 81 GHz to 2 THz. Two traveling wave tubes are in development at 83.5 GHz for satellite communications applications. Development was performed on an overmoded, suspended ladder circuit at 95 GHz for high power applications. Two sources are in development at 650 GHz. CCR is continuing development of a 600-700 GHz backward wave oscillator and is funded to develop a 650 GHz TWTA (including the power supply). This TWT design incorporates a sheet electron beam and a planar circuit. To address high peak power applications, CCR is developing a gyrotron to generate peak power pulses exceeding 5 kW from a few hundred GHz up to 2.0 THz.