We present characterization of the precision and accuracy of millimeter-wave serpentine waveguide traveling-wave circuits fabricated by CNC machining. The critical dimensions of circuit piece parts were measured using a digital optical microscope. Measured results are compared across 10 duplicate fabricated circuits and the design model in order to characterize the repeatability and overall fabrication tolerances. Results are reported in relative units, normalized to the nominal mid-band operating frequency in the millimeter-wave range. We observe that the achieved precision of critical dimensions is close to the inherent limit of the CNC machine.

We present the experimental demonstration of a traveling-wave tube (TWT) power amplifier operating in the ${W}$ -band (75–110 GHz) frequency range. The device is based on a serpentine waveguide (SWG) amplification circuit, a slow wave circuit type capable of high power and broad instantaneous bandwidth in the upper millimeter-wave range. A 20 kV, 140-mA round solenoid-focused electron beam powers the device. At 20 kV, we measure 215 ± 2 W peak RF output power at 93 GHz with 20.1 ± 0.15 dB saturated gain, pulsed at 0.1% duty. We observe 10-GHz instantaneous amplification bandwidth at 100-W minimum output power, covering the range 88–98 GHz. Operating at 20.8 kV, the TWT produces 285 ± 3 W at 91 GHz with 22.4 ± 0.15 dB gain, and 7 GHz, 3-dB bandwidth. The peak electronic efficiency measured is approximately 10%.

We present experimental characterization of a 20-kV, 130-mA thermionic electron gun, which is a component of a W-band traveling-wave tube (TWT) power amplifier. The electron beam is strongly focused by a 6.6-kG permanent magnet solenoid, producing a peak current density up to 800 A/cm 2 . The cathode temperature, focus electrode voltage, and modulating anode voltage are varied to characterize the emitted cathode current and beam transport through the beam tunnel of the TWT RF circuit. We observe the variation of the cathode current from 107 to 155 mA and peak beam transmission of 95% measured at the collector. Experimental results are compared to 2-D simulations with the electron gun and collector design code MICHELLE.

We report on efforts toward a monolithic Ka-band sheet-beam coupled-cavity TWT circuit. This new circuit serves as a drop-in replacement for a demonstrated device that was originally fabricated using the method of brazing stacked plates.

We present testing of a W-band traveling-wave tube (TWT) based on a serpentine waveguide circuit, powered by a 20 kV, 130mA electron gun. We measure peak output power of 215±2W at 93 GHz with 20.1±0.15dB saturated gain, and>100 W from 88-98 GHz, pulsed at 0.1% duty. Operating at 20.8 kV, the TWT produces 285±3W at 91 GHz with 22.4±0.15dB gain.

We present experimental characterization of a 20 kV, 130 mA thermionic electron gun in a W-band TWT. The gun heater voltage, focus electrode voltage, and modulating anode voltage are swept to characterize the performance of the emitted cathode current and beam transport through the RF circuit. We observe variation of the cathode current from 107 to 155 mA and peak beam transmission of 95% at the collector. Experimental results are compared to 2D MICHELLE simulations.

Millimeter and sub-millimeter wave vacuum electronic devices have been of great interest for a wide variety of applications, but fabrication of the sub-wavelength structures has been a persistent challenge. We present and compare fabrication approaches including CNC micromachining, 3D printed mold electroforming (3D PriME), and UV-photolithography for creating traveling wave tube circuits at 35, 95, 140 and 233 GHz, along with results. To date, CNC micromachining has yielded the best results up to 140 GHz.

To achieve higher output power while maintaining the compact size and weight of typical PPM-focused TWTs, we have explored options for incorporating a sheet or elliptical cross section electron beam in a coupled cavity or folded waveguide circuit. This approach allows significantly higher beam current to be propagated at a given voltage and magnetic field amplitude than is possible in a round beam. However, there are fundamental design constraints that limit the aspect ratio of the beam and the corresponding circuit designs that can be utilized. Here, some of these constraints will be discussed and designs presented for elliptical beam Ka-band TWTs that are capable of >2 kW output power with a 20 kV, 1A electron beam.

We present experimental characterization of W-band serpentine waveguide TWT circuits fabricated by CNC micro-endmill machining. Cold test measurements of S-parameters demonstrate excellent agreement with simulation and precise fabrication repeatability between multiple circuits. The circuit wavelength dispersion was measured in both W- and D-band waveguide bands, covering an octave in frequency, showing agreement with simulation from 85-170 GHz. We discuss details of the fabricated circuits and cold test results.

We present fabrication and testing of RF components for a W-band serpentine waveguide TWT. Broadband ceramic RF windows and loads exhibit reflection lower than -20 dB across the TWT operating band, nominally 87-100 GHz. End-to-end cold test of the entire tube assembly, including interaction circuit, shows a clean passband better than -15 dB, in agreement with simulation. We discuss details of the fabricated components and electromagnetic cold test results.

