The magnetic diagnostics foreseen for the Wendelstein 7-X (W7-X) stellarator are diamagnetic loops to measure the plasma energy, Rogowski coils to measure the toroidal plasma current, saddle coils to measure the Pfirsch-Schlüter currents, segmented Rogowski coils (poloidal magnetic field probes) to add information on the distribution of the plasma current density, and Mirnov coils to observe magnetohydrodynamic modes. All these magnetic field sensors were designed as classical pick-up coils, after the time integration of induced signals for 1/2 hour had been successfully demonstrated.The long-pulse operation planned for W7-X causes nevertheless significant challenges to the design of these diagnostics, in particular for the components located inside the plasma vessel, which may be exposed to high levels of microwave (electron cyclotron resonance) stray radiation and thermal radiation. This article focuses on the tests and modelling performed during the development of the magnetic diagnostics and on the design solutions adopted to meet the conflicting requirements. * Corresponding author, email: endler@ipp.mpg.de 1 All pick-up coils foreseen for the initial operation phase of W7-X and their signal cable sections inside the plasma vessel and the cryostat are now installed, and their electronics and data acquisition are under preparation.

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.

The operation of W7-X stellarator for pulse length up to 30 minutes with 10 MW input power requires a full set of actively water-cooled plasma facing components. From the lower thermally loaded area of the wall protection system designed for an averaged load of 100 kW/m² to the higher loaded area of the divertor up to 10 MW/m², various design and technological solutions have been developed meeting the high load requirements and coping with the restricted available space and the particular 3D-shaped geometry of the plasma vessel. 80 ports are dedicated alone to the water-cooling of plasma facing components and a complex networking of kilometers of pipework will be installed in the plasma vessel to connect all components to the cooling system. An advanced technology was developed in collaboration with industry for the target elements of the high heat flux (HHF) divertor, the so-called "bi-layer" technology for the bonding of flat tiles made from CFC NB31 onto the CuCrZr cooling structure. The design, R&D and the adopted technological solutions of plasma facing components are presented. At present, except the HHF divertor, most of plasma facing components has been already manufactured.

Foreseen to perform pellet investigations in the new stellarator W7-X, the former ASDEX Upgrade Blower Gun was revised and revitalized. The systems operational characteristics have been surveyed in a test bed. The gun is designed to launch cylindrical pellets with 2 mm diameter and 2 mm length, produced from frozen Deuterium D 2 , Hydrogen H 2 or a gas mixture consisting of 50% H 2 and 50% D 2 . Pellets are accelerated by a short pulse of pressurized helium propellant gas to velocities in the range of 100-250 m/s. Delivery reliabilities at the launcher exit reach almost unity. The initial pellet mass is reduced to about 50% during the acceleration process. Pellet transfer to the plasma vessel was investigated by a first mock up guiding tube version. Transfer through this S-shaped stainless steel guiding tube (inner diameter 8 mm; length 6 m) containing two 1 m curvature radii was investigated for all pellet types. Tests were performed applying repetition rates from 2 Hz to 50 Hz and propellant gas pressures ranging from 0.1 to 0.6 MPa. For both H 2 and D 2 , low overall delivery efficiencies were observed at slow repetition rates, but stable efficiencies of about 90% above 10 Hz. About 10% of the mass is eroded while flying through the guiding tube. Pellets exit the guiding tube with an angular spread of less than 14°.

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The critical issues in the development of diagnostics which need to work robust and reliable under quasisteady state conditions for discharge durations of 30 min and which cannot be maintained throughout the one week duration of each operation phase of the Wendelstein 7-X stellarator are being discussed.

All In-Vessel Components (IVCs) of W7-X are actively cooled. Inside the plasma vessel about 4 km of pipes will be installed, supplying water to the IVC. 226 cooling circuits with 78 variants are necessary. The cooling circuits enter the cryostat and the plasma vessel through ad hoc flanged penetrations called "plugins", which provide for the vacuum boundary between the plasma chamber and the torus hall atmosphere. The plug-ins are installed inside the W7-X ports. Some of the plug-ins are also used for the diagnostic cables. In total eighty plug-ins will be produced and installed. The inlet / outlet cooling lines are connected to the plug-ins using a welded hydraulic connector. The lay-out of the cooling lines is rather complex in consideration of the limited space and the routing between many component parts. Additionally the differential thermal expansion of the lines with respect to the supporting structures during the different operation scenarios had to be compensated by ad-hoc supports and adjustments in the flexibility of the lines.

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