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 summary of the NASA Jet Propulsion Laboratory's 675 GHz imaging radar is presented, with an emphasis on several key design aspects that enable fast, reliable through-clothes imaging of person-borne concealed objects. Using the frequency-modulated continuous-wave (FMCW) radar technique with a nearly 30 GHz bandwidth, sub-centimeter range resolution is achieved. To optimize the radar's range resolution, a reliable software calibration procedure compensates for signal distortion from radar waveform nonlinearities. Low-noise, high dynamic range detection comes from the radar's heterodyne RF architecture, low-noise chirp source, and high-performance 675 GHz transceiver. The radar's optical design permits low-distortion fast beam scanning for single-pixel imaging, and a real-time radar image frame rate of 1 Hz is now possible. Still faster speeds are on the horizon as multi-beam THz transceivers are developed.

THz Imaging Radar for Standoff Personnel Screening

For terahertz optoelectronics to find broader applications, more efficient sources and detectors are needed. Towards this end, Berry et al. demonstrate the use of plasmonic contact electrodes for both terahertz emitters and detectors, finding large enhancement over standard photoconductive devices.

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Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes

As of today, the fifth generation (5G) mobile communication system has been rolled out in many countries and the number of 5G subscribers already reaches a very large scale. It is time for academia and industry to shift their attention towards the next generation. At this crossroad, an overview of the current state of the art and a vision of future communications are definitely of interest. This article thus aims to provide a comprehensive survey to draw a picture of the sixth generation (6G) system in terms of drivers, use cases, usage scenarios, requirements, key performance indicators (KPIs), architecture, and enabling technologies. First, we attempt to answer the question of "Is there any need for 6G?" by shedding light on its key driving factors, in which we predict the explosive growth of mobile traffic until 2030, and envision potential use cases and usage scenarios. Second, the technical requirements of 6G are discussed and compared with those of 5G with respect to a set of KPIs in a quantitative manner. Third, the state-of-the-art 6G research efforts and activities from representative institutions and countries are summarized, and a tentative roadmap of definition, specification, standardization, and regulation is projected. Then, we identify a dozen of potential technologies and introduce their principles, advantages, challenges, and open research issues. Finally, the conclusions are drawn to paint a picture of "What 6G may look like?". This survey is intended to serve as an enlightening guideline to spur interests and further investigations for subsequent research and development of 6G communications systems.

The Road Towards 6G: A Comprehensive Survey

As of today, the fifth generation (5G) mobile communication system has been rolled out in many countries and the number of 5G subscribers already reaches a very large scale It is time for academia and industry to shift their attention towards the next generation At this crossroad, an overview of the current state of the art and a vision of future communications are definitely of interest This article thus aims to provide a comprehensive survey to draw a picture of the sixth generation (6G) system in terms of drivers, use cases, usage scenarios, requirements, key performance indicators (KPIs), architecture, and enabling technologies First, we attempt to answer the question of “Is there any need for 6G?” by shedding light on its key driving factors, in which we predict the explosive growth of mobile traffic until 2030, and envision potential use cases and usage scenarios Second, the technical requirements of 6G are discussed and compared with those of 5G with respect to a set of KPIs in a quantitative manner Third, the state-of-the-art 6G research efforts and activities from representative institutions and countries are summarized, and a tentative roadmap of definition, specification, standardization, and regulation is projected Then, we identify a dozen of potential technologies and introduce their principles, advantages, challenges, and open research issues Finally, the conclusions are drawn to paint a picture of “What 6G may look like?” This survey is intended to serve as an enlightening guideline to spur interests and further investigations for subsequent research and development of 6G communications systems

