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 H E 11 mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications. 12 achieving an ideal transmission line over distances orders of magnitude bigger than the propagated wavelength with a good power coupling to the source, low loss, and low dispersion remains challenging.Cylindrical corrugated waveguides are widely used to transmit frequencies of several tens of GHz.3, 13 These waveguides propagate hybrid electric (HE) modes, for which the lowest order H E 11 mode, as in fiber optics, 14 has a power loss of the order of 1% per 100 m, a low dispersion over bandwidth that can reach more than one octave, intrinsic filtering of spurious modes, and an efficient coupling (≈98%) to the TEM 00 "gaussian mode," which is the lowest order mode for free space propagation. 3, 15The corrugation geometry, Fig. 1(a), depends on the propagated wavelength λ and has dimensions and periodicity smaller than λ/2. 3,15,16 Up to now, the corrugation is achieved by conventional machining inside hollow tubes and reaches the current technological limits at several hundreds of GHz, over lengths of few cm and inner radii on the order of 1 cm. Beyond this limit, the period of the corrugations can no longer be lowered without seriously deteriorating the transmission performances.In this note we introduce the concept of stacked rings to manufacture H E 11 cylindrical corrugated waveguides with high mechanical accuracy that meet the geometrical requirements to propagate frequencies up to at least 7.5 THz over modules of tens of centimeters without mechanical limitations on the inner radii.Corrugations are built by alternately stacking two sets of metallic rings having the same external shape but different inner diameters and thickness inside a guiding pipe, Fig. 1. Rings are obtained from high precision laminated stainless steel sheets commonly available with thickness down to 10 ±1 μm, cut by electric discharge machining, guaranteeing a cut accuracy of ± 2 μm.Based on the idea of stacked rings, two 40 cm long sections of H E 11 corrugated waveguides were assembled, Fig. 2(a). By using 4000 rings with a thickness of 0.1 and 0.3 mm (d = 0.3 mm, D = 19.3 mm), a corrugation that fits the propagation of the H E 11 mode with a nominal frequency of 0.26 THz is achieved. This frequency corresponds to the electron resonance in state-of-the-art dynamic nuclear polarization enhanced nuclear magnetic resonance experiments 4 ongoing at the EPFL, which motivated this work.The robustness of the method has been tested by the realization of a 10 cm long section designed for the propagation of the H E 11 mode at 1.5 THz (p = 0.1 mm, d = 0.05, w = 0.05, D = 19.3). The prototypes have been built with auto-aligning connection flanges allowing a modular design, a vac...

A new model for simulating gyrotron oscillators in the monomode time-dependent linear self-consistent regime is presented. Starting from a nonlinear time-dependent monomode model, the linearization and the following simplification of the model, based on a moment approach, are described. This simplified model represents a numerically efficient model and allows to have a deeper physical insight, in particular, for regimes dominated by self-consistent effects such as for the gyro-backward wave instability. One specific case of a gyrotron cavity is studied in detail and compared with experimental results, with special attention to self-consistent effects and to the differences with a model using a fixed field profile. Self-consistent linear simulations are, amongst other applications, important for the design of frequency-tunable gyrotrons or high-power gyrotrons with cavities having a relatively low quality factor, but also for studies of parasitic oscillations as they may occur in beam ducts and/or in the launcher section following the interaction cavity.

An experimental study of a nanosecond pulsed regime in a THz gyrotron oscillator operating in a self-consistent Q-switch regime has been carried out. The gyrotron is operated in the TE(7,2) transverse mode radiating at a frequency of 260.5 GHz. The 5 W nanosecond pulses are obtained in a self-consistent Q-switch regime in which the cavity diffraction quality factor dynamically varies by nearly 2 orders of magnitude on a subnanosecond time scale via the nonlinear interaction of different mode-locked frequency-equidistant sidebands. The experimental results are in good agreement with numerical simulations performed with the TWANG code based on a slow time scale formulation of the self-consistent time-dependent nonlinear wave particle interaction equations.

An over-moded evacuated waveguide line was chosen for use in the transmission system for the proposed JET-enhanced performance project (JET-EP) electron cyclotron resonance heating (ECRH) system. A comparison between the quasi-optical, atmospheric waveguide and evacuated waveguide systems was performed for the project with a strong emphasis placed on the technical and financial aspects. The evacuated waveguide line was chosen as the optimal system in light of the above criteria. The system includes six lines of 63.5 mm waveguide for transmitting 6.0 MW(10 s) at 113.3 GHz from the gyrotrons to the launching antenna. The designed lines are on average 72 m in length and consist of nine mitre bends, for an estimated transmission efficiency of ∼90%. Each line is designed to include an evacuated switch leading to a calorimetric load, two dc breaks, two gate valves, one pumpout tee, a power monitor mitre bend and a double-disc CVD window near the torus. The location of waveguide support is positioned to minimize the power converted to higher-order modes from waveguide sagging and misalignment. The two gate valves and CVD window are designed to be used as tritium barriers at the torus and between the J1T and J1D buildings. The last leg of the waveguide leading to the torus has to be designed to accommodate the torus movement during disruptions and thermal cycles. All lines are also designed to be compatible with the ITER ECRH system operating at 170 GHz.

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