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Abstract: The stellarator Wendelstein 7-X (W7-X) is presently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The plasma density profile will be measured by two-colour interferometry where for each line of sight through the plasma the phase shifts of two far-infrared laser beams are compared. Due to the 3D shape of the superconducting coils in W7-X, opposite ports are not available. Instead, corner cube retro-reflectors (CCR) will be incorporated in the plasma vessel heat shield that reflect the beams back to the entrance window and diagnostic. This requires high accuracy of the reflecting surfaces which will be subjected to thermal loads from plasma radiation and therefore be heated up, displaced and deformed during operation. ANSYS is used to perform coupled thermo-mechanical analyses of the CCR, including plasma radiation, thermal conduction, and radiation between all parts. The resulting displacements provide the inputs to MATLAB routines which then yield local deformations, curvature radii, and tilting of the reflecting surfaces. This information is used to optimize the design of the CCR and their fixations. [Copyright &y& Elsevier]

Abstract: The stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald , uses 70 superconducting coils, arranged in 7 groups to create the magnetic confinement for the plasma. A wide variety of tests and investigations are performed in order to ensure the later safe operation of the device. Much attention is also paid to the proper insulation of all the parts. These measures are costly and time consuming but are necessary in order to avoid the severe consequences of faults–—especially short circuits–—during operation. If a short circuit would happen during an emergency switch-off, the discharge of any shorted inductance would be delayed, and the coupled magnetic flux of the discharging system would induce additional currents into this shorted part. The currents and forces developing in such a case depend not only on the short circuit resistance and the critical current of the superconductor, but also on the shorted inductance itself, its magnetic coupling to other inductances, and its position within the system. The paper describes the influences of these factors and presents simulation results for different fault scenarios involving short circuits across a coil group, a single coil, different double layers, and a single turn. Maximum currents result from a shorted outer double layer, maximum forces from a shorted coil group, depending on its position in the magnet system. [Copyright &y& Elsevier]

Abstract: At present the Wendelstein 7-X stellarator experiment is under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik. During operation the plasma will be confined by the magnetic field of superconducting coils which will store a magnetic energy of about 620MJ at 3T average field strength at the magnetic axis. In a fast shut down this energy will be mainly absorbed by discharge resistors which heat up in the process and change their resistances to about four times the initial value. These changing resistances not only decrease the time constants of the discharge process but also determine the voltages applied to the coils. The paper describes the simulation of the currents and voltages during shut down and gives an example for increased coil currents due to a shut down failure which would lead to higher magnetic forces acting on the coils. [Copyright &y& Elsevier]

Highlights: [•] The criterion for thermo-mechanical design of W7-X interferometer retro-reflector. [•] Thermo-mechanical analysis of retro-reflector with two different methods. [•] The most flexible part in the retro-reflector is spring washer. [•] Calculation of parallelism error between the incoming and reflected laser beams. [•] The parallelism error is much lower than the design limit 28 arcs. [Copyright &y& Elsevier]

Highlights: [•] Thermo-mechanical analysis of HHF divertor module TM-H09 shows that it can withstand heat loads of 10MW/m2 in steady state when the supports were optimized accordingly. [•] FE calculations indicate that the tiles of baffles and heat shields can withstand stationary heat loads of 250kW/m2, but that the pulse length of plasma operation must be limited depending on the number of full load (500kW/m2) cycles. [•] Gap requirements for wall panels during assembly were defined based on several FE calculations. [Copyright &y& Elsevier]

Abstract: Wendelstein 7-X, a superconducting fusion experiment, is presently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald. The 50 non-planar and 20 planar coils are combined in seven electrical circuits with ten coils of the same type each. After the test of the seven individual power supply modules have been done in the past, now tests were made to show the proper co-ordination of the seven modules. In W7-X all seven electrical circuits are magnetically coupled. Therefore a special test load was designed and built up in order to simulate the magnetic coupling as realistic as possible. In the experimental program of W7-X eight magnetic configurations are defined, requesting different currents in the seven coil groups. During the tests all configurations were successfully tested, including the proportional energizing and slow discharging of the coils. Each power supply is combined with a protection system for the respective superconducting coil group. In case of a fast discharge all seven circuits have to discharged simultaneously to avoid an over current in the delayed circuit, caused by the inductive coupling of the coil groups. Another part of the final tests was the check of the proper reaction of the system after simulated failures. Also this kind of tests were executed successfully. [Copyright &y& Elsevier]

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. 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 FROM AUTHOR]

Nitrogen is routinely used to control the power load to the divertor targets of tokamak fusion reactors. However, its chemical reactivity can have implications on design and operation of a fusion device. In this contribution experimental results from recent years on three topics are briefly presented. These are the formation of nitrides, the sputtering of beryllium in the presence of nitrogen and the production of ammonia. Laboratory experiments have shown that surface nitrides are formed both on beryllium and tungsten upon exposure to energetic nitrogen ions. Erosion rates of Be by energetic N ions are in good quantitative agreement with modeling. Erosion upon exposure of Be to a mixed N/D plasma is reduced with respect to a pure D plasma. Finally, the appearance of ammonia has been observed in mixed N/D plasmas as well as in the exhaust gas of AUG and JET. The production rate in AUG reached 5% of the injected N atoms in a series of three subsequent N 2 -seeded discharges. [ABSTRACT FROM AUTHOR]

Abstract: The stellarator fusion experiment Wendelstein 7-X (W7-X) is currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The main magnetic field will be provided by a superconducting magnet system which generates a fivefold toroidal periodic magnetic field. However, unavoidable tolerances can result in small deviations of the magnetic field which disturb the toroidal periodicity. In order to have a tool to influence these field errors five additional normal conducting trim coils were designed to allow fine tuning of the main magnetic field during plasma operation. In the frame of an international cooperation the trim coils will be contributed by the US partners. Princeton Plasma Physics Laboratory has accomplished several tasks to develop the final design ready for manufacturing e.g. detailed manufacturing design for the winding and for the coil connection area. The design work was accompanied by a detailed analysis of resulting forces and moments to prove the design. The manufacturing of the coils is running at Everson Tesla Inc; the first two coils were received at IPP. [Copyright &y& Elsevier]