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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.

The fabrication of slow wave structures for THz vacuum electronic devices presents substantial technology challenges and requires tight tolerances as the frequency increases. The fabrication of the 0.346 THz backward wave oscillator for plasma diagnostics is in the final phase. It is based on the Double Corrugated Waveguide, fabricated by high accuracy CNC milling. In this paper, the variation of the height due to the bonding process, with respect to the nominal value and effect of the electron beam misalignment along the propagation direction will be discussed, for predicting the eventual impact on performance.

Nuclear fusion energy is perhaps one of the most demanding challenges the scientific community is facing. Unfortunately, the plasma is affected by micro-turbulence, which is still not fully understood, but which can degrade plasma confinement. The 0.346 THz backward wave oscillator is the enabling device for a high-k plasma collective scattering diagnostic that will provide unprecedented insight on turbulence thereby contributing to the realization of fully operational fusion reactors. This paper describes the final fabrication phase of the 0.346 THz backward wave oscillator for the collective scattering diagnostic jointly performed in an international project, involving three leading institutions in vacuum electronics. The advancements in technology will open the route to new families of THz vacuum electron devices to enable new THz applications and provide industry with new advanced processes.

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.

A collaboration between UC Davis, Lancaster University, Beijing Vacuum Electronics Research Institute (BVERI), and the University of Electronic Science and Technology of China (UESTC) is developing a 346 GHz backward wave oscillator (BWO) for use as a high frequency RF source in plasma fusion diagnostics, medical, biological, and security imaging systems. Development of new electron gun optics that achieve almost 100% beam transmission based on particle-in-cell simulations is presented along with comparisons of the cold and hot beam. Also, a CVD diamond pillbox window design is presented. Additionally, the mechanical layout of the devices is being completed and prepared for fabrication.

A new approach to realize THz BWOs relaxing the assembly challenge is presented. An international consortium including UC Davis, Beijing Vacuum Electronics Research Institute (BVERI), and Lancaster University is involved in the design and fabrication of 0.346 THz BWOs to replace the bulky FIR laser at the plasma diagnostic at the NSTX-U fusion device. The use of a highly energetic beam permit and a wide channel, double corrugated waveguide permit to achieve about 4 W of output power at 0.346 THz.

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.

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.

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.

Vacuum electron devices are the most promising solution to generate power at Watt level at millimeter waves and terahertz frequencies. The three dimensional nature of metal structures required to provide an effective interaction between an electron beam and THz signal poses relevant fabrication challenges. At the increase of the frequency, losses are a relevant detrimental effect on performance. In particular, the skin depth, in the order of one hundred nanometers or less, constrains the maximum surface roughness of the metal surfaces below those values. Microfabrication techniques were proved in principle to achieve values of surface roughness at nanoscale level, but the use of different metals and affordable microfabrication techniques requires a further investigation for a repeatable quality of the metal surfaces. This paper will discuss on the nanoscale issues of metal waveguides for a 0.346 THz backward wave tube oscillator and a 0.22 THz traveling wave tube.

A Backward Wave Oscillator (BWO) based on Double Corrugated Waveguide (DCW) is presented to operate at 0.346 THz. The dimensions of the DCW are defined to have high interaction impedance and low losses at a beam voltage of 13 kV. An efficient coupler with reduced length is designed to connect the DCW to WR-2 standard rectangular waveguide. Particle in Cell (PIC) simulations are performed using CST particle studio. The results show that the BWO can produce 500 mW cw output power.

Applications such as fusion diagnostics, imaging and security systems require high frequency sources. As part of a joint international effort regarding novel THz BWOs, a double staggered grating sheet beam BWO at 346 GHz is underdevelopment and being fabricated. Design work has been done on various components, with nano machining and cold testing of the slow wave structure completed.

The development of collective scattering diagnostics is essential for understanding of the anomalous transport attributed to short scale length microturbulence which poses a threat to the development of nuclear fusion reactors. Signals in the sub-THz range (0.1 – 0.8 THz) with adequate power are required to probe the plasma. A joint international effort is therefore devoted to the design and realization of novel backward wave oscillators at 0.346 THz and above with output power in the 1 Watt range to replace the bulky, high maintenance optically pumped FIR lasers so far utilized for this plasma diagnostic.

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.

Two backward wave oscillator (BWO) designs are proposed for a 346 GHz source for fusion plasma diagnostics applicable for the MAST and NSTX-U devices. Both designs feature a double grating machined from bulk copper, using nano-machining technology.

THz backward wave oscillators (BWOs) are the most promising solution to realize portable THz imaging system to be applied in artwork inspection. The new microfabrication techniques applied to purposely devised interaction structures permit to achieve performance unrivalled by any other technology in the frequency range 0.1 - 1 THz. BWOs with output power up to 1 Watt are designed and in fabrication phase.

The DURIP funds have been spent on several ultra high vacuum systems important to the advance of high power microwave generation. Novel oxide cathodes have been fabricated in our vacuum-arc plasma deposition chamber that produce high emission densities of 20 A/cm2 and display low work functions of only 1.6eV, which were made possible by the low vacuum pressures produced by the equipment purchased under the DURIP grant. Two Cusp electron guns have been delivered to UC Davis by Northrop Grumman. These state-of-the-art electron gunds will be employed to drive UC Davis' efficient high harmonic gyrotrons and peniotrons.