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In quasi-optical transmission systems, it is usually required to transform a certain Gaussian beam into another one characterized by a different set of parameters. Mirrors with quadratic surface contour functions, such as paraboloidal, ellipsoidal, and toroidal mirrors, are widely used for such a transformation. However, quadratic surface contours do not provide sufficient conversion efficiencies needed for the transformation of off-axis Gaussian beams into desired Gaussian millimeter-wave field distributions. Therefore, it is necessary to develop a new method for the transformation of off-axis Gaussian beams. This paper presents such a method for the synthesis of beam-shaping mirrors for off-axis Gaussian beam transformation. Based on this new method, a matching optics unit consisting of two mirrors has been designed to transform a Gaussian-mode-like field distribution into a Gaussian beam with desired parameters.

Bremsstrahlung X-rays produced at the collector can be used for definition of electron energy distributions in gyrotrons and maybe in other high-power microwave devices. Such a possibility underwent early testing at the Karlsruhe Institute of Technology on two gyrotrons. Electron energy distributions calculated on the basis of X-ray spectra measured near the gyrotrons operated at different regimes have plausible shapes and demonstrate good correlation with expected characteristics of the electron beam, thus confirming the viability of the proposed diagnostic technique.

In this paper, we present the results of an experimental study on the influence of the lateral misalignment between the axes of the annular electron beam and the cavity on the performance of a 1-MW 140-GHz gyrotron. The subject of the study is of practical interest. On one hand, the discrepancy between numerical simulations of the gyrotron performance and experimental results could by partially attributed to the beam misalignment. On the other, it is in practice very difficult to reach the necessary tolerances in series production of high-power high-frequency (>100 GHz) gyrotrons. The problem actually will become more demanding with the increase of the operating frequency of the working mode because the required accuracy scales with the wavelength of the electromagnetic wave. It is shown that a large displacement of the annular beam from the cavity axis may lead to the excitation of a competing mod which is not excited under normal conditions. The excitation of the main mode is observed, however, at a beam-cavity displacement of up to 0.8 mm.

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained.

During the last years, electron cyclotron heating (ECH) was proven to be one of the most attractive heating schemes for stellarators because it provides net-current-free plasma startup and heating. Both the stellarator Wendelstein 7-X (W7-X), which is under construction at the Max-Planck-Institut fu umlr Plasmaphysik, Greifswald, Germany, and the International Thermonuclear Experimental Reactor (ITER) tokamak, which will be built in Cadarache, France, will be equipped with a strong ECH and current-drive system. Both systems are comparable in frequency and have continuous-wave capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for ITER). The commissioning of the ECH plant for W7-X is well underway; the status of the project and the first integrated full-power test results from two modules are reported and may provide valuable input for the ITER plant. The ten gyrotrons at W7-X will be arranged in two subgroups symmetrically to a central beam duct in the ECH hall. The RF wave of each subgroup will be combined and transmitted by a purely optical multibeam-waveguide (MBWG) transmission line from the gyrotrons to the torus. The combination of five 1 MW gyrotron beams to one beam line with a power of 5 MW reduces the complexity of the system considerably. The single- and MBWG mirrors and the polarizers have been manufactured. Cold tests of a full-size uncooled prototype line delivered an efficiency exceeding 90%. The microwave power will be launched to the plasma through ten chemical-vapor-deposited-diamond barrier windows and in-vessel quasi-optical plug-in launchers, allowing each 1-MW RF beam to be steered independently. The polarization, as well as the poloidal and toroidal launch angles, will be adjusted individually to provide optimum conditions for different heating and current-drive scenarios.

Summary form only given, as follows. Electron cyclotron resonance heating (ECRH) has proven during the last few years to be one of the most attractive heating schemes for stellarators, as it provides net current free plasma start tip and heating. Extensive measurements on stellarators at IPP Garching provide a solid physical and technological basis for ECRH systems. Therefore, ECRH will be the main heating method for the Wendelstein 7-X stellarator (W7-X) now under construction at Greifswald / Germany. A 10 MW ECRH system with continuous wave (CW) possibilities operating at 140 GHz will be built to meet the scientific goals of the stellarator at Greifswald with inherent steady-state capability at reactor relevant plasma parameters. A prototype gyrotron with an output power of 1 MW was developed in collaboration between European research laboratories, and European industries. The gyrotron is equipped with a single stage depressed collector for increasing the efficiency, an optimized quasi-optical mode converter and a CVD-diamond window. The prototype gyrotron has been successfully tested with an output power of 0.89 MW at a pulse duration of three minutes and all output power of 0.54 MW for about 15 minutes. The specified value of Gaussian beam output power exceeding 0.9 MW has been obtained for about 1 minute. Further results will be given.

The nine refereed papers in this special issue represent the high quality of the presentations and contributions of ICOPS 2008.