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

Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR.

Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

https://iopscience.iop.org/article/10.1088/0022-3727/47/37/374001/pdf

A review of terahertz sources

Dynamic nuclear polarization (DNP) increases the sensitivity of nuclear magnetic resonance (NMR) spectroscopy by using high frequency microwaves to transfer the polarization of the electrons to the nuclear spins. The enhancement in NMR sensitivity can amount to a factor of well above 100, enabling faster data acquisition and greatly improved NMR measurements. With the increasing magnetic fields (up to 23 T) used in NMR research, the required frequency for DNP falls into the THz band (140- 600 GHz ). Gyrotrons have been developed to meet the demanding specifications for DNP NMR, including power levels of tens of watts; frequency stability of a few megahertz; and power stability of 1% over runs that last for several days to weeks. Continuous gyrotron frequency tuning of over 1 GHz has also been demonstrated. The complete DNP NMR system must include a low loss transmission line; an optimized antenna; and a holder for efficient coupling of the THz radiation to the sample. This paper describes the DNP NMR process and illustrates the THz systems needed for this demanding spectroscopic application. THz DNP NMR is a rapidly developing, exciting area of THz science and technology.

THz Dynamic Nuclear Polarization NMR

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 ${\sim}{\rm 15}~{\rm GHz}$ 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.

Demonstration of a High Power, Wideband 220-GHz Traveling Wave Amplifier Fabricated by UV-LIGA

Significant progress in modeling and manufacturing technologies open wide possibilities for performance improvements of millimeter-wave vacuum electron devices. However, many practical aspects should be considered to realize reliable long-life high-power sources. These are: thermal and RF stability, materials and assembly sensitivity to manufacturing, electrical stresses, cathode poisoning prevention, thermal beam effects, and many others. We address the full spectrum of design and manufacturing aspects while developing state-of-the-art extended interaction klystrons (EIKs). EIKs provide unprecedented RF performance and reliability in a compact user-friendly package. This paper discusses EIK design methodology and manufacturing concepts stating self-imposed restrictions and design modifications enhancing power capability, bandwidth, and extending operating frequencies into the terahertz region.

Practical Aspects of EIK Technology

This paper reviews the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL) CloudSat Extended Interaction Klystron requirements, design, performance, and development of a long life model for the European Space Agency (ESA)/Japan Aerospace Exploration Agency EarthCARE program plus Ka-band Extended Interaction Klystrons for the NASA/JPL Jupiter Icy Moons Orbiter and ESA European Global Precipitation Measurement Missions.

State-of-the-art W-band extended interaction klystron for the CloudSat program

The extended interaction klystron is one of the few readily available devices, capable of providing the required performance. This presentation describes the extended interaction klystron technology and presents improvements and design modifications in order to enhance power capability, bandwidth and to extend the operating frequency to the sub-millimeter region. A ladder-type RF circuit supports high efficiency and thermal stability at millimeter and sub-millimeter frequencies, while operating with moderate electron beam voltages. These EIKs operate at frequencies from 25 GHz to 280 GHz in continuous or pulsed modes.

Advantages of the Extended Interaction klystron Technology at Millimeter and Sub-Millimeter Frequencies

This paper reviews the NASA/JPL CloudSat extended interaction klystron (EIK) requirements, design, performance, and development of a long life model for the ESA/NASDA EarthCARE program plus a Ka-band extended interaction klystron for the NASA/JPL Jupiter Icy Moons Orbiter (JIMO) mission. The key enabling technology for these missions is a high-power, space-qualified, 94 GHz EIK, that has been developed and fabricated by CPI under contracts from ESA, JPL and CSA. Two flight models were delivered to the Jet Propulsion Laboratory, which is responsible for the instrument payload. This EIK is the first space-qualified high-power W-band amplifier.

A novel model of the compact high power 264 GHz CW Extended Interaction Oscillator has been developed by CPI Canada. By combination of mechanical tuning and electronic switching between two operating modes, this EIO provides over 1 W of power in the range of 258 to 270 GHz. Liquid cooling is used to stabilize RF circuit temperature and prevent frequency detuning. Further development is ongoing to support THz operation with enhanced output power, which is currently limited by vacuum window capability.

High power CW 264 GHz tunable Extended Interaction Oscillator

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Practical Aspects of EIK Technology

State-of-the-art W-band extended interaction klystron for the CloudSat program

Advantages of the Extended Interaction klystron Technology at Millimeter and Sub-Millimeter Frequencies

State-of-the-art W-band extended interaction klystron for the CloudSat program

High power CW 264 GHz tunable Extended Interaction Oscillator