Wendelstein 7-X is a highly optimized stellarator that went into operation in 2015. With a 30 cubic meter volume, a superconducting coil system operating at 2.5 T, and steady-state heating capability ...of eventually up to 10 MW, it was built to demonstrate the benefits of optimized stellarators at parameters approaching those of a fusion power plant. We report here on the first results with the test divertor installed, during the second operation phase, which was performed in the second half of 2017. Operation with a divertor, and the addition of several new fueling systems, allowed higher density operation in hydrogen as well as helium. The effects that higher density operation had on both divertor operation and global confinement will be described. In particular, at high densities detachment was observed, and the highest fusion triple product for a stellarator was achieved.
Under construction for the stellarator project Wendestein 7-X is a neutral beam heating system based on RF driven positive ion sources. It is planned to start operation with 2 sources capable of ...injecting 5 MW of heating power in deuterium. This paper gives the current status and future plans of the construction of the injector boxes and subsequent installation in the experimental hall. The fruitful collaboration with the National Center for Nuclear Research in Swierk, Poland is also detailed. Lastly, results from an initial study on fast ions in Wendelstein 7-X will be given.
Abstract The paper reports for the first time the heat and particle exhaust at the plasma boundary through various edge diagnostics for the high-performance plasma obtained after pellet injection on ...Wendelstein 7-X. The plasma density at the edge is found to be reduced by a factor of 2 in the high-performance phase, supporting the previously reported density peaking at the plasma centre. The plasma beta effect on the magnetic topology is reflected by the appearance of the second strike line, which is well understood with simulation. However, during the rapid decay phase of the enhanced confinement, a transient localized heat flow of up to 16 MW m −2 is observed at the leading edge of a poorly cooled divertor component, which has not been understood but raises concerns about machine safety.
During the fabrication of the coils for the Wendelstein 7-X experiment the need for pressure dependent high voltage testing-Paschen tests-of the coils and components arose. These tests ensure the ...safety of the coils in case of an emergency shutdown due to a break in the vacuum of the cryostat. For that purpose the vacuum test facility at Babcock Noell Nuclear was modified in order to perform these tests. It is now possible to test coils and their components up to 17 kV and for pressures below 1.0middot10 -3 mbar. Completed coils, winding packs before insertion in the case and also several components have been tested up to now. It has been demonstrated that these Paschen tests are a powerful tool to detect insulation defects. The paper shows the effort which has been taken to set up the test facilities. It will show the preparation of the components under testing as well as present and discuss the results obtained up to now
A multi-purpose overview video system, based on EDICAM cameras, was set up at Wendelstein 7-X stellarator, in order to fulfill both machine protection and scientific observation purposes. Places of ...strong plasma-wall interaction, which can easily evolve to hot-spots, were detected by the EDICAM operators during plasma operation, based on intense local light emission. The EDICAM system was successfully used to avoid hot-spot formation during the commissioning of magnetic configurations with plasma operation. Featuring non-destructive readout capability, smaller areas of the torus interior could be monitored ca. two orders of magnitude faster, in parallel to the normal full frame overview. These fast measurements could be used to show the presence of plasma turbulence (filaments), also detected by other diagnostic systems.
The dependence of impurity transport on plasma parameters in the modular stellarator Wendelstein 7-AS was investigated by means of a laser blow-off technique. An increased impurity transport at ...higher heating power and lower magnetic field strength as well as no effect of the isotope composition on the impurity confinement was observed. The most critical scaling with respect to stationary operation at high density is the improved confinement of impurities at high densities, leading to a degradation of plasma energy by increasing radiation and to a loss of density control. This was attributed to a reduction of the impurity diffusion coefficient with density. After installation of island divertor modules, a transition from normal confinement into the high-density H-mode (HDH) at a certain power-dependent threshold density appeared. This transition is characterized by a strong reduction of the impurity confinement time and an increase in energy confinement time. In the HDH operational regime, access to even higher densities (4 × 10
20
m
−3
) than achieved before became possible under stationary operation conditions. Impurity transport measurements and model predictions indicate that the reduction of the impurity confinement in HDH is caused by not only a reduction of the inward convection in the core plasma but also possibly by changes in the edge transport. Comparison of experimental data with an axisymmetric transport model should elucidate the role of stellarator-specific transport aspects.
Deep particle fueling into a fusion-relevant plasma can be performed by the injection of cryogenic hydrogen or deuterium pellets. However, the penetration depth and fueling efficiency can be greatly ...limited when enhanced pellet ablation by fast particles occurs. Only a limited database exists for the penetration depth of ice pellets into stellarators. The penetration depth is measured during pellet injection into purely electron cyclotron resonance heated plasmas on the Wendelstein 7-AS (W7-AS) stellarator. These data are compared with the International Pellet Ablation Database (IPADBASE). Good agreement is found, if the neutral gas shielding model is applied for the scaling law of the penetration depth. The experimental data from W7-AS are used as a basis for planning a new pellet injection system for the Wendelstein 7-X stellarator, which is now under construction.
Paschen Problems in Large Coil Systems Lange, C.; Baldzuhn, J.; Fink, S. ...
IEEE transactions on applied superconductivity,
06/2012, Volume:
22, Issue:
3
Journal Article, Conference Proceeding
Peer reviewed
Open access
When a breakdown of the thermally insulating vacuum occurs in an energized high power coil system (e.g. a leakage of the He-cooling system or to the surrounding air) the coils warm up and have to be ...discharged fast to avoid destruction. Due to the coil inductance and the necessary discharge time, a high voltage is produced in the coil and the feeder system. Therefore an unspecified pressure coincides with the occurrence of a high voltage at the coil and feeder system. It is well known that under these conditions arcing can occur at a magnet when it is not perfectly insulated even for voltages 1 kV (Paschen-effect). The gas dependent breakdown voltage minimum occurs typically in the range of some 100 V at a pressure value that depends from geometry and temperature. For lower and higher pressures the breakdown voltage is increasing. For a high power magnet system an insulation is necessary that can withstand high voltage in the 10 kV range although any possible pressure is applied to the system. Usually a pressure independent solid insulation is used to solve this requirement. In order to test the reliability of this electrical insulation under all pressure conditions, a so-called Paschen test has to be performed. This Paschen test is a high voltage test with pressures varying between good vacuum and ambient pressure. In this paper examples for insulation failures on large coils found with Paschen testing are given. In a series of experiments with samples that have been prepared with intended insulation faults the necessity of Paschen testing is demonstrated to identify insulation problems that are not visible under good vacuum or ambient pressure. Therefore, a Paschen-test is also a useful high voltage diagnostic tool to localize insulation faults in solid insulated magnets.
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