Category Archives: THz

Low-loss Transmission Lines for High-power Terahertz Radiation

Nanni, E., et al., Low-loss Transmission Lines for High-power Terahertz Radiation. J. Infrared Millim. Te., 2012: p. 1-20.

http://dx.doi.org/10.1007/s10762-012-9870-5

Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE 11 mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE 11 mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE 11 mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.

Transmission Line for 258 GHz Gyrotron DNP Spectrometry

Bogdashov, A., et al., Transmission Line for 258 GHz Gyrotron DNP Spectrometry. J. Infrared Millim. Te., 2011. 32(6): p. 823-837.

http://dx.doi.org/10.1007/s10762-011-9787-4

We describe the design and test results of the transmission line for liquid-state (LS) and solid-state (SS) DNP spectrometers with the second-harmonic 258.6 GHz gyrotron at the Institute of the Biophysical Chemistry Center of Goethe University (Frankfurt). The 13-meter line includes a mode converter, HE11 waveguides, 4 mitre bends, a variable polarizer-attenuator, directional couplers, a water-flow calorimeter and a mechanical switch. A microwave power of about 15 W was obtained in the pure HE11 mode at the spectrometer inputs.

Transmission Line for 258 GHz Gyrotron DNP Spectrometry

Bogdashov, A., et al., Transmission Line for 258 GHz Gyrotron DNP Spectrometry. J. Infrared Millim. Te., 2011. 32(6): p. 823-837.

http://dx.doi.org/10.1007/s10762-011-9787-4

We describe the design and test results of the transmission line for liquid-state (LS) and solid-state (SS) DNP spectrometers with the second-harmonic 258.6 GHz gyrotron at the Institute of the Biophysical Chemistry Center of Goethe University (Frankfurt). The 13-meter line includes a mode converter, HE11 waveguides, 4 mitre bends, a variable polarizer-attenuator, directional couplers, a water-flow calorimeter and a mechanical switch. A microwave power of about 15 W was obtained in the pure HE11 mode at the spectrometer inputs.

Stacked rings for terahertz wave-guiding

de Rijk, E.; Macor, A.; Hogge, J.; Alberti, S.; Ansermet, J. Review of Scientific Instruments 2011, 82, 066102.

http://dx.doi.org/10.1063/1.3597579

We demonstrate the construction of corrugated waveguides using stacked rings to propagate terahertz frequencies. The waveguide allows propagation of the same fundamental mode as an optical-fiber, namely, the HE11 mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications.

Stacked rings for terahertz wave-guiding

de Rijk, E.; Macor, A.; Hogge, J.; Alberti, S.; Ansermet, J. Review of Scientific Instruments 2011, 82, 066102.

http://dx.doi.org/10.1063/1.3597579

We demonstrate the construction of corrugated waveguides using stacked rings to propagate terahertz frequencies. The waveguide allows propagation of the same fundamental mode as an optical-fiber, namely, the HE11 mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications.

Linearly Polarized Modes of a Corrugated Waveguide

E.J. Kowalski et al., Linearly Polarized Modes of a Corrugated Waveguide, IEEE Trans. on Mic. Theo. and Tech., 2010, 58(11), 2772-2780

http://dx.doi.org/10.1109/TMTT.2010.2078972

A linearly polarized $({rm LP}_{mn})$ mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the ${rm LP}_{mn}$ modes and the conventional modes $({rm HE}_{mn},{rm EH}_{mn},{rm TE}_{0n},{rm TM}_{0n})$ of the corrugated guide is shown.

The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal $(m)$ indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.

Linearly Polarized Modes of a Corrugated Waveguide

E.J. Kowalski et al., Linearly Polarized Modes of a Corrugated Waveguide, IEEE Trans. on Mic. Theo. and Tech., 2010, 58(11), 2772-2780

http://dx.doi.org/10.1109/TMTT.2010.2078972

A linearly polarized $({rm LP}_{mn})$ mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the ${rm LP}_{mn}$ modes and the conventional modes $({rm HE}_{mn},{rm EH}_{mn},{rm TE}_{0n},{rm TM}_{0n})$ of the corrugated guide is shown.

The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal $(m)$ indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.

Amplification of Picosecond Pulses in a 140-GHz Gyrotron-TravelingWave Tube

H.J. Kim et al., Amplification of Picosecond Pulses in a 140-GHz Gyrotron Travelling Wave Tube, Phys. Rev. Lett., 105(13), 135101-135104

http://dx.doi.org/10.1103/PhysRevLett.105.135101

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro- TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE06 mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz.

Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.

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