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The Wendelstein 7-X experimental device has completed the third plasma operation campaign in November 2018. The running, two-year shut down phase is being used to install new diagnostics, new in-vessel-components (steady-state divertor and cryopumps), and their auxiliary supply systems. In particular, the installation of the ten cryopumps requires a substantial upgradation of the cryosystems. The work package comprises the installation of a 55-m long new transfer line from the cryoplant to the new valve box and the installation of ten transfer lines with lengths up to 15 m from the valve box to the ten cryovacuum pumps (CVPs) in the plasma vessel. The tasks have been split into two main contracts in industry. Both contracts are presently running, the production of the transfer lines and the valve box per September 2019 is in an advanced status. The aim is to complete the production in early 2020 and the installation work by the end of 2020. A second task regarding the cryosystems is the upgrade with respect to the foreseen long pulse operation of W7-X. Therefore, new storage tanks for helium gas and for liquid nitrogen are necessary. The helium gas storage has been installed at the end of 2018 whereas the liquid nitrogen storage tank is presently under design. This article gives an overview about individual upgrade tasks of the W7-X cryosystems, highlights the special design requirements, and reports the status of the contracts. [ABSTRACT FROM AUTHOR]

The plasma fusion experiment Wendelstein 7-X (W7-X) uses a system of 50 nonplanar and 20 planar superconducting coils. These coils produce the magnetic field that is required to confine the plasma. Magnetic flux densities up to 3 tesla can be reached in the center of the plasma. Supercritical helium is used to cool down the coils to operating temperatures below 4 K. The coils are specified for currents up to 18 kA. A critical issue in all superconductors is the occurrence of quenches. These are unwanted local transitions from superconductivity to normal conductivity. If that happens, the coil current has to be discharged as fast as possible into a dump resistor. However, the strong current change will produce a self-induced high voltage (HV) with values up to several kilovolts. Therefore, the electrical coil insulation versus ground has to be HV proof to avoid a high-current arc in any circumstance. Worst case scenario is a quench that is induced by a loss of thermal insulation after a leak in the helium supply lines, because Paschen-minimum conditions could be given. This article describes some HV test procedures and techniques employed to test and qualify the coil system against that scenario. Some of these techniques are used today for the routine coil tests. Some other turned out as inefficient for daily use. Essential for a safe machine operation is the detection of insulation defects, but also on their localization along the superconductor, for the sake of a later repair. [ABSTRACT FROM AUTHOR]

The stellarator fusion experiment Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max Planck Institute for Plasma Physics, Germany. The W7-X superconducting magnet system basically consists of 50 nonplanar and 20 planar coils, including the interconnecting bus bars, and the support structure. The seven electrical circuits with 10 coils each in a series are supplied by 14 current leads (CLs) operating between the cryogenic and ambient temperature environments. A special feature of these feedthroughs is their upside-down orientation to save space in the vicinity of the machine. Basic electrical CL requirements are maximal steady-state currents of 18.2 kA and voltage strengths of 13 kV to ground. A W7-X CL consists of a copper conductor that also acts as a heat exchanger at the warm end side, in its continuation of a high-temperature superconductor part, and at the cold end side of a copper bar with integrated Nb3Sn rods. All components are fully contained within a CL vacuum chamber that is separated from the main W7-X cryostat vacuum. The high-voltage (HV) electrical insulation is built up of a glass tape epoxy resin system. Mechanical support of the CLs is achieved by a warm and cold glass fiber reinforced plastic flange. There are three He cooling circuits: 1) one for the bus bar and contact cooling; 2) one for the cold end of the CL (both at about 5 K); and 3) one for the CLs heat exchanger (entrance temperature about 50 K). After intensive tests of two prototypes, the series production has been established and completed. The CLs were tested at room temperature (HV, helium leaks, instrumentation, etc.) and with electrical currents up to the maximum current at cryogenic temperatures. This paper gives an overview of the basic CL design requirements and layout as well as on the fabrication and acceptance tests. Furthermore, a description of the assembly progress is given. [ABSTRACT FROM PUBLISHER]

Assembly of the superconducting stellarator Wendelstein 7-X is well advanced, and commissioning of the device is being prepared. A first draft of the commissioning tasks has been developed and will be discussed in this paper. [ABSTRACT FROM PUBLISHER]

The stellarator fusion experimental device Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik, Germany. The superconducting magnet system consists of 50 nonplanar coils, 20 planar coils, a superconducting bus system, and 14 current leads. It is organized in seven electrical circuits with ten coils, the bus system for the interconnection, and two current leads each. The magnet system is cooled by supercritical helium. It is enclosed in a cryostat with an outer diameter of 16 m formed by the plasma vessel and the outer vessel. There are five different types of nonplanar coils having dimensions of 3.5 \times 2.5 \times\break 1.5\ \meters in maximum and a weight of about 5.5 tons. The two different types of the planar coils are nearly circular coils with a diameter of up to 4.5 m and a weight of about 3 tons. The superconducting bus bar system connects the coils to each other and provides the connection to the current leads inside the cryostat. All types of coils and the bus system use the W7-X superconductor, a forced flow cable-in-conduit superconductor with 243 copper stabilized NbTi strands with an outer aluminum-alloy jacket. The current leads provide the transfer of the electrical current from the room temperature bus bar system outside the cryostat to the superconducting parts inside the cryostat. Their special feature is the upside-down orientation with the cold end at the top. [ABSTRACT FROM PUBLISHER]