Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission ͑TEE͒ and secondary electron emission ͑SEE͒, in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor ␦. An anomaly of ␦ in detached recombining plasmas is discussed.

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.