Micro-fabricable terahertz sheet beam amplifier integrated with broadband metamaterial circuit

Abstract

A compact, high power terahertz (THz) sheet beam traveling wave tube (TWT) amplifier is being developed for communication, sensing, and imaging applications. This research is aimed at realizing MEMS fabrication/integration of a CW coherent radiation source with 1000 W-GHz power-bandwidth product. In order to achieve this goal, we investigate (1) electric negative index metamaterial (ENG, surface plasmon) embedded electronic circuit for broad spectral coverage, (2) on-chip MEMS fabrication/ integration techniques, (3) tungsten scandate nano-composite cathode for high current density beam loading, and (4) electron gun and magnet system for high aspect ratio sheet beam transport. Relevant to broadband circuit design, particle-in-cell (PIC) simulations showed that an ENG-circuit, comprised of a staggered double grating array interacting with a 20 keV and 0.25 A sheet beam, produces 150 to 275 W, corresponding to 3 - 5.5 % efficiency, at 0.22 THz over ~ 30 % bandwidth with greater than 12 dB/cm growth rate. The circuit characterization experiment at Ka-band agrees well with both a theoretical model and simulation analysis in dispersion and attenuation plots, showing a 25 % dynamic bandwidth and 0.15 dB/cm attenuation. UV LIGA and high precision CNC machining on the 0.22 THz circuit fabrication have accomplished ~ 2 - 3 μm dimensional tolerance and ~ 30 - 50 nm surface roughness. Its preliminary cold-test showed -3.1 dB signal transmission at 0.22 THz. Currently, we are preparing for another test to obtain a flat band response. A scandate nano-composite (Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -W) cathode, fabricated by cryogenic milling and sol-gel process, has successfully emitted 115 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (space charge limited) at 1150 °C and 50 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 1050 °C for 6,000 hrs as required to produce the requisite high current density electron beam. It is currently being attempted to be incorporated in the sheet beam electron gun by high precision machining. We also succeeded in beam-tests of a 25 : 1 sheet beam, the world-highest aspect ratio, with ~ 40 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cathode loading, which leads to 0.25 A, corresponding to 6 : 1 compression rate, in the beam tunnel. We plan to transport the beam over the 2 cm long interaction circuit with high transmission by systematic design of a permanent magnet stack.