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The soft x-ray tomography diagnostic in the stellarator Wendelstein 7-X consists of twenty pinhole cameras, up–down symmetrically arranged in a poloidal, triangular cross-section of the plasma vessel. The x-ray emissivity is measured with 16 bit amplitude resolution at 2 MHz sampling rate along 360 lines-of-sight by silicon photodiode arrays. In the recent operation campaign data acquisition (DAQ) has been working reliable for the conducted plasma pulse lengths <1 min, however the DAQ system are ready for the foreseen 30 min plasma pulse lengths of upcoming campaigns. The bandwidth of the preamplifiers is ≈200 kHz and the sensitive energy range is approximately 1–12 keV. The measurements indicate the up–down symmetric emissivity distribution in the triangular poloidal cross-section. First tomographic reconstructions of different magnetic field configurations are consistent with the theoretically calculated flux surface topology.

The engineering and design of the soft X-ray Multi Camera Tomography System (XMCTS) in Wendelstein 7-X stellarator (W7-X) must fulfill several additional requirements compared to short pulse machines. The XMCTS has to withstand irradiation and electron cyclotron microwave loads in addition to being ultra high vacuum compatible, having low magnetic permeability and using low neutron activation materials (e.g. Co ≤ 2000 ppm). A further difficulty is the limited space inside the plasma vessel, which requires special engineering solutions.After detailed design development, supported by finite element analyses, prototypes have been manufactured and tested. At the end all test results have successfully proven that the components fulfill the requirements and that reliable and stable measurements will be possible with the XMCTS diagnostics during W7-X operation.The paper describes the design and the technological development, in particular on the electric multipin feedthrough (UHV barrier between in vessel detectors and the preamplifiers), the active cooling of the electronic components (reducing dark current/ noise increase), the pneumatic shutter (protection of the detectors from sputtering and during baking) and the fiber optics illumination system (calibration of the detectors).

The paper presents an overview of the design, finite element (FE) analysis results, tests, and assembly strategy of the bolted connection between the coils of neighboring W7-X modules. The design is based on an accurately machined bridge and allows the accommodation of expected misalignments of the coil positions up to ±23 mm and 1 deg. The joint is capable to cope with forces up to 1.3 MN and moments up to 0.2 MNm. Loads are transmitted by a combination of form lock provided by tapered coil block shoulders, and by friction on the bottom of the blocks. Special friction-enhancing foils are inserted between the bridge bottom surfaces and coil blocks to ensure a friction factor above 0.5. Non-linear FE analyses with elastic-plastic material models show that local plastification and even slippage in spite of the initial high friction are unavoidable but stay within an acceptable margin. In parallel, machining and assembly tests have been carried out to check and simplify the design further, and to develop the manufacturing strategy.

Wendelstein 7-X (W7-X) is the world's largest superconducting nuclear fusion experiment of the optimized stellarator type. In the first Operation Phase (OP1.1) helium and hydrogen plasmas were studied in limiter configuration. The heating energy was limited to 4 MJ and the main purpose of that campaign was the integral commissioning of the machine and diagnostics, which was achieved very successfully. Already from the beginning a comprehensive set of diagnostics was available to study the plasma. On the path towards high-power, high-performance plasmas, W7-X will be stepwise upgraded from an inertially cooled (OP1.2, limited to 80 MJ) to an actively cooled island divertor (OP2, 10 MW steady-state plasma operation). The machine is prepared for OP1.2 with 10 inertially cooled divertor units, and the experimental campaign has started recently.The paper describes a subset of diagnostics which will be available for OP1.2 to study the plasma edge, divertor and scrape-off layer physics including those already available for OP1.1, plus modifications, upgrades and new systems. The focus of this summary will be on technical and engineering aspects, like feasibility and assembly but also on reliability, thermal loads and shielding against magnetic fields.

The stellarator Wendelstein 7-X is being prepared for long pulse operation. This includes diagnostics for investigation of plasma wall interaction processes. A versatile optical observation system has been developed for local characterization of the divertor plasma and the divertor target surface. The optical systems consist of two endoscopes each with perpendicular fields of view and the opportunity of tomographic reconstruction. Mirror based optics has been chosen in order to assure good optical properties independent of the wavelength. A narrow field of view allows for high spatial resolution while rotation of the first mirror covers the full poloidal divertor sections. An integrated shutter mechanism and a vacuum window far back minimize coating of optical components. For assessment of change of light transmission, a relative calibration function is implemented. The output light is split into wavelength ranges. Both, cameras equipped with narrow band filters as well as spectrometers are connected. The first endoscope was mounted at W7-X after successfully passing mechanical, optical and functional tests.

Long pulse operation considerably increases the thermal load on in-vessel components. Diagnostic frontends formerly employed at short pulse machines therefore have to be considerably re-designed for installation in the stellarator W7-X that is currently being built at Greifswald, Germany. The strategy applied to cope with the thermal load is threefold: to reduce the influx of heat on the component, to conduct the heat inside the component to suitable heat sinks and to choose suitable materials for sensitive components. The first is achieved by the shielding against microwave stray radiation, plasma radiation, thermal radiation and particle fluxes and by absorbing residual microwave stray radiation in the immediate vicinity of sensitive components. The second task, suitable heat conduction, enforces severe restrictions on the use of any thin parts like foils or meshes. Thirdly, in order for a component to survive the residual loads, materials must be chosen that absorb only a small fraction of the microwave stray radiation flux, conduct heat well enough, and survive high temperatures and large temperature gradients. Examples are provided from bolometry, magnetic diagnostics, soft X-ray diagnostics and Thomson scattering. Measurements of microwave stray radiation effects are presented, in particular the effectiveness of several shielding concepts.

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