The development of electron cyclotron resonance heating (ECRH)-electron cyclotron current drive (ECCD) as a tool for suppression of plasma instabilities requires that the millimeter-wave beams used...

A New Launcher for Real-Time ECRH Experiments on FTU

To combine powers from a set of gyrotrons and to switch the combined wave beam between a number of output channels, various kinds of multiplexers can be used. Especially, narrow-band frequency diplexers in connection with small frequency-shift keying of gyrotrons can be used to switch the millimeter-wave power between two output channels. This technique can, for example, be used for fast beam steering for synchronous stabilization of rotating neoclassical tearing modes in tokamaks. Beam steering can be performed by a multistage multiplexer, provided that phase-controlled sources are available. In the paper, various concepts for fast directional switches as well as their integration into transmission lines are discussed. Calculations and low-power measurements of prototypes are presented. A resonant diplexer experiment is at present being prepared to be tested at high power in the electron cyclotron resonance heating (ECRH) system for Wendelstein 7X (W7-X). Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments are shown. Finally, future prospects for the application of diplexers in large ECRH systems are discussed.

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Fast switching and power combination of high-power electron cyclotron wave beams: principles, numerical results and experiments

A method was developed to calculate the reflection of microwaves at corrugated metallic surfaces along with the expected ohmic losses and phase shifts. The implemented FDTD algorithm treats the wall as a plasma with a cutoff frequency far above the microwave frequency. This results in a self-consistent solution of the currents together with the E/M fields. After describing the algorithm, comparisons with loss and phase measurements are shown and an optimized corrugation profile is suggested.

Numerical Calculation of Reflection Characteristics of Grooved Surfaces with a 2D FDTD Algorithm

Extension of steering angle in a square corrugated waveguide antenna

Strong anomalous spectra were systematically observed in collective Thomson scattering (CTS) at 140 GHz in the high-field tokamak FTU, a proof-of-principle experiment of CTS on thermal density fluctuations performed with propagation below electron cyclotron (EC) resonance with the final aim of demonstrating high-density CTS in the extraordinary mode. Following the results of two experimental campaigns expressly performed to investigate them, these spectra are ascribed to a gyrotron perturbation caused by a back-reflected signal originating in the beam injection port, where the electron cyclotron layer, the upper-hybrid layer and the right-handed cutoff layer unavoidably crossed by the probing beam are activated by a breakdown plasma sustained by the beam itself. The degree of generality ascribable to our results and the constraints they set on present and future CTS experiments with propagation below electron cyclotron resonance are discussed. A viable solution in terms of a transmit antenna robust against the risk of gyrotron perturbation is suggested.

Critical issues highlighted by collective Thomson scattering below electron cyclotron resonance in FTU

For the upper electron cyclotron wave launcher on the international thermonuclear experimental reactor (ITER), the use of a 'remote steering antenna' based on the imaging properties of square waveguides is planned. To characterize launchers of this type, low-power experiments on a four-side corrugated square waveguide, with a scanning mirror at the input of the waveguide, were performed in the frequency range 140–160 GHz.It is shown that elliptical polarization, which is necessary for the electron cyclotron current drive (ECCD), can be transmitted without depolarization and that the usable steering range of the antenna is at least 20°.Low-power measurements demonstrate that mitre bends can be integrated into the waveguide, practically without extra loss. Detailed calorimetric measurements for this set-up confirm this statement for the polarization perpendicular to the scanning plane, whereas excess loss is measured for the parallel polarization. Various modifications of mitre bends are investigated and results are discussed.

Performance of a remote steering antenna for ECRH/ECCD applications in ITER using a four-wall corrugated square waveguide

For the upper ECW launcher on ITER, the use of a “remote steering antenna” based on the imaging properties of rectangular waveguides is planned. To characterise launchers of this type, low-power experiments on a 4-side corrugated square waveguide with a scanning mirror at the input of the waveguide were performed in the frequency range of 140 to 160 GHz. It is shown, that elliptical polarisation needed for ECCD can be transmitted without depolarisation. The usable steering range of the antenna is at least -10” < cp 5 +lo”. Experiments with the scanning plane in the diagonal of the square waveguide led to a negligible increase the scanning range, and lobes in the upward and downward direction are detected at larger scanning angles. Mitre bends can be integrated into the waveguide without prohibitive extra loss, which is demonstrated by far-field measurements. Detailed calorimetric measurements for this set up confirm this statement for the polarisation perpendicular to the scanning plane, whereas extra loss is measured for the parallel polarisation. Results are discussed and are compared to theory.

Performance of a remote steering antenna for ECRH/ECCD applications using 4-Wall corrugated square wave-guide

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Performance of a remote steering antenna for ECRH/ECCD applications in ITER using a four-wall corrugated square waveguide

Performance of a remote steering antenna for ECRH/ECCD applications using 4-Wall corrugated square wave-guide