Low-Loss Transmission Lines for High-Power Terahertz Radiation

Abstract

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.

Fig. 1

Illustration of a corrugated metallic waveguide which is optimized with a radius a ≫ λ, groove depth d ≈ λ/4, period p ≈ λ/3 and groove width w < p/2.

Fig. 2

(a) The 250 GHz transmission line connecting the gyrotron output (left) to the NMR spectrometer (right). (b) The measured Gaussian beam emitted from the end of an 8 mm corrugated waveguide and radiated to a distance of 12 mm from the waveguide end.

Fig. 3

(a)The 460 GHz transmission line connecting the gyrotron output (left) to the NMR spectrometer (right). (b) The measured Gaussian beam emitted from the end of a 19 mm ID corrugated waveguide and radiated to a distance of 10 cm from the waveguide end.

Fig. 4

Ohmic losses in overmoded circular corrugated waveguide for rectangular brass (conductivity 1.56 × 10⁷ mhos/m) corrugations as predicted by our theory (solid line) and the impedance approximation method (dashed line). Numerical simulations with HFSS are shown with blue dots.

Fig. 5

The magnitude of the electric field from HFSS simulations for one period of a rectangular corrugation with a = 9.5 mm, d = 0.227 mm, p = 0.3175 mm, t/p = 0.5 and ideal brass conductivity at (a) 105 GHz, (b) 158 GHz, (c) 250 GHz and (d) 330 GHz.

Fig. 6

(a)Groove shapes simulated in HFSS with d = 0.227 mm and p = 0.3175 mm given in the groove coordinate system (ξ, z) used in Fig. 1. (b) Ohmic losses in corrugated waveguide simulated in HFSS compared to the impedance approximation method.

Fig. 7

(a) Photos of the 330 GHz tap and waveguide cross sections. Chips from cutting are present on the second tap tooth from left. (b) Theoretical prediction of power loss due to axial offset (black line) and tilts (dashed and dashed/dotted lines) at waveguide junctions for the 19 mm diameter 330 GHz corrugated waveguide.

Fig. 8

(a) VNA setup for 330 GHz waveguide transmission line loss measurements. (b) Schematic of the setup for transmission line losses with removable short or load.

Fig. 9

Predicted (black line) and measured (red dot) signal from the VNA as a function of waveguide length.

Fig. 10

(a) Measured transmission through 19 mm ID corrugated waveguide at 250 GHz as a function of waveguide length. (b) HE₁₁ waveguide transmission loss measurements (blue dot: radiometer, black square: vector network analyzer) and HFSS model (red triangle and line: trapezoidal groove) for ideal brass conductivity and a = 9.5 mm.