Claudio Paoloni

Corrugated Rectangular Waveguide Tunable Backward Wave Oscillator for Terahertz Applications

A tunable backward wave oscillator (BWO) for terahertz applications is proposed. The use of a corrugated rectangular waveguide as the slow-wave structure permits relevant performance together with full compatibility with microfabrication technologies. The design, done using an analytical electromagnetic model, is fully verified by 3-D particle-in-cell simulations. A 20% tuning bandwidth is obtained at a central frequency of 1 THz, demonstrating more than 100-mW output power.

Double-Corrugated Rectangular Waveguide Slow-Wave Structure for Terahertz Vacuum Devices

A novel rectangular-corrugated waveguide is proposed for submillimeter and terahertz vacuum devices. Two parallel corrugations that are enclosed in a rectangular waveguide create a beam channel that supports an interaction with a cylindrical electron beam. A notable advantage of the double-corrugated rectangular waveguide slow-wave structure (SWS) is the extension of well-established cylindrical beam technology to corrugated waveguide SWSs. The structure is also fully realizable with the most recent microfabrication techniques and is easily assembled. A detailed study to describe the electromagnetic behavior of the presented SWS is performed by 3-D electromagnetic simulation. A 650-GHz backward-wave oscillator and a 227-GHz traveling-wave tube are designed and simulated, by 3-D particle-in-cell code, to highlight the great potential of the double-corrugated rectangular waveguide for submillimeter frequency vacuum devices.

Accurate analysis of helix slow-wave structures

In this paper, the helix slow wave structure (SWS) of traveling wave tubes (TWTs) has been analyzed. Dielectric supporting rods of arbitrary cross section have been considered in this analysis. The inhomogeneous dielectric loading factor has been accounted for by modeling the discrete support with a number of continuous dielectric tubes of appropriate effective dielectric permittivity. The helix tape model has been used for the field analysis. Furthermore the thickness of the helix tape has been considered. A rigorous solution of the field equations, including the contribution of the space harmonics, was performed to evaluate the phase velocity and the interaction impedance up to millimeter-wave frequencies. The nonuniformity of radial propagation constant over the structure cross section has been also included. With respect to other approaches, a closed-form expression of the field constants has been obtained. A study to choose the optimum number of space harmonics and dielectric tubes to be used in the analysis, has shown how the results are more sensitive to the number of space harmonics than to the number of dielectric tubes, beyond a certain number of the latter. The validity of this theory has been proved by comparison between measurements and simulations for helix SWS with different dimensions, rod shapes and operating frequency band.

Design and Realization Aspects of 1-THz Cascade Backward Wave Amplifier Based on Double Corrugated Waveguide

The design and fabrication challenges in the first ever attempt to realize a 1-THz vacuum tube amplifier are described. Implementation of innovative solutions including a slow-wave structure in the form of a double corrugated waveguide, lateral tapered input and output couplers, deep X-ray LIGA fabrication process, and a cascade architecture of the backward wave amplifier are discussed. New knowledge in the field of terahertz vacuum devices brought by intensive simulations and development of advanced fabrication and assembly processes of the micro-structures is highlighted.

Nanoscale Surface Roughness Effects on THz Vacuum Electron Device Performance

Vacuum electron devices are the most promising solution for the generation of watt-level power at millimeter wave and terahertz frequencies. However, the three-dimensional nature of metal structures required to provide an effective interaction between an electron beam and THz signal poses significant fabrication challenges. At increasing frequency, losses present a serious detrimental effect on performance. In particular, the skin depth, on the order of one hundred nanometers or less, constrains the maximum acceptable surface roughness of the metal surfaces to be below those values. Microfabrication techniques have proven, in principle, to achieve values of surface roughness at the nanometer scale; however, the use of different metals and affordable microfabrication techniques requires further investigation for a repeatable quality of the metal surfaces. This paper compares, for the first time, the nanoscale surface roughness of metal THz waveguides realized by the main microfabrication techniques. In particular, two significant examples are considered: a 0.346-THz backward wave tube oscillator and a 0.263-THz traveling wave tube.

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.

Double Corrugated Waveguide for G-Band Traveling Wave Tubes

A novel wide-band traveling wave tube (TWT) based on the double-corrugated waveguide (DCW) is proposed for operating in IEEE G-band (110-300 GHz). The DCW has been conceived for relaxing critical technological issues in terahertz vacuum tube realization, such as assembly, and to support a cylindrical electron beam. The study of the properties of the DCW in the forward wave regime demonstrates wide band performance suitable for TWTs fabrication at millimeter wave-terahertz frequencies. A traveling wave tube with 18-dB gain and up to 3.7 W output power over a bandwidth of about 30 GHz at 225 GHz as central frequency is demonstrated, assuming 13-kV beam voltage and 30-mA beam current.

Nano-CNC Machining of Sub-THz Vacuum Electron Devices

Nano-computer numerical control (CNC) machining technology is employed for the fabrication of sub-THz (100-1000 GHz) vacuum electron devices. Submicron feature tolerances and placement accuracy have been achieved and surface roughness of a few tens of nanometers has been demonstrated providing high-quality radio frequency (RF) transmission and reflection parameters on the tested circuit structures. Details of the manufacturing approach are reported for the following devices: W-band sheet beam (SB) klystron, two designs of a 220-GHz SB double-staggered grating traveling wave tube (TWT), 263-GHz SB TWT amplifier for an electron paramagnetic resonance spectrometer, 346-GHz SB backward wave oscillator for fusion plasma diagnostics, 346-GHz pencil beam backward wave oscillator, and 270-GHz pencil beam folded waveguide TWT self-driving amplifier. Application of the nano-CNC machining to nanocomposite scandate tungsten cathodes as well as to passive RF components is also discussed.

Analytical Design Method for Corrugated Rectangular Waveguide SWS THz Vacuum Tubes

The corrugated rectangular waveguide is particularly suitable for microfabrication of slow-wave structures for vacuum amplifiers in THz frequency range. An analytical model for the corrugated rectangular waveguide has been extended to compute the cold parameters (dispersion and interaction impedance). A validation procedure demonstrates the validity of the model in THz frequency range. A study on the positioning of the electron beam in the slow-wave structure, based on the interaction impedance evaluation has been carried out. Monte Carlo analysis of a corrugated rectangular waveguide traveling wave tube (TWT), performed by the use of the proposed model demonstrates its effectiveness in the design at THz frequencies, without the need of 3-D simulators in this design phase.

Photonic Crystal-Structures for THz Vacuum Electron Devices

The technology of photonic crystals (PhCs) is investigated here to improve the performance of THz vacuum electron devices. Compared with conventional metallic waveguides, the PhC arrangement alleviates typical issues in THz vacuum electron tubes, i.e. difficult vacuum pumping process and assembling, and improves the input/output coupling. A slow-wave structure (SWS) based on a corrugated waveguide assisted by PhC lateral walls and the efficient design of a PhC coupler for sheet-beam interaction devices are demonstrated. Based on the proposed technology, a backward-wave oscillator (BWO) is designed in this paper. Cold parameters of the novel PhC SWS as well as 3-D particle-in-cell simulations of the overall BWO are investigated, obtaining more than 70-mW-peak output power at 0.650 THz for beam voltage of 11 kV and beam current of 6 mA.