Larry R. Barnett

Phase-Shifted Traveling-Wave-Tube Circuit for Ultrawideband High-Power Submillimeter-Wave Generation

A novel slow-wave vacuum electron device circuit, consisting of a half-period-staggered double-vane array and a high-aspect ratio sheet electron beam, has been conceived for a high-power wideband submillimeter-wave generation. A particle-in-cell simulation, which is based on a finite-difference time-domain algorithm, has shown that this circuit has a very wide intrinsic bandwidth (in excess of 50 GHz around the operating frequency of 220 GHz) with a moderate gain of 13 dB/cm. Moreover, the saturated conversion efficiency is predicted to be 3%-5.5% over the operating band corresponding to an output power of 150-275 W, assuming a beam power of 5 kW. Of particular importance, this structure is based on the TE-fundamental mode interaction, thereby avoiding the complex over moding instabilities that usually cause spurious signal oscillation in conventional high-aspect-ratio structures. This planar circuit has simple 2-D geometry that is thermally and mechanically robust as well as being compatible with conventional microfabrication techniques. This concept is expected to open numerous opportunities in potential applications of versatile electronic devices in the low-millimeter- and submillimeter-wave regions.

Intense wideband terahertz amplification using phase shifted periodic electron-plasmon coupling

We report efficient wideband amplification of terahertz frequency electromagnetic waves from sequential electron-plasmon coupling along a half-period staggered double grating structure. Numerical eigenmode calculations show that the fundamental plasmonic band has a strong symmetric TE-mode, with a significant longitudinal electric field component, that is confined to the electron beam channel. A particle-in-cell simulation analysis with a single frequency excitation, combined with a short pulse broad spectrum drive, confirms that this terahertz plasmonic circuit produces three or four orders of magnitude power amplification with up to 30% bandwidth and 3%–5.5% saturated energy conversion efficiency.

Terahertz vacuum electronic circuits fabricated by UV lithographic molding and deep reactive ion etching

The 0.22 THz vacuum electronic circuits fabricated by UV lithography molding and deep reactive ion etching processes are under investigation for submillimeter wave applications. Eigenmode transient simulations show that, accounting for realistic values of our currently achievable fabrication tolerances, the transmission, and dispersion properties of the operation modes of a TE-mode, staggered, double grating circuit are maintained within less than 1 dB and 2% deviation, respectively. Scanning electron microscopy and atomic force microscopy analyses of the fabricated circuit samples demonstrate that both of the microelectromechanical system fabrication approaches produce circuits with ±3–5 μm dimensional tolerance and ∼30 nm surface roughness.

Strongly confined plasmonic wave propagation through an ultrawideband staggered double grating waveguide

In the course of research on high power terahertz vacuum electronic radiation sources, it was discovered that an intense plasmonic wave propagates through the channel between a half-period-staggered pair of TE-mode gratings. Experimental measurements were in good agreement with both a theoretical model and simulation analysis within 2% over the passband. This phase-shifted geometric modulation supports a dynamic bandwidth exceeding 25% with signal attenuation of less than 0.15dB∕cm (at Ka-band). Finite-difference time-domain analysis revealed that this low dispersion and low-loss optical response is ascribed to point-contact hopping motion of widespread resonant waveguide modes with the same phase velocity.

System Design Analysis of a 0.22-THz Sheet-Beam Traveling-Wave Tube Amplifier

The primary constituents of a 0.22-terahertz (THz) sheet-beam traveling-wave tube (TWT) amplifier, composed of a staggered double grating array waveguide, have been designed for broadband THz operation (~ 30%) using the fundamental passband (TE-mode). Currently, we are looking into the possibility of a pulsed low-duty test of this device as a proof of principle (POP) and have been making efforts to construct the system. The optimally designed input coupler has ≤ 1 dB insertion loss at 0.22 THz with ~ 75 GHz (34%) 1-dB matching bandwidths. A thin mica RF window provides a coupling bandwidth spanning multiple octaves. The collector is designed to have a jog for collecting the spent beam along the RF path coupled to the output RF window. Computer simulations show that the collector hybridized with a WR-4 window has ~ 60 GHz matching bandwidth with ~ - 0.5 dB insertion loss at 0.22 THz. The hybrid periodic permanent-magnet design combined with the quadrupole magnet (PPM-QM), intended for low-duty pulse operation in a proof-of-concept experiment, allows the elliptical sheet beam from an existing gun (25 : 1 aspect ratio) to unoptimized gun to have 73% beam transmission. The POP pulsed test is designed to be matched to our existing system, which will thereby tolerate beam transmission. However, a proper gun for the sheet-beam tunnel of the designed circuit will provide much better transmission. In our prior works, we successfully proved at W-band that the magnet design provided >; 99% beam transmission of a 10:1 aspect ratio sheet beam. Most of the TWT circuit components have been designed, and currently, a full simulation modeling effort is being conducted.

