Besides oversized waveguides, optical systems can be used for the transmission of millimetre waves. Especially for high frequencies above 100 GHz optical systems are advantageous. The millimetre waves are transmitted as a collimated gaussian beam, where the diffraction is compensated by iterative reflection from focusing mirrors. Such a beam waveguide yields low loss (typ. 10 %/100 m) and - provided special configurations of the mirrors - broadband transmission of the power.
At IGVP, beam waveguides and optical antenna systems for high- and low-power applications are developed. An example is the transmission system for the Electron Cyclotron Resonance Heating at the Stellarator W7-X. The microwave generator has a modular design and consists of 10 gyrotrons at 140 GHz with an output power of 1 MW CW each. With 2 mirrors each, the beams of the gyrotrons are matched to the system; 2 other mirrors allow optimum setting of the polarisation of the millimetre waves. The main part of the transmission is performed with two multi-beam wave guides (MBWGs), which can transmit up to 6 beams at 140 GHz (and optionally another beam at 70 GHz). Near to the torus, the beams are separated by plane-mirror arrays and are focused via vacuum barrier windows and individually steerable antennas into the plasma.
The figure illustrates the underground transmission duct with individual matching and polarizing optics, a beam combination array and an MBWG installed at the wall of the duct. The inserts show the design of a water-cooled mirror as well as a low-power measurement of the beam patterns at the end of a prototype MBWG.
In this context, various measurement and beam diagnostic devices, like grating and waveguide directional couplers which are integrated into the reflectors, calorimeter for power measurement, beam profile recording by infrared thermography and alignment systems.
A further field is the development of diffractive optics, which will be used inside the plasma vessel to provide optimum multi-pass transmission through the plasma and thus maximum heating efficiency. These reflection gratings have to be fitted to the shape of the plasma vessel and therefore need 3-D grating structures which have to be optimized numerically.