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A dedicated optical system is designed for the breakdown of the first ITER plasma, when the machine will not be equipped with the equatorial ECRH-launcher. Seven beams from the upper launcher will be transmitted via several in-vessel mirrors through the resonance layer to a beam-dump. The whole system was simulated with the PROFUSION package to obtain the beam parameters, estimate spill-over effects and determine the required polarization in the launcher in order to have X-mode polarization in the resonance.

Reflecting gratings have been installed in the vacuum vessel of ASDEX Upgrade for all beamlines of the electron cyclotron resonance heating system. Potentially unabsorbed millimetre wave power after the first pass through the plasma is redirected towards the plasma centre. This increases the efficiency of heating schemes with reduced single pass absorption like O-2 or X-3. In order to monitor beam position and power, thermocouples were installed into the gratings. A numerical model was developed to evaluate the beam intensity during short pulses from the thermocouple measurement in a non-stationary environment. An experiment was carried out, where only the X-3 resonance is present in the plasma, and the millimetre wave beam shine-through was measured successfully as a function of the central plasma electron temperature. This allows to deduce the X-3 absorption experimentally. Scanning the launching angles, it seems possible to measure the 2D beam cross section after the first pass through the plasma.

High power mm waves for fusion plasma heating need to be elliptically polarised to ensure good absorption in the plasma. In some scenarios, electron cyclotron resonance heating (ECRH) at higher harmonics (X3 and O2) is used, but this has significant shine-through because of low single pass absorption. Grating reflectors at the inboard strike point form a holographic mirror that reflects the beam back into the plasma. This paper investigates the optical properties and ohmic losses of both the polariser and the reflectors with the 3D fullwave code IPF-FD3D. The reflection properties of a reflector for ASDEX Upgrade and the improved ohmic losses of a waveguide polariser were confirmed.

Transmission of high-power millimeter waves for ECRH is often realised with oversized corrugated circular waveguides. Coupling from the gyrotron source to the waveguide is typically done via matching mirrors in free space. Small alignment errors of the system lead to the excitation of higher-order modes inside the waveguide beside the main transmission mode HE11. Those modes have comparably higher losses and can in worst case result in local fields exceeding the breakdown limit of the medium inside the waveguide. For alignment control over the whole pulse duration of the gyrotron, a set of hole-array couplers placed into a miter bend mirror probes the field inside the waveguide. The arrays are designed to detect the marker modes for beam offset and tilt (LP(e=o)11 )as well as for beam waist mismatch (LP02). In addition, a main mode coupler sensitive mostly for the HE11 content is used as a power monitor. By maximizing the signal of the power monitor and minimizing the content of marker modes, a first-order optimization of the coupling from free space to the waveguide can be achieved. Signal processing of the 140 GHz information is done at kHz range after downmixing, using a frequency shifted part of the power monitor signal. As the measurement system is placed in a miter bend mirror, it can also be easily installed at various locations along the transmission line to check for possible misalignments of the waveguide connections between miter bends. Simulation and low power experimental results will be shown.

The electron cyclotron resonance heating system at ASDEX Upgrade (AUG) is currently being extended to eight similar Gyrotrons in total. Each Gyrotron operates at 105 and 140 GHz and is designed for up to 1 MW millimetre wave output power. A substantial part of the AUG program will focus on experimental conditions, where the plasma density may be above the X-2 cut-off density at 140 GHz. In order to cope with the high density, the heating system will operate in the O-2 mode scheme with potentially incomplete absorption in the first pass. Reflecting gratings installed into the heat shield on AUG’s inner column allow for a controlled second pass of the beam’s unabsorbed fraction. Thermocouple measurements serve to control the beam position on the grating. The beam geometry is being finalized for the launchers #1-4. Beam propagation is simulated with the TORBEAM code and previous high density experiments are used as a database. The geometry is optimized using three criteria: central deposition, high absorption and robustness of the beam dump after the second pass. The experimental conditions, and the plasma electron density in particular, may vary such that the Gaussian beam parameters of the incoming beam on the grating deviate from the design values. It is proposed to model the effect of the grating with an equivalent ellipsoidal mirror. Laboratory measurements are shown, which support this model.

The ECE diagnostic at W7-X in its standard mode of operation measures in X2 mode polarization with a 32 channel radiometer in the frequency band around 140 GHz for central magnetic field 2.5T. The radiometer is calibrated by a noise source and the overall system absolutely calibrated by means of a hot-cold source placed outside the torus in front of a Gaussian telescope optics with identical geometry and transmission line as it is installed for the measurements in the plasma vessel. The system is supplemented with a 16 channel zoom device with 4 GHz span for higher frequency resolution at a suitable radial range and a Michelson interferometer for the characterization of higher harmonics sharing the same line of sight.

An existing framework for the design of reflection gratings was reworked. It takes the astigmatic complex beam parameters and the orientations of the beam axes of the incident and reflected beams as input and synthesizes a grating, which transforms the incident beam into the reflected beam. This is done by decomposing the 3D problem into a series of 2D reflections of plane waves. The 2D grating profiles are optimized in parallel on multiple computers. Finally, the 3D grating is derived using a simplified interpolation scheme.

The contributions on technology are summarized.

The ASDEX Upgrade electron cyclotron resonance heating operates at 105 GHz and 140 GHz with flexible launching geometry and polarization. In 2016 four Gyrotrons with 10 sec pulse length and output power close to 1 MW per unit were available. The system is presently being extended to eight similar units in total. High heating power and high plasma density operation will be a part of the future ASDEX Upgrade experiment program. For the electron cyclotron resonance heating, an O-2 mode scheme is proposed, which is compatible with the expected high plasma densities. It may, however, suffer from incomplete single-pass absorption. The situation can be improved significantly by installing holographic mirrors on the inner column, which allow for a second pass of the unabsorbed fraction of the millimetre wave beam. Since the beam path in the plasma is subject to refraction, the beam position on the holographic mirror has to be controlled. Thermocouples built into the mirror surface are used for this purpose. As a protective measure, the tiles of the heat shield on the inner column were modified in order to increase the shielding against unabsorbed millimetre wave power.