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In this paper, results of different models are compared for calculating effective, cold-circuit (beam-free) phase velocities and interaction impedances of folded waveguide (FW) slow wave circuits for use in millimeter-wave traveling wave tubes (TWT). These parameters are needed for one-dimensional (1-D) parametric model simulations of FW traveling wave tubes (FWTWTs). The models investigated include approximate analytic expressions, equivalent circuit, three-dimensional (3-D) finite difference, and 3-D finite element. The phase velocity predictions are compared with experimental measurements of a representative FW circuit. The various model results are incorporated into the CHRISTINE1D code to obtain predictions of small signal gain in a 40-55 GHz FWTWT. Comparing simulated and measured frequency-dependent gain provides a sensitive, confirming assessment of the accuracy of the simulation tools. It is determined that the use of parametric 1-D TWT models for accurate, full band predictions of small signal gain in FWTWTs requires knowledge of phase velocity and impedance functions that are accurate to <0.5% and <10%, respectively. Saturated gain predictions, being approximately half as sensitive to these parameters, appear to require correct specification of phase velocity and interaction impedance to within /spl sim/1% and 20%, respectively. Although all models generate sufficiently accurate predictions of the interaction impedance, not all generate sufficiently accurate predictions of the effective axial phase velocity.

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Accurate parametric modeling of folded waveguide circuits for millimeter-wave traveling wave tubes

This article gives a brief overview of the common vacuum electronic tube amplifiers used in high-power transmitters. Only three types of devices [travelling wave tubes (TWTs) including helix and coupled-cavity types, microwave power modules (MPMs), and klystrons] are covered, with emphasis on the recent advance in the millimeter-wave band.

Vacuum tube amplifiers

Northrop Grumman Corp. (NGC) has continued to develop a VE-based folded waveguide (FWG) technology as part of DARPA's terahertz imaging focal plane array technology (TIFT) source development program. The THz source consists of a thermionic electron gun, a FWG regenerative oscillator circuit, a depressed collector, and a 10 kG Nd-Fe-B permanent magnet solenoid to confine the electron beam. Significant improvements have been made to the compact THz source, increasing the output power, efficiency, and operational duty cycle.

A compact, high power, 0.65 THz source

Northrop Grumman has developed a folded-waveguide vacuum electronic high power amplifier at 233 GHz. The amplifier demonstrates output power over 79 W (49 dBm) from 232.6–234.6 GHz, and greater than 50 W (47 dBm) over a 2.4 GHz instantaneous bandwidth. Saturated gain is approximately 23–24 dBm over this bandwidth, with an input drive of 26–27 dBm required for saturated output power. The device operating point is 20.95kV, 114mA for a frequency range centered at 233.6 GHz, but is capable of producing 50W at frequencies up to 237.8 GHz by varying the operating voltage. Typical instantaneous (3 dB) operating bandwidth ranges from 2.4– 3.0 GHz for a beam voltage range of 20.5–21 kV. The amplifier uses a compact permanent magnet solenoid for beam focusing and confinement, and typically achieves 95– 98% beam transmission through the folded-waveguide circuit and into the single-section depressed collector. This device has demonstrated operational duty cycles up to 50% with liquid cooling and using Northrop Grumman's Microwave Power Module compact integrated power conditioner technology. A more compact version of this power amplifier is being developed with emphasis on reducing the size of the permanent magnet solenoid.

233 GHz high Power amplifier development at Northrop Grumman

A 0.670 THz Power Module based on a vacuum electronic (VE) power amplifier and a solid-state power amplifier (SSPA) has produced >100 mW, operating from 0.640–0.685 THz in a bread-board demonstration. The driver amplifier is a power-combined, MMIC SSPA module based on InP HEMT technology. The compact VE power amplifier is based on a DRIE-fabricated folded waveguide slow-wave circuit, together with a high current density thermionic cathode and a single-stage, depressed collector for increased overall device efficiency. The VE power amplifier achieved 21.5 dB of gain, 15 GHz of operational bandwidth, a collector efficiency of 93% and an overall device efficiency of 0.44%.

K. Kreischer

Papers

Accurate parametric modeling of folded waveguide circuits for millimeter-wave traveling wave tubes

Vacuum tube amplifiers

A compact, high power, 0.65 THz source

233 GHz high Power amplifier development at Northrop Grumman

A 100 mW, 0.670 THz power module