This contribution reviews the currently applied wall conditioning methods in fusion devices with special emphasis on wall conditioning in the presence of a permanent magnetic field by applying RF discharges at the ion- and electron cyclotron range of frequencies (ICRF resp. ECRF). The review is built upon the results of tokamaks JET, TEXTOR, TCV, ASDEX Upgrade and JT60-U. Stellarators experience is based on W7-X, U2-M and LHD. The roles of traditional wall conditioning methods such as baking, glow discharge conditioning (GDC) and low-Z wall coatings on superconducting devices will also be discussed. Wall conditioning is essential to increase the plasma performance in fusion devices by reducing the release of volatile plasma impurities from the first wall due to plasma-surface interactions, and to control the recycling of hydrogenic fuel fluxes [1]. In particular, ITER relies on conditioning to mitigate the tritium inventory build-up in the plasma-facing materials [2]. Current research efforts on RF conditioning aim at developing reliable RF plasma production methods for tokamaks as well as stellarators using ICRF and ECRH systems to produce a currentless plasma. RF conditioning plasmas, in reactive or noble gases, are characterized by a significant higher density than glow discharge plasmas. This enhances desorption of volatile species during conditioning, although requires pulsed plasma operation to reduce their re-ionisation. The location and size of the plasma-wetted area is determined by the shape of the confining magnetic field, allowing targeted interaction with limiters or divertor. For example, stellarator He-ECRH plasma, produced by localised power absorption at the EC resonance layers, was shown to effectively desaturate the divertor targets from hydrogen on W7-X. ECRH plasma production in the tokamak vacuum magnetic field, with much reduced confinement, is hampered by a low single pass absorption. To increase the single pass absorption, high densities and temperatures at the resonance layer are required, and hence considerable launched power. Strong and volumetric collisional absorption at low density and temperature is characteristic for ICRF plasmas, relaxing the power requirements for these plasmas and simultaneously improving the discharge homogeneity. While not necessarily accessing the same areas, the amount of retained fuel in the plasma facing materials accessible to ICRF conditioning is larger than that of L-mode plasmas by a factor 2, and approaches that of GDC in isotopic exchange experiments on the JETILW. RF wall conditioning discharges in future devices such as ITER and JT60-SA are studied with help of the codes TOMATOR, KIPT-RF and RFDINITY. A brief description of these simulations and the underlying physical principles will be presented.

An optimized hard X-ray (HRX) spectrometer was designed to collect information from Bremsstrahlung emission in the MeV range runaway electrons (RE) generated during disruptions. The detector is based on a cerium doped lanthanum bromide scintillator crystal (LaBr3:Ce) coupled with a photomultiplier tube. The diagnostic allows for measurements of high hard X-ray fluxes in excess of 1 MHz with a wide dynamic range up to 20 MeV. The diagnostic was tested at the tokamak ASDEX Upgrade. The results achieved are promising and suggest the possibility of inferring information on the runaway electron energy distribution in tokamaks using deconvolution techniques.

An advanced technology has been developed and employed for the main circuit breakers (CB) of the quench protection circuits (QPC) of the superconducting coils of JT-60SA: it consists in a Hybrid mechanical-static CB (HCB) composed of a mechanical Bypass switch (BPS) for conducting the continuous current, in parallel to a static circuit breaker (SCB) based on integrated gate commutated thyristor (IGCT) for current interruption. It was the result of a R&D program carried out since 2006 to identify innovative solutions for the interruption of high dc current, able to improve the maintainability and availability of the CB. The HCB developed for the JT-60SA QPC is the first realization of a dc circuit breaker based on this design approach for interrupting current of some tens of kA with reapplied voltage of some kV. It also represents the first application of hybrid technology with IGCT for protection of superconducting magnets in fusion experiments. The paper aims at giving a comprehensive overview of the main R&D activities devoted to the development of this new technological approach; then, the key aspects of the design, manufacturing and testing of the QPCs for JT-60SA, successfully completed in Naka Site in summer 2015 are presented. Finally, the significance of this research is discussed and the possible future developments, in particular in view of DEMO fusion reactor, are outlined.

The FE and analytical electromagnetic computations carried out to assess the effectiveness of a set of coils in locally correcting the magnetic field configuration of a nuclear fusion device are reported in the paper. The electric parameters required for the design of the control system of the coils were also evaluated. Some of the results are compared with measures on the system after its installation on the device.