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, plasma diagnostics, healthcare and many others will gain substantial features if high power at millimeter waves 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 manufacturing challenges due to tight tolerances. 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 device, 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.

We report on hot test measurements of a wide-bandwidth, 220-GHz sheet beam traveling wave tube amplifier developed under the Defense advanced research projects agency (DARPA) HiFIVE program. Nano-computer numerical control (CNC) milling techniques were employed for the precision fabrication of double vane, half-period staggered interaction structures achieving submicrometer tolerances and nanoscale surface roughness. A multilayer diffusion bonding technique was implemented to complete the structure demonstrating wide bandwidth (>50 GHz) with an insertion loss of about -5 dB achieved during transmission measurements of the circuit. The sheet beam electron gun utilized nanocomposite scandate tungsten cathodes that provided over 438-A/cm ² current density in the 12.5:1 ratio sheet beam. An InP HBT-based monolithic microwave integrated circuit preamplifier was employed for TWT gain measurements in the stable amplifier operation region. In the wide-bandwidth operation mode (for gun voltage of 20.9 kV), a gain of over 24 dB was measured over the frequency range of 207-221 GHz. In the high-gain operation mode (for gun voltage of 21.8 kV), over 30 dB of gain was measured over the frequency range of 197-202 GHz. High-power tests were conducted employing an extended interaction klystron.

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.

The efficient transport of high current density electron beams is essential for the success of millimeter wave and terahertz vacuum devices. To achieve high current density sheet electron beam transmission, an advanced periodic cusped magnet-tunable quadrupolar magnet (PCM-TQM) is proposed to improve the transmission factor of the sheet beam. Through numerical a nalysis, the TQM is shown to provide the ability to be compatible with a larger initial transverse velocity spread and match the current density increasing caused by beam bunching. This new PCM-TQM focusing system is employed to transport a 19 kV, 0.15 A, ~8.5:1 aspect ratio sheet beam with an elliptical cross section of 0.60 mm (width) × 0.07 mm (height), whose current density exceeds 400 A/cm ² , through a 45-mm long circuit, whose beam tunnel is 0.7 mm × 0.12 mm. CST PARTICLE STUDIO is used to model the beam transport system and verify the analytical results. With the tunable feature of the QM poles, the “no RF” and “with RF” beam transmission factors under the focusing magnetic field of the advanced PCM-TQM are predicted to achieve 99.3% and 97.6%, respectively.

The fabrication of high purity copper using additive manufacturing has proven difficult because of oxidation of the powder feedstock. Here, we present work on the hydrogen heat treatment of copper powders for electron beam powder bed fusion (EB-PBF), in order to enable the fabrication of high purity copper components for applications such as accelerator components and vacuum electronic devices. Copper powder with varying initial oxygen contents were hydrogen heat-treated and characterized for their chemistry, morphology, and microstructure. Higher initial oxygen content powders were found to not only reduce surface oxides, but also reduce oxides along the grain boundaries and form trapped H₂O vapor inside the particles. The trapped H₂O vapor was verified by thermogravimetric analysis (TGA) and residual gas analysis (RGA) while melting. The mechanism of the H₂O vapor escaping the particles was determined by in-situ SEM heated stage experiments, where the particles were observed to crack along the grain boundaries. To determine the effect of the EB-PBF processing on the H₂O vapor, the thermal simulation and the validation of single melt track width wafers were conducted along with melting single layer discs for chemistry analysis. A high speed video of the EB-PBF melting was performed in order to determine the effect of the trapped H₂O vapor on the melt pool. Finally, solid samples were fabricated from hydrogen-treated copper powder, where the final oxygen content measured ~50 wt. ppm, with a minimal residue hydrogen content, indicating the complete removal of trapped H₂O vapor from the solid parts.

Development of high frequency, high-average power linear beam devices is hindered by the slow progress in advancement of suitable mechanical design and manufacturing techniques. To compliment the significant progress in electromagnetic modeling and design of high-frequency electronics, the mechanical design and manufacturing approach employed for the development of a 10 kW continuous wave W-band sheet beam klystron is reported. The reported design enabled demonstration of 10-kW, 20-ms duration pulses with 17-dB gain and 97% (hot) beam transmission. This high-average power design was based on the demonstration of a 56 kW short pulse (1 μs) device with 54 dB of gain. The high-average power design features a multilayer strengthened material structure with hybrid microcooling and macrocooling circuits for enhanced heat flux handling. Microcooling channels fit between circuit cavities providing heat removal as well as thermal-stress relief for reduced cavity deformation. The design achieves a 1.5-safety factor on yield and less than 2-μm deformation at the cavities. The presented design and manufacturing approach can be applied to the development of other high-average power high-frequency linear beam devices.