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Understanding of the anomalous transport 1 attributed to short-scale length microturbulence through 2 collective scattering diagnostics is key to the development of 3 nuclear fusion energy. Signals in the subterahertz (THz) range 4 (0.1-0.8 THz) with adequate power are required to map wider 5 wavenumber regions. The progress of a joint international effort 6 devoted to the design and realization of novel backward-wave 7 oscillators at 0.346 THz and above with output power in the 1 W 8 range is reported herein. The novel sources possess desirable 9 characteristics to replace the bulky, high maintenance, optically 10 pumped far-infrared lasers so far utilized in this plasma 11 collective scattering diagnostic. The formidable fabrication 12 challenges are described. The future availability of the THz 13 source here reported will have a significant impact in the field of 14 THz applications both for scientific and industrial applications, 15 to provide the output power at THz so far not available. AQ:1 AQ:2 AQ:3 16 Index Terms-Backward-wave oscillator (BWO), double-17 corrugated waveguide (DCW), double-staggered grating (DSG), 18 plasma diagnostic, terahertz (THz).19

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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.

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