Millimeter Wave Traveling Wave Tubes for the 21st Century

Traveling wave tubes are rapidly evolving to provide unprecedented power level in comparison to solid state devices in the millimeter waves region of the spectrum (80 – 300 GHz) thus enabling a wide range of applications. Wireless communications, imaging, security, plasma diagnostics, healthcare and many others will gain substantial features if high power in the millimeter wave region would be available from compact sources. The development of fabrication technologies is proving crucial for introducing new topologies and structures for millimeter wave vacuum electronic devices, compatible with the dimensions dictated by the short wavelength that poses substantial challenges due to tight tolerances and surface quality. This review paper will provide an overview of the principles, evolution and state of the art of one of the most widely utilized vacuum electronic devices, the traveling wave tube (TWT). The wide band, high gain features of TWTs make those devices the most promising solutions for high power at millimeter waves and THz frequencies

Array concepts for solid-state and vacuum microelectronics millimeter-wave generation

The authors have proposed that the increasing demand for contact watt-level coherent sources in the millimeter- and submillimeter-wave region can be satisfied by fabricating two-dimensional grids loaded with oscillators and multipliers for quasi-optical coherent spatial combining of the outputs of large numbers of low-power devices. This was first demonstrated through the successful fabrication of monolithic arrays with 2000 Schottky diodes. Watt-level power outputs were obtained in doubling to 66 GHz. In addition, a simple transmission-line model was verified with a quasi-optical reflectometer that measured the array impedance. This multiplier array work is being extended to novel tripler configurations using blocking barrier devices. The technique has also been extended to oscillator configurations where the grid structure is loaded with negative-resistance devices. This was first demonstrated using Gunn devices. More recently, a 25-element MESFET grid oscillating at 10 GHz exhibited power combining and self-locking. Currently, this approach is being extended to a 100-element monolithic array of Gunn diodes. This same approach should be applicable to planar vacuum electron devices such as the submillimeter-wave BWO (backward wave oscillator) and vacuum FET

Imaging Polarimeter Arrays for Near-Millimeter Waves

An integrated-circuit antenna array has been developed that images both polarization and intensity. The array consists of a row of antennas that lean alternately left and right, creating two interlaced sub-arrays that respond to different polarizations. The arrays and the bismuth bolometer detectors are made by a photoresist shadowing technique that requires only one photolithographic mask. The array has measured polarization at a wavelength of 800 µm with an absolute accuracy of 0.8° and a relative precision of 7 arc min. and has demonstrated nearly diffraction-Iimited resolutiort of a 20° step in polarization

Millimeter-wave diode-grid phase shifters

Monolithic diode grids have been fabricated on 2-cm square gallium-arsenide wafers with 1600 Schottky-barrier varactor diodes. Shorted diodes are detected with a liquid-crystal technique, and the bad diodes are removed with an ultrasonic probe. A small-aperture reflectometer that uses wavefront division interference was developed to measure the reflection coefficient of the grids. A Phase shift of 70° with a 7-dB loss was obtained at 93 GHz when the bias on the diode grid was changed from -3 V to 1 V. A simple transmission-line grid model, together with the measured low-frequency parameters for the diodes, was shown to predict the measured performance over the entire capacitive bias range of the diodes, as well as over the complete reactive tuning range provided by a reflector behind the grid, and over a wide range of frequencies form 33 GHz to 141 GHz. This shows that the transmission-line model and the measured low-frequency diode parameters can be used to design an electronic beam-steering array and to predict its performance. An electronic beam-steering array made of a pair of grids using state-of-the-art diodes with 5-Ω series resistances would have a loss of 1.4 dB at 90 GHz

Millimeter-Wave Diode-Grid Frequency Doubler

Monolithic diode grid were fabricated on 2-cm^2 gallium-arsenide wafers in a proof-of-principle test of a quasi-optical varactor millimeter-wave frequency multiplier array concept. An equivalent circuit model based on a transmission-line analysis of plane wave illumination was applied to predict the array performance. The doubler experiments were performed under far-field illumination conditions. A second-harmonic conversion efficiency of 9.5% and output powers of 0.5 W were achieved at 66 GHz when the diode grid was pumped with a pulsed source at 33 GHz. This grid had 760 Schottky-barrier varactor diodes. The average series resistance was 27 Ω, the minimum capacitance was 18 fF at a reverse breakdown voltage of -3 V. The measurements indicate that the diode grid is a feasible device for generating watt-level powers at millimeter frequencies and that substantial improvement is possible by improving the diode breakdown voltage

