Terahertz Traveling Wave Tube Research Articles

Investigation of Ridge-Loaded Folded Rectangular Groove Waveguide Slow-Wave Structure for High-Power Terahertz TWT

A novel slow-wave structure (SWS) called ridge-loaded folded rectangular groove waveguide has been proposed for developing the wideband high-power terahertz (THz) traveling-wave tube (TWT). A new kind of attenuator suitable for this kind of SWS has also been proposed, which can successfully suppress the self-excited oscillations and keep the harmonics to a very low level. An input and output waveguide transition structure which is appropriate for this new kind of SWS is put forward. It can be found from the simulation results that the ridge-loaded SWS can enhance the electric field intensity in the electron beam tunnel so that the average interaction impedance of this ridge-loaded folded rectangular groove waveguide SWS at 0.34 THz is 30.1% higher than that of the folded rectangular groove waveguide SWS. Moreover, the particle-in-cell simulation results reveal that with the sheet electron beam parameters of 27 kV and the 250 mA, the average output power, the gain, and the electronic efficiency of the ridge-loaded folded rectangular groove waveguide TWT at the frequency of 0.34 THz can reach 39 W, 26.99 dB, and 0.59%, respectively. Therefore, the ridge-loaded folded rectangular groove waveguide SWS should be considered as a promising slow-wave circuit for high-power wideband THz radiation source.

Features of the Development of Electron-Optical Systems for Pulsed Terahertz Traveling-Wave Tubes (Review)

We present an analysis of the current state and development of pulsed amplifying traveling-wave tubes for terahertz radiation operation at frequencies of at least 200 GHz, as well as the development prospects of the principles of creating electron-optical and magnetic systems. The possibility of using field emission cathodes based on carbon nanotubes for constructing an electron-optical system with the compression of a sheet beam is discussed. A numerical simulation of a field emission electron gun forming a sheet electron beam for traveling-wave tubes of the terahertz range is carried out.

Development of an extremely thin-diamond window for terahertz traveling wave tubes

This paper describes the design and fabrication of an extremely thin-diamond window for terahertz traveling wave tubes (THz TWTs). To reinforce the mechanical properties of the micro-crystalline diamond (MCD) incorporated with ultra-nano-crystalline diamond (UNCD), a seven-layered composite diamond having a total thickness of 60μm has been fabricated and studied. SEM studies showed that UNCD growth on MCD resulted in surface roughness improvement via smoothing sharp corners and straight edges as well as filling in valleys of MCD grains. Further comparison studies of the same thickness samples demonstrated that the roughness for the composites were ~25–50% lower than those of the MDCs, depending on the number of UNCD layers. More importantly, seams or gaps resulting from irregularly shaped MCD grains were found to be terminated by UNCD growth, which is believed to attribute to the lateral growth mechanism of UNCD. The seven-layered composite diamond film was cut into window disks without lapping and polishing, which were then made into window assemblies. The mechanical testing of the window assembly showed no occurrence of window cracking or other damages due to the mechanical stress from pressure difference, while leak tests found no apparent air leakage resulting from the composite material. These testing results show such a thin composite diamond possesses adequate mechanical properties for the application of THz wave TWT windows. This work provides a novel approach for the fabrication of extremely thin-diamond windows for THz TWTs.

Improved Design and Microfabrication of ${H}$ -Plane and ${E}$ -Plane Loaded Rectangular Slow-Wave Structure for THz TWT Amplifier

This paper reports further investigation on a recently proposed ${H}$ -plane and ${E}$ -plane loaded slow-wave structure (SWS) for terahertz traveling-wave tube (TWT) amplifier. An improved input–output coupler is designed to enable easy fabrication by microfabrication technology, UV-lithography, electroplating, and molding (LIGA), and deep reactive-ion etching. The coupler shows very low reflection coefficient, ${S}_{11} dB over a frequency range 360–450 GHz. To improve the beam-wave interaction and enhance the saturated output power of the TWT, the SWS with tapered design is implemented. By tuning the period, the wave is resynchronized with the beam at the end of the SWS, resulting in more than 60% increase in the saturated output power across the 80-GHz bandwidth. In addition, the sensitivity of the output power of the TWT to the fabrication tolerance of the individual geometrical parameters is also studied in detail. It is found that the output power reduces by 80% for approximately 2% variation in the synchronized beam voltage. The KMPR-based UV-LIGA technique is adopted to fabricate the ${H}$ -plane and ${E}$ -plane loaded SWS. The measured lateral dimensional accuracy within $\sim 2~\mu \text{m}$ and rms roughness of metal sidewall surface less than 80 nm is obtained.