High-power millimeter-wave amplifiers are required for various new and emerging applications, such as point-to-multi-point wireless high-data-rate communications over multi-km ranges for residences and businesses. Driven largely by these requirements, both civilian and defense-related, researchers over the last decade have dramatically increased the power and bandwidth of a variety of vacuum electronic amplifiers in the millimeter-wave regime. The Naval Research Laboratory (NRL) has played a key role in the development of the science and technology that has made possible this increased performance. One attractive class of devices to arise from these efforts is the power booster, a relatively low-gain but very high-power amplifier that can produce significantly higher power than state-of-the-art amplifiers driven by comparable electron beams. At Ka band, NRL power boosters have produced from 1.5 to 4 kW of peak power from electron beams of ~20 kV and up to 600 mA. The 3-5 dB gain of these amplifiers allows compact designs and high interaction efficiency. For example, with a multistage depressed collector the 4 kW output power device can achieve a power added efficiency of >40%.

We present development progress for a 233 GHz hybrid serpentine waveguide TWT amplifier employing microfabrication techniques based on UV-LIGA. Maximum output power from the circuit is predicted to exceed 140 W from a 20 kV, 124 mA electron beam. Updates include progress of the circuit microfabrication and testing, window tuning, and electron gun.

Recent advances in the development of millimeter wave vacuum electronic devices have been made possible by powerful, specialized design tools. We summarize them here and point out areas where improved models and codes will be needed.

NRL has developed and demonstrated a unique sheet-beam electron gun, which produces a 3.5-4-A, 0.3 × 4 mm electron beam at ~20 kV. This electron beam has been successfully used to power multi-kW amplifiers at both Ka and W bands. The devices developed thus far employ a permanent magnet solenoidal field of 6 - 8 kG to transport the electron beam through the interaction circuit. While a periodic permanent magnet (PPM) structure would be much more compact, some compromises in performance are required to utilize PPM focusing. We will discuss some of the design considerations and requirements for high-power sheet beam amplifiers and review some key features of the amplifiers demonstrated thus far.

The principal challenge for creating vacuum electron devices in the millimeter-wave (mmW) frequency range is accurate fabrication of slow-wave circuits and other electromagnetic features with tight tolerance. Ultraviolet Photolithography and Electroforming (UV-LIGA) techniques are presented that allow tight tolerance control for slow wave circuits for the mmW and sub-mmW bands. We show how these techniques were applied at the W- and G-bands.

Summary form only given. An extended-interaction oscillator (EIO), driven by a 20-kV, 4-A sheet electron beam having a cross section of 0.3 × 4 mm, is described. This new EIO is an outgrowth of our recently demonstrated sheet-beam-driven, 94-GHz extended interaction klystron (EIK) amplifier, which has produced a peak power of over 7.5 kW.1, 2 The size and weight of these devices are dictated by the permanent magnet solenoid used to focus the electron beam through the circuit, and they scale approximately as L2B, where L and B are the length a magnitude of the uniform field region. The oscillator configuration consists of a single 11-gap cavity that is 0.9 cm long, so the full circuit is about half the length of the amplifier. Consequently, the oscillator is both much simpler (no input coupler and driver required) and about 25% the size and weight of the amplifier. Overall dimensions are 18 × 22 × 8 cm, with a weight of less than 16 kg. Neptune3 particle-in-cell simulations predict a peak output power of 9.5 kW from the oscillator. The power is maximized by varying the number of gaps in the cavity, by tapering the gap spacing within the cavity, and by adjusting the size and positioning of the coupling iris. Oscillator design considerations for peak and average power and comparisons with the amplifier will be discussed.

NRL's unique sheet-beam electron gun, which produces a 3.5-4-A, 0.3 × 4 mm electron beam at ~20 kV, has been successfully used to power multi-kW amplifiers at both Ka and W bands. The 94-GHz narrow-band EIK uses a 3-cavity circuit to produce a peak output power of over 7.5 kW, with saturated gain of ~35 dB. The Ka-band amplifier uses a double staggered ladder coupled-cavity circuit to achieve a peak output power >10 kW at 34 GHz, with a 3-dB bandwidth of 5 GHz and saturated gain of ~15 dB. The performance of both amplifiers is in very good agreement with models and simulations.

The general electromagnetic properties and design methodology for serpentine/folded-waveguide (FW) amplifiers are presented. In addition, hybrid-waveguide circuit topologies, which permit greater design flexibility than the basic serpentine/FW topologies, are also introduced, and their dispersion characteristics are discussed. Experimental validation of design methodology and tools is provided via test results of the recently demonstrated wideband 220-GHz serpentine amplifier, which embodies the design methodology described herein. Particular attention will be paid to the comparison between code prediction and experimental data, which are in excellent agreement.

A technological breakthrough is embodied in the successful demonstration of an extended interaction klystron (EIK) amplifier, which has produced over 7.5 kW of peak output power at W-band (94 GHz). An efficiency of ~17% has been achieved with a depressed collector. The EIK is driven by a 20-kV, 4-A sheet beam in a permanent magnet solenoid, with 99% beam current transmission from gun to collector. Key features that contribute to the success of this device are: tight beam focusing and correspondingly narrow beam tunnel, which are made possible by the solenoidal focusing and which provide high interaction impedance and high gain per unit length and the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.