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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–Schluter 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. 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.

The stellarator Wendelstein 7-X (W7-X) has a divertor consisting of 10 units installed inside the plasma vessel (PV). It was decided not to install the long pulse high-heat flux (HHF) divertor targets at the first two years stage of W7-X operation and to start with an adiabatically cooled test divertor unit (TDU) and shorter plasma pulses operation. This allows to accumulate operation experience with much simpler components, and as a result to adjust accurately the actively cooled HHF divertor which replaces the TDU for the stationary operation. Finite element (FE) analyses have been performed for better understanding of thermo-mechanical problems of divertor targets, and to guide the design of the TDU and HHF divertors. This paper presents the detailed results of the temperature response, the deformation and thermal stress of the divertor components.

For long-pulse plasma operation of the W7-X stellarator, one of the most serious challenges for the design of in-vessel diagnostic systems is the thermo-mechanical problem. Thermal load from convective losses and from plasma radiation can reach up to 500 kW/m 2 at some locations close to plasma. The typical thermal load from plasma radiation alone ranges from 10 kW/m 2 to 100 kW/m 2 as derived from 3D Monte-Carlo simulations. Most plasma facing diagnostic components require active cooling for the long-pulse plasma operation. Especially the design of optical components must be optimized to minimize thermal deformations. Finite element analyses (FEA) are performed for better understanding of thermo-mechanical problems on plasma facing components of diagnostics, and to guide the design of the diagnostic system. As examples the paper presents the FEA results of the Corner-Cube Retro-Reflectors for the CO 2 -laser interferometer and the movable pop-up probe arrays in the divertor.

The purpose of the Wendelstein 7-X cryostat is to thermally insulate the magnet system (operating at cryo-temperature) by high vacuum and super insulation, to provide and enclose the working space for the plasma, and to serve as a bearing structure for the system components that have to be placed in the immediate vicinity of the plasma, such as the first wall, the divertor and some of the diagnostic devices. Concerning structural components only, the cryostat is composed of the plasma vessel, the outer vessel and the ports that interconnect the vessels. The cryostat together with the machine base comprises the so called cryostat system, and this is the system that is under consideration in this paper. Due to both the geometrical complexity and the complexity of loading, individual subsystems of the cryostat system could not be analyzed separately. For that reason, a global finite element model of the whole cryostat system had to be created. The paper presents an overview of finite element modeling, simulations with ANSYS performed on this system, and the corresponding calculation results.

The Wendelstein 7-X (W7-X) modular stellarator is in the assembly phase at the Max-Planck-Institut fur Plasmaphysik (IPP) in Greifswald, Germany. The goal of the project is to demonstrate that this type of machine is a viable option for a fusion power-plant. The “pentagonal” magnet system of the machine encompasses 50 non-planar and 20 planar superconducting coils with sophisticated support structure. Structural reliability of components as well as resulting deformations and displacements during various modes of operation have to be considered not only for the magnet system but also throughout the whole cryostat whose main components are the plasma vessel, outer vessel, ports, and thermal insulation. A reliable prediction of the W7-X structural behaviour is only possible by employing complex finite element (FE) simulations with a hierarchical set of FE models. A special strategy has been developed and implemented for the task. The design is basically completed, main parameters are defined, and most of the W7-X components are manufactured. Therefore, the focus in the analysis is being shifted to the creation of parametric FE models which allow performing fast analyses of possible non-conformities, changes in the assembly procedure, and future exploration of operational limits. This paper gives an overview of the implemented analysis strategy, the applied safety margins, and focuses on the most remarkable results.

Deuterium retention in tungsten increases substantially when it is pre-exposed to low-energy He plasma at a high temperature (1600 K), while it decreases when pre-exposed at a low temperature (700 K). The increase of deuterium retention could be attributed to the submicron-sized He bubbles and secondary interstitial defects formed on the tungsten surface. Occupation of trap sites by He atoms during the He pre-exposure at low temperature could decreases the effective number of available trap sites for deuterium atoms, which would decreases the deuterium retention on the tungsten surface.

In order to prepare for tungsten coating to be applied to the low field side poloidal guard limiters, 1 μm W-coated CFC limiter tests have been performed in ASDEX Upgrade. The test limiters were exposed to H-mode deuterium plasmas with the neutral beam heating at maximum 5 MW for approximately 1 s. The power flux deposited on the test limiter was calculated by solving the 3-D heat conduction equation by using the measured time dependent surface temperatures as boundary conditions. The experimental results show that the heating with more radial NI beams leads to a higher local deposited power with a longer radial decay length related to banana particle loss. Strong localized W erosion was found at the tip of the test limiter due to arcing, which obviously depends on the neutral beam injection (NBI) geometry.

Heat deposition onto plasma facing components outside the divertor by ELMs and fast particles has been investigated in ASDEX Upgrade. Heat fluxes up to tens of megawatts per square meter are found during ELMs on limiters and a few megawatts per square meter in between. The total deposited power on non-divertor structures can be up to 10% of the heating power and has to be considered for high power next step experiments. The mechanisms behind the observed heat load patterns are qualitatively understood. The main contributions come from fast ions created by neutral beam injection (and ion cyclotron minority) heating, and from ELMy energy bursts from the plasma edge.

A systematic study on the formation mechanism of micron-sized He bubbles and holes in powder metallurgy tungsten due to helium ion irradiation with an ion energy below 30 eV and a particle flux above 1022 m−2 s−1 has been performed in the linear divertor plasma simulator NAGDIS-II. Holes are formed with incident helium ion energy above 5 eV, which could be related to the surface barrier potential energy for He penetrating into tungsten. Tungsten surface temperature strongly influences the number and size of hole. Above 1600 K, bubbles and/or holes with several hundreds nano-meter diameter appear on the tungsten surface. Single crystal tungsten, which has much fewer intrinsic defects than powder metallurgy tungsten, was also irradiated by He plasmas. There is no qualitative difference in the hole formation between the two grades of tungsten. Bubble and hole formation mechanisms are discussed based on the experimental results.

A space charge effect on thermoelectron emission current from a tungsten plate has been experimentally investigated in a high density helium plasma. Experimental result shows that the space charge limited current from the tungsten target plate, which is heated by the plasma irradiation, tends to be saturated with an increase in a sheath voltage, which can be predicted by the developed new formula based on the boundary condition of zero electric field on the material surface.

Tungsten (W) specimens were irradiated by low energy ( 10 22–23 m −2 s −1 ) helium plasma to investigate the incident ion energy dependence of helium bubble formation. Experimental results indicate the existence of the incident ion energy threshold for the bubble formation. The threshold energy around 15 eV could associate with the surface potential for He ions entering to the W surface.