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.

Vacuum Electronic High Power Terahertz Sources

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 ${\sim}{17\%}$ 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: 1) 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 2) the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.

Demonstration of a Multikilowatt, Solenoidally Focused Sheet Beam Amplifier at 94 GHz

Do you design and build vacuum electron devices, or work with the systems that use them? Quickly develop a solid understanding of how these devices work with this authoritative guide, written by an author with over fifty years of experience in the field. Rigorous in its approach, it focuses on the theory and design of commercially significant types of gridded, linear-beam, crossed-field and fast-wave tubes. Essential components such as waveguides, resonators, slow-wave structures, electron guns, beams, magnets and collectors are also covered, as well as the integration and reliable operation of devices in microwave and RF systems. Complex mathematical analysis is kept to a minimum, and Mathcad worksheets supporting the book online aid understanding of key concepts and connect the theory with practice. Including coverage of primary sources and current research trends, this is essential reading for researchers, practitioners and graduate students working on vacuum electron devices.

Microwave and RF Vacuum Electronic Power Sources

The millimeter-wave spectrum, generally considered to range from 30 to 300 GHz, is one of the most actively used spectrums today, with applications such as high-data-rate terrestrial and space communications, radar, and electronic warfare. The helix traveling-wave tube (TWT) amplifier has long been a critical component for enabling modern millimeter-wave systems. This paper presents an overview of several recently developed high-power millimeter-wave helix TWTs which have extended the helix TWT performance envelope. In particular, the power-frequency-bandwidth product for these devices has increased by a factor of 4 to over 100 depending upon the application. These helix TWTs operate at power levels up to 1-kW peak power or 500-W average or continuous-wave power and over frequencies from 18 to 46 GHz, with efficiencies up to 60%. The helix TWT performance improvements discussed here have not only kept pace with improvements in rival technologies such as GaN solid-state power amplifiers but also pushed helix TWTs into competition with traditionally higher power and higher frequency vacuum electron devices such as extended-interaction klystrons and coupled-cavity TWTs. Indeed, the modern high-power millimeter-wave helix TWT offers system engineers a singularly attractive amplifier technology with an outstanding combination of small size, light weight, high efficiency, good linearity, high reliability, and large bandwidth.

Latest Advancements in High-Power Millimeter-Wave Helix TWTs

This is Part I of a two-part report on design and manufacturing methods used at SLAC to produce accelerator klystrons. Chapter 1 begins with the history and applications for klystrons, in both of which Stanford University was extensively involved. The remaining chapters review the theory of klystron operation, derive the principal formulae used in their design, and discuss the assumptions that they involve. These formulae are subsequently used in small-signal calculations of the frequency response of a particular klystron, whose performance is also simulated by two different computer codes. The results of calculations and simulations are compared to the actual performance of the klystron.

/pdf/high-power-klystrons-theory-and-practice-at-the-stanford-4pi6u2u4uw.pdf

High Power Klystrons: Theory and Practice at the Stanford Linear Accelerator CenterPart I

The stable transport of high-perveance, low-voltage sheet electron beams is a key requirement for the successful development of compact, high-power sheet beam amplifiers. We describe a beam stick to demonstrate the transport of such a beam (19.5 kV, 3.5 A) in a solenoidal magnetic field of about 8.5 kG. The beam stick consists of a novel sheet beam gun having single-plane convergence of a factor of ∼30, a permanent magnet solenoid, a 1.8-cm-long × 5 mm wide × 0.4 mm high beam tunnel, and an isolated collector. The engineering design was based closely upon MICHELLE and MAGIC-3D simulations of a W-band extended interaction klystron.

3.4: Sheet beam stick for low-voltage W-band extended interaction klystron (EIK)

Key performance characteristics that guide the selection of an amplifier for various applications are gain, power, linearity, and efficiency. In this paper, these parameters are contrasted for two amplifier configurations: a single-beam amplifier with stages placed in series vs. a multi-beam amplifier with cascading stages. Based on MAGIC-3D simulations, while gain and efficiency are comparable, the novel cascading amplifier configuration provides significant improvements in terms of power for a given level of linearity, both AM/AM and intermodulation distortions, over those of a conventional serial approach.

Linearity performance of multi-stage TWT amplifiers: Cascade vs. series

A sheet-beam Extended Interaction Klystron (EIK) is an attractive MMW amplifier because of its enhanced power capability, compactness, and relatively high gain per unit length. However, an intrinsic problem with a sheet beam amplifier is its susceptibility to multimoding, which results because the width of the device is typically larger than a wavelength. Even in its ideal configuration, a sheet beam slow wave structure can support a number of closely spaced modes. Furthermore, small imperfections or asymmetries can excite a fundamental TE 10 mode in the rectangular beam tunnel, which can cause oscillation or otherwise degrade performance. We have performed 3D PIC simulations (using MAGIC and ICEPIC) to study these effects in several different EIK structures. We will describe methods for minimizing the deleterious effects of multimoding and imperfections, present results of the simulations, and compare the two PIC codes.

3.5: Sheet beam EIK sensitivity to multimoding and circuit imperfections

A sheet-beam Extended Interaction Klystron (EIK) is being developed at W-band to demonstrate the enhanced power capability, compactness, and relatively high gain per unit length that have been predicted for the device. The EIK is powered by a 19.5-kV, 3.3-A sheet beam having dimensions of 0.32 mm × 4 mm. A beamstick for the device has achieved 98% current transport efficiency over a distance of 2 cm in a uniform magnetic field of 8.5 kG. Particle-in-cell simulations using MAGIC-3D and ICEPIC have predicted saturated output power of almost 10 kW from the EIK. A 3-cavity version of the device is predicted to have a saturated gain of >30 dB with an interaction efficiency of ∼16%. The spent-beam energy distribution is compatible with a depressed collector that can increase the overall efficiency to >30%.

W-band sheet beam Extended Interaction Klystron (EIK)

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 TWT
 1
, 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.

Multi-kW 35 GHz TWT power booster development

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