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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°. [ABSTRACT FROM AUTHOR]

Abstract: The WENDELSTEIN 7-X (W7-X) superconducting stellarator experiment i.a. consists of 50 non-planar coils (npc) and 20 planar coils (pic). Before assembly the function of every coil is tested and examined at the French partner-institute CEA, Saclay. The testing follows a certain and constant routine laid out in a master-document (Overview); the main characteristical results are then recorded in appropriate reports. This presentation will point out the importance of good documentation, give an example of how it might be organized and show its respective needs of time and manpower. After testing some of the main data is collected in various tables and analysis-software for a first assessment and statistical analysis. The results will be presented as an example of a comparative first-step-database. [Copyright &y& Elsevier]

Abstract: The W7-X magnet system consists of 50 non-planar coils of five different types and 20 planar coils of two different types. Factory tests of the non-planar coils are carried out at the manufacturer site of Babcock-Noell, Germany, and for the planar coils at Tesla Engineering, UK. They consist of electrical insulation checks, mass flow measurements, leak tests and sensor checks. In the test facility of CEA Saclay, France, each coil is cooled down to ∼5K and operated at nominal current. At least one coil of each type is quenched by increasing the inlet temperature. Results of the mass flow measurements and the quench tests are presented. The manufacturing and testing progress is reviewed and the impact of technical failures is discussed. In conclusion, the scope of the tests allows a very strict quality control. This experience is highly beneficial for the construction and testing of similar components for future superconducting fusion experiments. [Copyright &y& Elsevier]

• W7-X achieved several of its project goals during the first operation phases. • Boronisation reduced the impurity content strongly and allowed high-density operation. • Stationary full detachment with strongly reduced target heat flux was demonstrated. • The next operation phase will feature water cooling of in-vessel components. • W7-X is a key project to verify the physics models for stellarator reactor. The Wendelstein 7-X project is aimed at demonstrating that an optimised stellarator is an attractive candidate for a fusion reactor. This requires the achievement of a number of technical and physics goals. Several of these goals have already been achieved in the first three experimental campaigns. We shall exemplify this by a number of results. One important goal is the demonstration of quasi-steady-state operation at high plasma density and temperature for half an hour, which encompasses the physical and technical requirements of stable operation with density and impurity control, cw heating, water-cooled targets, particle exhaust, and an appropriate control and data acquisition system for long-pulse operation. In the previous experimental campaigns, with uncooled targets, stationary discharges for up to 25 s with 5 MW heating power and up to 100 s with 2 MW heating power were achieved. For the next operational phase, to start in 2022, water-cooled targets are being installed, water cooling will be provided for all first-wall components, and a number of further upgrades to plasma heating, vacuum and fuelling systems as well as diagnostics will become available. With this equipment, the further goals of the project will be tackled, including the stepwise extension of discharge intervals to the multi-minute range. The experimental results so far have confirmed the neoclassical theory underlying several of the optimisation goals. At the same time, they showed that an optimisation is also required with respect to anomalous transport. In the future operational phases we aim at building a solid theoretical, experimental and technical foundation for the design of a next-step stellarator device. [ABSTRACT FROM AUTHOR]

Abstract: WENDELSTEIN 7-X (W7-X) is a superconducting stellarator which uses 50 non-planar coils for the main confinement field and 20 planar coils for the magnetic configuration. All 70 coils are subject to an extensive test program after manufacturing and before assembly, realised at the cryomagnetic test facility at CEA Saclay. The tests are part of a well-advanced quality assurance plan attesting the specified coil performance. The test program is mainly divided in warm and cold tests. Various tests have to be performed such as visual inspections, hydraulic measurements, electrical tests and the check of sensors. The most elaborate test is the cryogenic temperature margin test using current up to 17.6kA including the quench or fast discharge test and the evaluation of the specified interlayer joint resistance. Up to now 75% of the coils have been tested and accepted. The last acceptance tests at CEA are scheduled to be performed beginning of 2009. The test program has been enhanced permanently in its course, due to further improvement as well as considering aspects of the time schedule. This paper will present a short description of the test program done at CEA, their aim and background. [Copyright &y& Elsevier]

Abstract: The Wendelstein 7-X (W7-X) is an advanced optimized stellarator under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik. Its magnet system consists of 70 superconducting coils, i.e. five types of 10 non-planar (NPC) and two types of 10 planar (PC) individuals each. In case of an emergency shut-down like in succession of a quench, the voltage across a NPC may raise up to 450V according to the specification. Since all the 10 coils of a type are connected in series, the total voltage could thus reach 4.5kV. Therefore, the coils had to be provided with a corresponding electrical insulation. Its test voltage levels were fixed at 10.4kV and 7.2kV, respectively, for the NPCs and PCs. The coil acceptance tests take place in an ad hoc facility at CEA, Saclay where a large variety of trials are performed, including the verification of the electric insulation quality. Several DC high voltage tests (DHVT) are carried out to assess the proper manufacture of the ground insulation which stretches across the winding pack, conductor terminals, interlayer joints, and the numerous quench detection wires including their connections. The DHVTs are performed under different conditions: before cool down at room temperature and atmospheric pressure, then at cryogenic temperature and, finally, under vacuum at room temperature. Since the coils are necessarily connected to the test facility, the corresponding components like wires, plugs, or connections are also energized by the nominal voltage. Thus a flaw in the external test bed may invalidate the outcome of the experiment. It is therefore very important to determine whether the origin of a current leak is in the coil or in the test facility. To do so, it is necessary to have deep knowledge of the whole arrangement. The paper summarizes the experimental observations, data analysis, and consequent improvements which were implemented at CEA test facility. [Copyright &y& Elsevier]

The research on divertors for stellarators is at the beginning. Extensive studies are being prepared on large helical device (LHD) and W7-X. W7-AS is now being operated with an open island divertor (ID) which serves as a test bed for the W7-X divertor. The divertor enables access to a new NBI-heated, high-density operating regime with improved confinement properties. This regime—the high-density H-mode (HDH)—displays no evident mode activity, is extant above a threshold density and characterized by flat density profiles, high-energy- and low-impurity-confinement times and edge-localized radiation. Impurity accumulation, normally associated with ELM-free H-modes, is avoided. Quasi-steady-state discharges with &nmacr;e up to 4×1020 m-3, edge radiation levels up to 90% and plasma partial detachment at the divertor targets can be simultaneously realized. The accessibility to other improved confinement modes in W7-AS (conventional H-mode and OC-mode) is not restricted by the divertor. The results provide a promising basis for future experiments, in particular on W7-X, and recommend the ID as a serious candidate for solving the plasma exhaust problem in stellarators. [Copyright &y& Elsevier]