Modeling Investigation of an Ultrawideband Terahertz Sheet Beam Traveling-Wave Tube Amplifier Circuit

Extensive numerical analysis has demonstrated that a terahertz (H-band) sheet beam traveling-wave tube (TWT) amplifier circuit, composed of a staggered double grating array waveguide, has very broad bandwidth (~30%) of the fundamental passband (TE mode) with a 7:1 aspect ratio sheet beam without excitation of n = 1 space harmonic backward-wave modes. Particle-in-cell (PIC) simulations utilizing MAGIC3D and CST PS predict that the designed circuit produces ~150-300-W output power, corresponding to ~3%-5.5% intrinsic electronic efficiency (~35-38-dB saturated gain from 50-mW input driving power), over ~25% bandwidth, which is in good agreement with CHRISTINE 1-D code predictions. Simulations, using a perfectly matched layer boundary (~ -30-dB return loss), show that the circuit stably operates without noticeable oscillation. With a more realistic matching condition (~ -9.5-dB return loss), it becomes unstable. However, simulations show that the incorporation of an attenuating sever with tapered conductivity suppresses the instability in tube operation.

Electron beam transport analysis of W-band sheet beam klystron

The formation and transport of high-current density electron beams are of critical importance for the success of a number of millimeter wave and terahertz vacuum devices. To elucidate design issues and constraints, the electron gun and periodically cusped magnet stack of the original Stanford Linear Accelerator Center designed W-band sheet beam klystron circuit, which exhibited poor beam transmission (≤55%), have been carefully investigated through theoretical and numerical analyses taking advantage of three-dimensional particle tracking solvers. The re-designed transport system is predicted to exhibit 99.76% (cold) and 97.38% (thermal) beam transmission, respectively, under space-charge-limited emission simulations. The optimized design produces the required high aspect ratio (10:1) sheet beam with 3.2 A emission current with highly stable propagation. In the completely redesigned model containing all the circuit elements, more than 99% beam transmission is experimentally observed at the collector locate...

Particle-In-Cell Simulation Analysis of a Multicavity W-Band Sheet Beam Klystron

A W-band sheet beam klystron is being developed as a portable coherent radiation source for active denial system application. The interaction circuit design employs eight stagger-tuned cavities (multigap structure) and a 12:1-aspect-ratio sheet electron beam (74 kV and 3.6 A) to produce 50-kW peak power (2.5 kW average) and 40-dB gain with 200-MHz instantaneous bandwidth. The output cavity is designed to have a quasi-optical (QO) external coupler utilizing optical wave superposition. The circuit design has been optimized by using a 1-D disk-model code and a 3-D particle-in-cell (PIC) solver. The iterative simulation analysis predicts that a five-gap configuration is the optimum structure for a QO-output cavity because it provides sufficient output power and stable single frequency operation without mode competition. The 3-D PIC simulation predicts that the designed circuit produces stable 50-kW output power from a 4-W input driving signal, with 40-dB gain, at 94.5 GHz. The frequency sweep predicts a 3-dB bandwidth of 150 MHz in 2π-mode operation. The numerical simulation results agree well with the small-signal analysis, thereby providing confidence in the predicted output performance of the QO klystron amplifier module.

UV Lithography and Molding Fabrication of Ultrathick Micrometallic Structures Using a KMPR Photoresist

By using a novel negative-tone photoresist, KMPR, we have investigated ultraviolet (UV) lithographic microelectroforming fabrication of ultrathick metallic microstructures ( ¿ 400 ¿m). Scanning coating spin speed together with the film thickness and uniformity has been characterized at low spin speed from 1000 to 200 r/min. Based on the film profile characterization, the single-spin lithography conditions for a 400-¿m-thick electroforming mold are optimized by scanning process parameters of UV exposure energy and bake temperature and time. SEM-measured dimensional accuracy and sidewall verticality of the optimized thick KMPR mold are ¿ ¿ 3 ¿m and 90° ±1°, respectively, which are comparable to those of SU8 molds. The SEM analysis of the patterned film and the electroformed structure has shown that the submillimeter-thick KMPR features have ~ 5-10:1 aspect ratio. The sidewall surface roughness of the copper deposition is locally measured to be about ~ 50-100 nm by atomic force microscopy, which is significantly smoother than that resulting from other mechanical machining approaches. This novel photoresist enables the lithographic molding microfabrication process to mass produce plastic and metallic microcomponents for various microelectromechanical systems applications.

Scandate Dispenser Cathode Fabrication for A High-Aspect-Ratio High-Current-Density Sheet Beam Electron Gun

A high-current-density scandate tungsten dispenser cathode was used for the demonstration of a 25 : 1-aspect-ratio 750-A/cm2 -current-density sheet beam for the Defence Advanced Research Project Agency High-Frequency Integrated Vacuum Electronics (HiFIVE) program intended for the realization of a wideband ( ~ 30%) 220-GHz traveling wave tube. The elliptical cathode with homogeneous microstructure was made from 1-2-μm-size tungsten powder added with nanosized Scandia using the sol-gel method; it has a current density of up to 160 A/cm2 at 1050 °C. A sheet beam gun analyzer was built to test the terahertz sheet beam gun and determine the size and current density of a sheet electron beam produced by the impregnated scandate tungsten dispenser cathode. A sheet electron beam with an aspect ratio of 12.5 : 1 with a current density exceeding 375 A/cm2 has been obtained using a BVERI impregnated scandate dispenser cathode without magnetic compression; further magnetic field compression would give the final current density of 750 A/cm2.