**5. Conclusions and future plans**

A dedicated test facility has been constructed for studying RF sheath effects [24]. For this purpose plasma conditions representative of the edge of a tokamak are needed. A helical antenna creates plasmas with the required parameters. Helium and argon operation are used routinely, also hydrogen is foreseen, as well as gas mixtures. Plasma densities of the order of 1017–1018 m−3 and temperatures around 5–10 eV have been obtained. The main limiting factor is the generator power (3 kW) of the helical antenna. Performance optimisation is possible by adapting the magnetic field strength and topology; the detailed study of the helical plasma source will be the subject of future research.

A simple ICRF antenna is installed and operational, it is coupled to a broadband generator with frequencies in the range of 100 kHz to 100 MHz and a maximal power of 1 kW. If needed in the future a much more powerful generator can be used, when it will be coupled to the ASDEX RF system, with an available power up to the MW level.

Attention is also given to developing diagnostics for characterising the plasma parameters and electric fields, especially in the vicinity of the ICRF antenna, since they are the key ingredients for the sheath theories and modelling codes. Different probes and spectroscopic methods are used.

With the probes the behaviour of the plasma electron density and the three magnetic field components of the injected RF fields are measured under different operating conditions. Modelling is on-going using the COMSOL multi-physics environment [25, 26]. The measurements will also serve more advanced sheath and edge simulation codes in the future.

For measuring electric fields in the plasma caused by the RF antenna sheaths two approaches are followed. Passive optical emission spectroscopy monitors Stark effects on spectral lines with a high-resolution spectrometer, provided that the local electric fields are strong enough to overcome the broadening of the lines. Doppler-free saturation spectroscopy is more powerful; a laser beam depletes the ground state, eliminates the line broadening effects and makes smaller electric fields visible. However, the more complicated set-up, with a careful alignment of laser beams, makes the measurements much more challenging. After a first test on a glow discharge plasma, the design of the optical path and the installation of the laser at IShTAR have started.
