The diagnostics developed for the IShTAR (Ion Cyclotron Sheath Test ARrangement) facility are oriented towards measurements of plasma parameters and electric fields in the vicinity of a Radio Frequency (RF) antenna in order to provide input for sheath modelling codes [1, 2, 3]. The plasmas are created by a helicon antenna operated at a frequency of 11.76 MHz and with a power up to 3kW (the maximum power coupled to the plasma is around 2.7 kW). Plasma characterisation and optimisation A schematic view of IShTAR is presented in [1]. Improvements have been made for more complete density measurements in the helicon source and the main vessel. An additional array of RF compensated probes has been installed to allow for a better characterisation of the plasma parameters. In this way an optimised performance regime was found. By adjusting the magnetic topology the plasmas density can be increased by a factor 3 to 5, which is beneficial to the sheath studies. It is believed to be linked to the helicon wave propagation, but further investigation is needed to confirm the results. A current scan in helium plasmas is shown in figure 1. The current in the big magnetic field coils around the main chamber is varied between 200 and 800 A. The current in the small coils around the helicon source is kept constant at a level of 300 A. The corresponding magnetic field topology for the 200 A and 800 A cases are presented resp. in the figures 1 (b) and (c). It can be seen that for stronger magnetic fields the maximum densities are higher, but the radial profiles are more peaked. The RF antenna location is at radius r = −20 cm. The most homogenous and broad profile in this example is obtained for a coil current of ±650 A. In order to benchmark the electron density measurements from the Langmuir probes an interferometer has been designed. The schematic picture and hardware are shown in figures 2 (a) and (b). The probing frequency of 47 GHz, associated to an intermediate frequency of 144 MHz, is suitable for the typical plasma densities of IShTAR. The first results indicate a peak electron density in the helicon source of the order of 2 · 10 17 m −3 (figure 2(c)). The order of magnitude is in agreement with the results found by the Langmuir probes. Systematic scans for different plasma parameters and input power levels of the helicon antenna are planned.