Submillimeter-wave antennas on thin membranes

Submillimeter-wave antennas with bismuth microbolometer detectors have been fabricated on 1-μm thick silicon-oxynitride membranes. This approach results in better patterns than previous lens-coupled integrated circuit antennas, and eliminates the dielectric loss associated with the substrate lens. Measurements on a wideband log-periodic antenna at 700 GHz, 380 GHz and 167 GHz, and on a 700 GHz log-periodic imaging array, show no sidelobee and a 3-dB beamwidth between 40° and 50°. Also, the effective area can be increased by 5 dB by the use of a back-shorting mirror. Possible application areas are superconducting tunnel junction receivers for radio astronomy and imaging arrays for plasma diagnostics

Fabrication of the 0.346 THz BWO for Plasma Diagnostic

Nuclear fusion is probably the most demanding challenge the scientific community is facing. The plasma is a delicate material that has to be properly shaped to achieve a high efficiency fusion process. Unfortunately, the plasma is affected by micro-turbulences still not fully understood, detrimental for the reactor functioning. The diagnostic of plasma is a fundamental technique that needs advanced approaches for a full mapping of the plasma behavior. The 0.346 THz backward wave oscillator is the enabling devices for a high-k plasma diagnostic that will provide unprecedented insight on turbulences leading to full operational fusion reactors. This paper describes the final fabrication phase of the 0.346 THz BWO for plasma diagnostic jointly performed in an international project, involving three leading institutions in vacuum electronics

Fabrication of the 0.346 THz BWO for Plasma Diagnostic

Nuclear fusion is probably the most demanding challenge the scientific community is facing. The plasma is a delicate material that has to be properly shaped to achieve a high efficiency fusion process. Unfortunately, the plasma is affected by micro-turbulences still not fully understood, detrimental for the reactor functioning. The diagnostic of plasma is a fundamental technique that needs advanced approaches for a full mapping of the plasma behavior. The 0.346 THz backward wave oscillator is the enabling devices for a high-k plasma diagnostic that will provide unprecedented insight on turbulences leading to full operational fusion reactors. This paper describes the final fabrication phase of the 0.346 THz BWO for plasma diagnostic jointly performed in an international project, involving three leading institutions in vacuum electronics

THz backward-wave oscillators for plasma diagnostic in nuclear fusion

Understanding of the anomalous transport attributed to short-scale length microturbulence through collective scattering diagnostics is key to the development of nuclear fusion energy. Signals in the subterahertz (THz) range (0.1–0.8 THz) with adequate power are required to map wider wavenumber regions. The progress of a joint international effort devoted to the design and realization of novel backward-wave oscillators at 0.346 THz and above with output power in the 1 W range is reported herein. The novel sources possess desirable characteristics to replace the bulky, high maintenance, optically pumped far-infrared lasers so far utilized in this plasma collective scattering diagnostic. The formidable fabrication challenges are described. The future availability of the THz source here reported will have a significant impact in the field of THz applications both for scientific and industrial applications, to provide the output power at THz so far not available

THz Backward-wave oscillators for plasma diagnostic in nuclear fusion

Summary form only given. The understanding of plasma turbulence in nuclear fusion is related to the availability of powerful THz sources and the possibility to map wider plasma regions. A novel approach to realize compact THz sources to be implemented in the plasma diagnostic at NSTX experiment (Princeton Plasma Physics Laboratory, USA) is reported.Two novel 0.346 THz Backward-Wave Oscillators (BWOs) have been designed and are presently in the fabrication phase. One BWO is based on the Double Staggered Grating (DSG) that supports a sheet electron beam to provide a high output power; the second BWO is based on the Double Corrugated Waveguide (DCW) that supports a cylindrical electron beam generated by a conventional Pierce gun. The performance of both the BWOs was computed by Particle-in-cells (PIC) simulations. The DSG-BWO provides about 1W of output power with a beam current of 10 mA and a beam voltage of 16.8 kV. The DCW-BWO provides 0.74W output power with 10 mA beam current and 13 kV beam voltage. The DSG and the DCW have been realized by state of the art prototype nano-CNC milling machine (DMG Mori-Seiki) that permits one to achieve performance, in term of cost and surface finishing, unavailable with any other technology. It is the first time that this technique is applied to structures above 0.3 THz. The high output power of both the BWOs demonstrates the importance of novel approaches in the emerging field of THz vacuum electron devices