Abstract. The objective of Wendelstein 7-X is to demonstrate steady state operation at -values of up to 5%, at ion temperatures of several keV and plasma densities of up to 210 20 m -3 . The second operational phase foresees a fully steady state high heat flux divertor. Preparations are underway to cope with residual bootstrap currents, either by electron cyclotron current drive or by high heat flux protection elements. The main steady state heating system is an electron cyclotron resonance heating facility. Various technical improvements of the gyrotrons have been implemented recently. They enable a reliable operation at the 1 MW power level. Some of the technical issues preparing plasma diagnostics for steady state operation are exemplified. This includes the protection against non-absorbed microwave radiation.

MeV electrons were produced with less than 6 kW of non-resonant 2.45 GHz microwave heating at magnetic fields of 0.3-0.5 T with no applied loop voltage at the WEGA stellarator. The maximum toroidal plasma current, carried by the electrons, was 1 kA. Broadband radiation, similar to synchrotron radiation, was measured in the 0.5-120 GHz range. X-ray radiation from the plasma and directed γ-ray emission from collisions of the electrons with nearby plasma facing components was measured as well. The acceleration process was modeled by a tail formation due to stochastic interaction with the RF-field at the antenna mouth. The characteristic time for tail formation could be reproduced with power modulation experiments.

A progress report on recent experiments in the WEGA stellarator is given dedicated to diagnostic development -especially for the 0.5 T operation of 28 GHz ECR heated plasmas -and magnetic field line visualization experiments in the vacuum field. Furthermore, results from transport and turbulence studies in the plasma edge and first measurements inside magnetic islands are presented with the help of 1D and 2D Langmuir probe arrays. Additionally, the prototype installation of the Wendelstein 7-X control system is under continuous tests and further development at WEGA.

Abstract. A new set of 80 optimized magnetic diagnostics has been designed, built and installed in the HSX stellarator to enable a more accurate reconstruction of the plasma equilibrium compared to a previously installed unoptimized set. The optimization of the positions of the diagnostic coils was based on two different methods. The first technique searches for diagnostics with a high signal effectiveness. This quantity is determined by how much the variance of the reconstructed parameters, which are used to describe the equilibrium, changes depending on a change in the variance of the measurement. The results of this method are combined with a study that determines which diagnostics have the largest signal change if only a single parameter is varied. Calculations show that the new set of diagnostics reduces the uncertainty in the reconstructed plasma pressure and plasma current profile compared to the previous magnetic diagnostics and a virtual internal diagnostic set which was not optimized. With a reduction in the signal uncertainties, a further improvement can be achieved for the optimized diagnostic set, demonstrating the advantage of a careful diagnostic design process. It is shown that it is possible to design diagnostic coils which are only sensitive to either the bootstrap current or the Pfirsch-Schlüter current.

This paper gives an overview of recent results on the application of electron Bernstein waves for plasma heating and diagnostic purposes at the WEGA stellarator. By applying a two-step mode conversion process it was possible to reach over-dense argon and helium plasmas with central densities of above 0.97 × 10 19 m −3 at a magnetic field strength of 0.5 T fully sustained by electrostatic Bernstein waves. An unexpected feature observed during the experiments was the detection of a strong increase of the radiation temperature associated with the existence of a super-thermal electron component in the keV-range during this heating period.