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.

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.

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 20.9kV), 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.

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.

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.

An international consortium including the University of California, Davis (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 in a wide channel double corrugated waveguide has been found to improve the performance at the Watt level and facilitate the alignment and the assembly.

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.

Research conducted in parallel with the construction of various vacuum electronic devices, including a 220 GHz Sheet Beam Traveling Wave Tube (SBTWT), a 263 GHz SBTWT, and two 346 GHz Backward Wave Oscillators (BWOs), has demonstrated that data garnered from the manufacturing process helps improve the performance, reduce the time to build, and lower the cost of subsequent devices. The data collected from metrology, microscopy, and manufacturing control are also critical to improving future device builds and provide powerful insight into the nature of fabricating high performance vacuum electronics.

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.

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.

A 75-110GHz (W band) high-Q tunable band pass filter is demonstrated. The band pass filter has low insertion loss, a Q of approximately 200 and can be tuned across the entire W waveguide band. This filter has a variety of uses including rejecting spurious signals generated from non-linear frequency multiplication and improving signal to noise ratio at the front end of a receiver. The tuning is achieved by actuating a resonant cavity by using a sub-micron resolution stepper motor.

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 NSTX upgrade has provided the impetus to redesign and improve the High-k Scattering System. Improvements will include increased kθ coverage to target Electron Temperature Gradient (ETG) modes. Improved kθ resolution is realized by increasing the probe frequency from 280 to 693 GHz. Current solid state sources cannot deliver sufficient power at this frequency. Availability of current sources is limited to about 300 GHz and 30 mW; therefore, an optically pumped Far Infrared (FIR), formic acid laser has been chosen which can deliver upwards of 100 mW at 693 GHz. A second high frequency source is needed to provide local oscillator (LO) power for an array of quasi optical mixers. Additional lasers are not desirable since they will add complexity to the system. We propose to design and build a 346 GHz Backward Wave Oscillator (BWO) for use with subharmonic mixers. Simulations predict that average output power of 1 W is achievable. Circuit dimensions on the order of 100 microns, with submicron tolerances, make the fabrication of the slow wave structure especially challenging. UC Davis will use nano CNC machining technology to construct the slow wave structure. Additional work includes assessing the feasibility of scaling down this BWO for 600+ GHz operation and/or employing a frequency doubler for higher frequency applications.

Precision fabrication and RF testing of a 220 GHz GHz sheet beam TWTA based on the double vane half-period staggered slow wave structure design[1]is reported. NanoCNC Milling technology[2] was employed to precision fabricate the entire TWTA circuit in bulk copper, incorporating input and output couplers, sever ports, and the slow wave structure with matching. To accommodate an overhead input/output coupler design, the TWTA circuit was fabricated in a three layer process with dimensional tolerance of within ~1-2 μm and surface roughness ~50 nm. The TWTA circuit was diffusion bonded (at UC Davis) within an accuracy of less than 10 μm between two circuit halves. Initial TWTA cold tests employing a BWO based scalar network analyzer showed an in-band insert ion loss of ~ -5 dB with a bandwidth exceeding 50 GHz. The in-band return loss was <; -12 dB. These measurements were subsequently confirmed using an Agilent PNA-X VNA. The hot test setup is ready to test the TWTA being baked at CPI and the latest test results will be presented at the conference and added to the abstract.