Showing papers by "Felix Schauer published in 2019"
TL;DR: The Wendelstein 7-X superconducting stellarator was used for the first high-performance plasma operation as discussed by the authors, achieving densities of up to 4.5 GHz with helium gas fueling.
Abstract: The optimized superconducting stellarator device Wendelstein 7-X (with major radius $R=5.5\,\mathrm{m}$, minor radius $a=0.5\,\mathrm{m}$, and $30\,\mathrm{m}^3$ plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of $1-4.5\cdot 10^{19}\,\mathrm{m}^{-3}$ with central electron temperatures $5-10\,\mathrm{keV}$ were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. Plasma densities up to $1.4\cdot 10^{20}\,\mathrm{m}^{-3}$were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of $8\cdot 10^{19}\,\mathrm{m}^{-3}$ a temperature of $3.4\,\mathrm{keV}$ with $T_e/T_i=1$ was accomplished, which corresponds to
$nT_i(0)\tau_E=6.4\cdot 10^{19}\,\mathrm{keVs}/\mathrm{m}^3$ with a peak diamagnetic energy of $1.1\,\mathrm{MJ}$. The discharge behaviour has further improved with boronization of the wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above $1\cdot 10^{20}\,\mathrm{m}^{-2}$ line integrated density and $T_e=T_i=2\,\mathrm{keV}$ central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.
154 citations
01 Sep 2019
TL;DR: In this article, the authors presented the design and the manufacturing technology of the CVPs and the adjacent periphery, as well as results of the quality assessment such as integral He leak testing at 160°C and cryogenic temperatures (77 K).
Abstract: The cryo-vacuum pump (CVP) system, consisting of 10 units distributed symmetrically inside the Wendelstein 7-X plasma vessel, will be installed together with the 10 units of the actively cooled high heat flux divertor. One pump each is located below the corresponding divertor, and positioned as close as possible to the flux line strike points in order to allow efficient control of plasma density, and for screening impurities. Each CVP is divided into two parts, interconnected by a transfer line, to ensure access for divertor diagnostic integration. The CVP panels are operated with supercritical helium at 3.3–3.8 K to pump discharge gases such as H2 and D2. They are protected against thermal radiation by black-oxide finished stainless steel chevrons cooled by liquid nitrogen at 77 K. In front of this LN2-shield is a water-baffle which protects the CVP against plasma and ECRH stray radiation. It consists of copper chevrons with zero overlap, mounted on a water-cooled steel pipe. These chevrons are coated with an Al2O3-TiO2 layer. In addition, an uncooled copper shield covers the gap between water-baffle and LN2 chevrons in order to prevent stray radiation to take this path to the He-cooled panel. The cryogenic fluids are supplied via a dedicated port plug-in which is thermally insulated by a LN2-cooled cryo-shield and superinsulation. All 10 CVPs are already manufactured and successfully leak tested under hot and cold conditions in the workshops of IPP Garching. This paper presents the design and the manufacturing technology of the CVPs and the adjacent periphery. Results of the quality assessment such as integral He leak testing at 160 °C and cryogenic temperatures (77 K) are also discussed.
10 citations
TL;DR: In the Wendelstein 7-X (W7-X) superconducting coils as mentioned in this paper, the magnetic flux densities up to 3 tesla can be reached in the center of the plasma.
Abstract: 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.