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 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 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 an overview of the latest advances in the code development and modeling using parallel version of the large-signal code TESLA-Z, based on impedance matrix approach. Capabilities and application of the code to the modeling of advanced, broadband Multiple-beam Klystrons (MBKs) are discussed in details. Predictions of the code are compared with the set of measured data available for a few experimental MBKs.

We present the fabrication and testing of a novel 4 electron beam TWT designed for operation at low voltage (6.2 KV). Presented are cold test and bead pull data from this Ka-band circuit fabricated with micromachining methods.

We present the results of a study of the stability of a two-gap input cavity used in an experimental broadband multiple-beam klystron (MBK) with 18-beams and 7-cavities, which was designed for projected > 500 kW maximum output power and 13% bandwidth (NRL MBK-3). The device was experimentally observed to be prone to self-excitation at beam voltages below its design operational value of ~42 kV. Postexperiment examination of the two-gap input cavity revealed traces of breakdown on its face. Subsequent 3D CEM code simulations showed the presence of a high Q higher-order mode (HOM) TM11. To explore this HOM instability in greater detail, we applied the 2D parallel large-signal code TESLA-MB, which represents a parallel extension of the klystron code TESLA suitable for the accurate modeling of multiple-beam devices.

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 the first results of linearity measurements made on a Ka-band traveling-wave tube (TWT) driver-booster combination consisting of high power, broadband coupled-cavity TWT (CCTWT) cascaded with a folded-waveguide power booster TWT (FWTWT).

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.

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.

We present the demonstration of the first traveling wave tube circuit fabricated using ultraviolet photolithography. The complete amplifier demonstrated over 60 W output power at 214.5 GHz with 15 GHz bandwidth in the small signal regime at 14 dB gain. A high-gain 233 GHz amplifier is now under development and predicted to reach over 100 W output power. We present several microfabrication advancements that will be utilized to create the new device.

The design of a high-gain single-stage TWT amplifier is presented. This amplifier will be driven by a 124 mA 20 kV electron gun currently under development. Output power is expected to be more than 80 W across the 231.5 - 235 GHz band at 113 mW of input power. Predicted saturated output power is more than 140 W, which can yield 23% overall efficiency with the use of a single-stage depressed collector.

We present the first vacuum electronic traveling wave amplifier to incorporate an interaction circuit fabricated by ultraviolet (UV) photolithography and electroforming, demonstrating over 60 W of output power at 214.5 GHz from a 12.1 kV, 118 mA electron beam. The tube also achieved an instantaneous bandwidth of similar to 15 GHz in G-band in the small signal regime. The all-copper circuit was fabricated in two layers using a UV-transparent polymer monofilament embedded in the photoresist to form the beam tunnel prior to electroforming. Effects arising from fabrication errors and target tolerances are discussed. This microfabrication technique and demonstration paves the way for a new era of vacuum electron devices that could extend into the 1-2 THz range with advances in high-current-density electron guns.

We describe our progress on the development of a Ka-band TWT driver-booster combination to produce >1kW over a 5 GHz band centered at 35 GHz. The driver is an existing > 500 Watt broadband coupled-cavity TWT1, employing a 2 stage depressed collector. The power booster is a newly designed sever-less folded waveguide TWT designed to provide 3-4 dB of additional gain. The booster also uses a 2 stage depressed collector and a slightly modified version of the same electron gun used in the driver. A photo of the driver and booster under test is shown in Fig. 1. The principal challenges that had to be met in a booster design were: (1) achieving the required output power and bandwidth, (2) ensuring stable booster operation under both small and large signal conditions and (3) presenting a good output match to the driver so that the driver remains stable.

A sheet-beam coupled-cavity traveling wave tube has produced over 10 kW of peak power at a center frequency of 34 GHz, with a 3-dB bandwidth of almost 5 GHz. The power of this amplifier is an order of magnitude higher than state-of-the-art conventional amplifiers of comparable frequency, bandwidth, and operating voltage (<;20 kV). This unprecedented performance is made possible by a unique, NRL-developed sheet electron beam along with a novel slow-wave interaction structure. High-current, low-voltage operation provides high gain per unit length and allows an interaction structure <; 5-cm long to be used to achieve the desired gain of 15 dB at saturation. Measured performance agrees well with 3-D particle-in-cell simulations.

We present development plans for a 233 GHz, serpentine waveguide vacuum electron amplifier employing an embedded monofilament microfabrication technique based on UV-LIGA. Output power from the circuit is predicted to exceed 140 W in conjunction with a newly developed electron gun at 20 kV and 124 mA. Design, fabrication and integration progress will be discussed.

Progress on the demonstration of a high power broadband three-beam Ka-band cascaded traveling-wave tube (TWT) prototype is presented. The amplifier will be driven by an electron gun (0.6 A × 3, 20 kV) adapted from our 18-beam multiple-beam klystron gun design. Both serpentine and folded-waveguide versions of the cascaded-TWT have been evaluated. The expected peak RF output power is 4.5 kW with 100 W of drive power at a frequency of 30 GHz. The minimum output power is 3 kW over a 5 GHz frequency band (4 kW over > 2 GHz).

To meet the need to transmit increasingly massive volumes of data, both the defense and commercial sectors are turning to higher operational frequencies to take advantage of larger signal bandwidths while concurrently requiring increased amplifier power to achieve the necessary signal-to-noise ratios over large transmission distances. In response to these needs, the last decade has seen a leap in performance of a variety of millimeter-wave devices. The Naval Research Laboratory (NRL) is the principal U.S. Department of Defense R&D center focused on the development of the science and technology behind new millimeter-wave high power solid-state and vacuum electronic devices. Selected examples of NRLs research projects are described with an emphasis on high power millimeter-wave vacuum electronic devices.

The design of a W-band serpentine TWT with >200 W of power over a 4 GHz bandwidth (>100 W over 7 GHz) is presented. The amplifier is driven by a 122mA, 20 kV electron beam generated by a slightly modified version of the demonstrated 670 GHz beamstick at a reduced magnetic field. The design was performed by both the established MAGIC-3D and the recently validated NRL code Neptune, with good agreement between the two codes. Predicted RF peak power is 245 W, corresponding to 10% electronic efficiency.