Hanyan Li

Nanoscale Surface Roughness Effects on THz Vacuum Electron Device Performance

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.

THz Backward-Wave Oscillators for Plasma Diagnostic in Nuclear Fusion

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.

Development of W-band Folded Waveguide Pulsed TWTs

W-band TWTs are key devices for high-frequency and high-power amplifiers used in high-resolution imaging applications. This paper presents the design, simulation, and fabrication of W-band pulsed TWTs. For higher power and broader bandwidth, TWTs have been developed using all-metal folded waveguide (FWG) slow-wave structures, diamond-disc windows, and horseshoe-shaped carbonized alumina attenuators. The test results show that W-band FWG pulsed TWTs can reach over 100 W output power with 6.7 GHz bandwidth and over 33 dB saturated gain at 20% duty cycle, while the maximum output power exceeds 200 W.

Progress of G band folded waveguide TWT

A folded waveguide traveling-wave tube (TWT) with PPM system operating at 220 GHz with 10 W of output power and 10 GHz bandwidth is being developed. This paper reports the status of fabrication and test.

Study on loss of folded waveguide structures of 220 GHz and 340 GHz

The loss of the folded waveguide structure plays a key role in the design of 220 GHz and 340 GHz TWTs. It can only be derived from experiments so far since both theory and empirical data are limited. Folded waveguide structures of 220 GHz and 340 GHz have been fabricated and tested. The experimental effective conductivity is 2.1-2.8×107 S/m at 220 GHz and 1.9-2.4×107 S/m at 340 GHz, respectively.

Development of G band folded waveguide TWTs

A compact folded waveguide traveling-wave tube (TWT) with PPM system operating at 220 GHz with 10 W of output power and 10 GHz bandwidth is being developed. This paper reports the test results of the latest prototype.

UV-LIGA microfabrication for high frequency structures of a Y-band TWT 2nd harmonic amplifier

In this paper, we fabricate copper high frequency structures for a Y-band(170-220GHz) TWT 2nd harmonic amplifier using UV LIGA technology. The two halves have been bonded and assembled in the amplifier. The measurement results of the amplifier show that over 100mW harmonic output power with most of the 11.4GHz of bandwidth at 1% duty cycle have been achieved, and the maximum harmonic output power reaches 500mW.

Investigation of high frequency vacuum devices using micro-fabrication

Investigation of high frequency vacuum devices using microfabrication is reported. TWT and BWO design and simulation is discussed. Design is based on theoretical analysis and equivalent circuit method.

I. The progress of 346GHz BWOs

346GHz backward wave oscillations (BWOs) have been considered as a key device for fusion plasma diagnostics, whose development is a joint project under collaboration among Beijing Vacuum Electronics Research Institute (BVERI), University of California Davis (UC Davis) and Lancaster University. In this paper, we report on some progress being made in the development of a 346GHz BWO.

Backward wave oscillator for high power generation at THz frequencies

The progress in microfabrication techniques and three dimensional electromagnetic simulations have enabled the fabrication of vacuum electron devices up to 1 THz. In particular, the backward wave oscillator is a compact and powerful THz vacuum source, based on the transfer of energy from an electron beam to an electromagnetic (EM) wave propagating in a slow wave structure. The paper reports the design and fabrication challenges to realize a near-THz Backward Wave Oscillator for plasma diagnostics in nuclear fusion. In particular, the beam optics and confinement as well as the slow wave structure are described. Manufacturing as well as device implementation and demonstration considerations are discussed. The double corrugated waveguide is used with a cylindrical electron beam producing an output power on the order of 1 W.