A Ridge-Loaded Sine Waveguide for $G$ -Band Traveling-Wave Tube

A novel slow-wave structure (SWS), named ridge-loaded sine waveguide (RLSWG), has been proposed to develop the wideband high-power terahertz traveling-wave tube (TWT). The slow-wave characteristics of the RLSWG SWS, including dispersion properties and interaction impedance, are analyzed by using the 3-D electromagnetic simulation software Ansoft high frequency structure simulator (HFSS). From our calculation, the average interaction impedance of the RLSWG SWS at 0.22 THz is 42.2% higher than the conventional SWG SWS. Meanwhile, the simulation results demonstrate that the RLSWG SWS possesses low ohmic losses and reflection. Moreover, the particle-in-cell (PIC) simulation results reveal that, with the cylindrical electron beam of 20.9 kV and 45 mA, the output power and electron efficiency of the RLSWG TWT at the typical frequency of 0.22 THz can reach 52.1 W and 5.54%, respectively. In addition, the 3-dB bandwidth of the RLSWG TWT exceeds 25 GHz. Compared with the SWG TWT, the RLSWG TWT has the shorter tube length and can generate the larger output power.

Experimental investigation of electron guns for THz microwave vacuum amplifiers

Single-sheet and multiple beam electron emitters based on thermionic minicathodes for terahertz traveling-wave tubes have been studied. Data are presented for impregnated blade thermionic cathode with dimensions 0.1 × 0.7 mm and a maximum current density of 114 A/cm2 in a pulsed mode. A variant of the five-beam electron gun with 0.25-mm-diameter cylindrical minicathodes in cells of a control grid is proposed that provides a current density of 85.5 A/cm2 at a grid potential of 900–1000 V.

Design Sheet Beam Gun for THz Traveling-Wave Tube

The Sheet Electron Beam Vacuum Electron Device is an Attractive Choice for Generating High Power Millimeter/terahertz Wave Radiation. the Sheet Electron Beam Gun is a Key Component for the Sheet Beam Vacuum Electron Device. in this Paper, a Novel Sheet Electron Beam Gun was Proposed for a Terahertz Traveling-Wave Tube. the Theories of Sheet Beam Gun are Deduced Based on the Round Beam Gun Theories. the Track of 24.5kV, 1A, 0.4mm8mm Sheet Beam is Gained through 3D Particle-in-Cell Simulation and the Theories are Verified. the Investigation Results Show that, the Design Method of the Sheet Beam Gun is Easy and Reliable.

Demonstration of an Acid-Spun Single-Walled Nanotube Fiber Cathode

Field emission dc cold cathodes continue as an important area of research for uses such as electron microscopy, novel X-ray sources, vacuum electronic devices, terahertz sources, and high-power microwave tubes. Each of these applications typically requires high current densities with high-brightness electron beams driven by cathodes exhibiting long lifetime in the presence of deleterious conditions such as ion back bombardment and excessive heating. The Air Force Research Laboratory (AFRL) now investigates cathodes operating in dc mode for use in a terahertz traveling wave tube (TWT). The TWT requires an electron beam of 50 μm in diameter or less, at 10s of kiloelectronvolt energy with energy spreads of less than 10 eV. While AFRL has tested numerous cathodes in this regime, this paper reports on the first demonstration of a dc cathode utilizing a highly aligned carbon nanotube (CNT) rope for the electron emitter. The rope consists of individual single-walled CNTs that have been subjected to a nitrogen-enhanced acid etch and then spun into a rope configuration. Thus, the single rope emitter has an overall diameter of 100 m and a length of 1.5 mm. We report on preliminary results from this cathode, in particular the fabrication of the cathode, the dc cathode test system, and the cathode operation up to a voltage of 5 kV. The cathode operates stably to within 0.6% with a 5-mm anode-cathode gap at 5 keV and 1.0-mA current for hundreds of hours. Finally, we provide estimates of the cathode parameters such as the effective field enhancement factor (βeff) and emitting area (A) through a Fowler-Nordheim plot and comparison of the experimental data with simulations utilizing the particle-in-cell code Improved Concurrent Electromagnetic Particle-in-Cell.

Full-Scale Three-Dimensional Electromagnetic Simulations of a Terahertz Folded-Waveguide Traveling-Wave Tube Using ICEPIC

This paper discusses simulation and modeling of the slow wave structure of a folded-waveguide terahertz traveling wave tube (TWT) using the Improved Concurrent Electromagnetic Particle In Cell (ICEPIC) software. This is the first time ICEPIC has been used for simulation of a TWT amplifier. Cold test simulations compare favorably with analytical models; at 368 GHz, the on-axis interaction impedance is 7.8 Ω. Hot test (beam included) ICEPIC simulations were used to determine the effects of space charge on the gain calculations. At 368 GHz, the normalized beam plasma frequency from ICEPIC simulations is Ωp = 0.56. Analysis of our ICEPIC simulations at 368 GHz indicates a normalized beam plasma frequency 75% larger than an analytical model we improvised from a sheath helix model taken from the literature. The hot test ICEPIC simulations at 368 GHz for a 64 periods long slow wave structure and a 10 mA, 25 kV electron beam produce small signal gain of 27 dB. The small-signal fractional 3-dB bandwidth of the TWT is 2.9%. The saturated fractional 3-dB bandwidth is 3.2%. Large signal simulations indicate that the saturated power at 368 GHz is 39.4 dBm and the saturated gain is 21.8 dB. A snapshot of a cross section of the electron beam shows bunching in space and a corresponding modulation in the velocity of the electron beam.