Extended-interaction klystrons (EIKs) are resonant cavity-based circuits. First proposed by Chodorow and Wessel-Berg [1], the EIK has evolved into a compact device of choice for the THz regime. Similar to klystrons, EIKs are comprised of an input cavity, which imparts a velocity modulation on the beam to start the beam bunching process, one or more idler cavities to improve beam bunching (gain) and/or bandwidth, and an output cavity to extract RF power from the optimally bunched beam. The key difference between a standard klystron and an EIK is that EIK cavities have several interaction gaps versus one for a standard klystron, which raises the cavity interaction impedance. The higher cavity interaction impedance is a key factor in compensating for the unavoidable lower beam current at higher operating frequencies. This is because beam current tends to scale as ~1/ f 2 for any given beam-forming approach.

Millimeter-wave (mmW) traveling-wave tube (TWT) devices are of interest for applications requiring amplifier output power in the >100 W range over instantaneous bandwidth of several GHz, but fabrication of the sub-wavelength slowwave structures has been a persistent challenge. We discuss the development of TWT device design and fabrication methods to advance the output power and bandwidth capability and improve the SWaP-C (size, weight, power, cost) of devices in the Ka-band to W-band frequency ranges.

We describe the design, development, and initial testing of a very compact, multiple-beam folded-waveguide traveling wave tube (TWT) operating in Ka -band at $\sim $ 6 kV. A proof-of-principle four-beam, $\sim $ 100 mA/beam device has been fabricated and tested, and a 16-beam version of the device has been designed and is predicted to produce an output power up to 1 kW. Here, we describe the design approach we devised and implemented to overcome two fundamental challenges: 1) generating and transporting the tightly spaced beams required to operate in the millimeter-wave regime and 2) developing folded waveguide circuits that can provide high interaction impedance for each of the beams as well as stable operation in a dense high-order-mode environment.

We present experimental hot testing of a novel low voltage, multi-electron beam, serpentine Ka-band traveling wave tube amplifier utilizing a cathode in an immersed magnetic field. The amplifier is designed to run at 430 mA of total beam current spread across four beamlets in an immersed, zero-compression flow. At present, the device is characterized at 225 mA of cathode current with 95% beam transmission at a maximum output power of 68 W. Using a four-segment beam collector, the beams are measured individually and exhibit good current sharing with a standard deviation of about 3 mA.

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.

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.

In this paper we present a practical design of Multiple Beam Traveling Wave Tube (MB-TWT) with Folded Waveguide (FW) Slow Wave Structure (SWS) operating at low voltages, 6.2 kV, with output power about 300 Wand central frequency of 29 GHz completed by using NRL large signal design codes [1], [2]. Stability analysis with respect to excitation of high order spurious modes has been done with NRL 3D PIC code Neptune [3].

Folded Waveguide (FW)mini- TWT based on four beams operating at 6.2 kV with total beam power 2.7 kW has been designed by using NRL design codes. The FW slow wave structure (SWS) has been fabricated and "cold" tested. The results of measurements are compared with design. The status of experimental demonstration of the mini- TWT will be presented.

Design methodology for low voltage high power multiple beam Traveling Wave Tube (TWT) amplifiers with folded waveguide (FW) slow wave structure (SW) has been developed. Four beam multiple beam (MB) FW TWT operating at 6.2 kV with power per beamlet about 75 W and total power of 300 W has been designed and studied by using NRL large signal codes TESLA-FW and CHRISTINE-FW and NRL GPU based PIC code Neptune. Recent status of experimental demonstration of MB mini TWT in NRL also will be presented.

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%.

NRL large signal design codes have been developed recently to be suitable for accurate simulations of multiple beam traveling wave tubes (MB-TWT). MB-TWTs are attractive novel devices with enhanced power-bandwidth characteristics. However, they are more vulnerable to spurious oscillations than single beam TWTs. The results of performance and stability analysis of MB-TWTs with 3D EM codes, NRL 3D PIC code Neptune and recently developed 1D and 2.5D design codes TESLA-FW and CHRISTINE-FW are presented and discussed.

Multiple beam traveling wave tubes (MB-TWTs) are attractive novel devices with enhanced power-bandwidth characteristics 1 . However, they are more vulnerable to spurious oscillations than single beam TWTs. Therefore, design codes for MB-TWTs should be capable of accurate prediction of MB-TWT operation in steady-state regime as well as prediction of spurious oscillations onset for different modes. Complexity of multiple beam slow wave structures is also one of challenging factors requiring for computational efficiency of design codes.

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.

In any TWT design, understanding of potential spurious oscillation characteristics is critical to achieve optimal amplifier performance and, more importantly, to avoid the unpleasant surprise of discovering problems after the fact. This paper serves to illustrate these oscillation characteristics and to discuss potential mitigation approaches with special emphasis on serpentine/folded-waveguide TWTs. The relationship between folded-waveguide TWTs and coupled-cavity TWTs is also discussed, particularly, with respect to "drive-induced" oscillations.

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.

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.