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Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because of oscillations encountered at higher voltages in this initial embodiment. Preliminary test at Vb=60 kV and Ib=3.7 A yielded 59 kW saturated output power at 92.2 GHz with 42 dB gain, 26.6% efficiency, and a 3 dB bandwidth of 1.2 GHz (1.3%).

In this paper, we describe micro-fabrication, RF measurements, and particle-in-cell (PIC) simulation modeling analysis of the 0.22 THz double-vane half period staggered traveling wave tube amplifier (TWTA) circuit. The TWTA slow wave structure comprised of two sections separated by two sever ports loaded by loss material, with integrated broadband input/output couplers. The micro-metallic structures were fabricated using nano-CNC milling and diffusion bonded in a three layer process. The 3D optical microscopy and SEM analysis showed that the fabrication error was within 2–3 μm and surface roughness was measured within 30–50 nm. The RF measurements were conducted with an Agilent PNA-X network analyzer employing WR5.1 T/R modules with a frequency range of 178-228 GHz. The in-band insertion loss (S21) for both the short section and long section (separated by a sever) was measured as ∼−5 dB while the return loss was generally around ∼−15 dB or better. The measurements matched well with the S-matrix simulation...

Previous theoretical and numerical studies [Y. M. Shin and L. R. Barnett, Appl. Phys. Lett. 92, 091501 (2008) and Y. M. Shin et al., Appl. Phys. Lett. 93, 221504 (2008)] have reported that a planar micro-channel with an asymmetric corrugation array supports strongly confined propagation of broadband THz plasmonic waves. The highly broad spectral response is experimentally demonstrated in the near-THz regime of 0.19–0.265 THz. Signal reflection and transmission tests on the three designed micro-channels including directional couplers resulted in a full-width-half-maximum bandwidth of ∼50-60 GHz with an insertion loss of approximately −5 dB, which is in good agreement with simulation data. These micro-structures can be utilized for free electron beam and electronic/optic integrated devices.

Particle-in-cell simulations of a staggered double grating array traveling wave tube intended as a wideband amplifier for terahertz communications, sensing, and imaging applications showed that, for an electron beam power of 5 kW, it produces 150–275 W, corresponding to 3%–5.5% electronic efficiency, at 0.22 THz with over ∼30% bandwidth and with greater than 12 dB/cm growth rate. The circuit has been fabricated by both UV lithography and high precision computer-numerical-control machining with ∼2–3 μm dimensional tolerance and ∼50 nm surface roughness. A scandate nanocomposite (Sc2O3–W) cathode for the electron beam source has successfully emitted 120 A/cm2 (space charge limited) at 1150 °C and 50 A/cm2 at 1050 °C for 8000 h as required to produce the requisite high current density electron beam.

The 0.22 THz vacuum electronic circuits fabricated by UV lithography molding and deep reactive ion etching processes are under investigation for submillimeter wave applications. Eigenmode transient simulations show that, accounting for realistic values of our currently achievable fabrication tolerances, the transmission, and dispersion properties of the operation modes of a TE-mode, staggered, double grating circuit are maintained within less than 1 dB and 2% deviation, respectively. Scanning electron microscopy and atomic force microscopy analyses of the fabricated circuit samples demonstrate that both of the microelectromechanical system fabrication approaches produce circuits with ±3–5 μm dimensional tolerance and ∼30 nm surface roughness.

Electron beam quality is essential to the performance of millimeter-wave gyroamplifiers, particularly the gyrotron traveling-wave tube amplifier, which is extremely sensitive to the electron velocity spread and emission uniformity. As one moves up in power and frequency, the quality of the electron beam becomes even more critical. One aspect of the electron beam formation technology which has received relatively little attention has been the performance analysis of the electron beam itself. In this study, a 100 kV, 8 A magnetron injection gun with a calculated perpendicular-to-parallel velocity ratio of 1.4 and axial velocity spread of 3.5% has been designed, tested, and analyzed. It is shown that the equipment precision and a fully relativistic data analysis model afford sufficient resolution to allow a verification of the theoretical predictions as well as a quantitative inference to the surface roughness of the cathode used.

In the course of research on high power terahertz vacuum electronic radiation sources, it was discovered that an intense plasmonic wave propagates through the channel between a half-period-staggered pair of TE-mode gratings. Experimental measurements were in good agreement with both a theoretical model and simulation analysis within 2% over the passband. This phase-shifted geometric modulation supports a dynamic bandwidth exceeding 25% with signal attenuation of less than 0.15dB∕cm (at Ka-band). Finite-difference time-domain analysis revealed that this low dispersion and low-loss optical response is ascribed to point-contact hopping motion of widespread resonant waveguide modes with the same phase velocity.

We report efficient wideband amplification of terahertz frequency electromagnetic waves from sequential electron-plasmon coupling along a half-period staggered double grating structure. Numerical eigenmode calculations show that the fundamental plasmonic band has a strong symmetric TE-mode, with a significant longitudinal electric field component, that is confined to the electron beam channel. A particle-in-cell simulation analysis with a single frequency excitation, combined with a short pulse broad spectrum drive, confirms that this terahertz plasmonic circuit produces three or four orders of magnitude power amplification with up to 30% bandwidth and 3%–5.5% saturated energy conversion efficiency.