The aim is to obtain the microwave power density in the vessel as a function of location. This allows to assess component heating and exposure of diagnostics. It can also serve to optimise launching efficiency and as data for modelling of plasma absorption. We consider the vacuum vessel as a resonator, or more precise: as a number of coupled resonators, in which sources and sinks are present.

Sources are non-absorbed microwave power and synchrotron radiation. Putting realistic and accurate numbers on the sources is not trivial. While the injected power is known, the non-absorbed fraction depends on plasma parameters and the accuracy at which the correct launching polarisation can be selected. In the case of synchrotron radiation, plasma emission and absorption must be assessed over the overall plasma volume.

Sinks are the plasma itself and “loss areas”, i.e. surface areas with a loss coefficient that can vary from unity (aperture) to almost zero (good conductor, e.g. copper). The use of loss areas does not take into account losses in ceramics (dielectrics) but these losses - while often huge for the particular component - are such a small fraction in the overall power balance that they are not considered.