Reduced field Scenario with X3 heating in W7-X

In the present work, an ECRH scenario with reduced magnetic field 1.75 T is considered. For 140 GHz, this field corresponds to X3 heating. The high mirror-ratio magnetic configuration, B01/B00 ≃ 0.24, was considered as one from most attractive for long-pulse operation with low bootstrap current. Since X3 wave mode can be effectively absorbed only in sufficiently hot plasmas, a preheating stage is necessary, and the requirements for target plasmas suitable for starting X3 have been studied. Different ways to establish target plasmas are also discussed, in particular, augmenting X3 heating with X2 beams at 105 GHz

Physical model assessment of the energy confinement time scaling in stellarators

The International Stellarator Confinement Database (ISCDB) is a joint effort of the helical device community. It is publicly available at http://www.ipp.mpg.de/ISS and http://iscdb.nifs.ac.jp. The validity of physics models is investigated employing ISCDB data. Bayesian model comparison shows differences in the confinement scaling of data subgroups. Theory-based assessment of pure neoclassical transport regimes, however, indicates scalability which is supported by experimental results in specific W7-AS scenarios. Therefore, neoclassical simulations are employed for predictive purposes in W7-X, accounting for effects due to power deposition, plasma profiles and the ambipolar radial electric field. Neoclassical case studies for W7-X are presented as examples for the neoclassical predictions to be considered as an upper limit of plasma performance

Publisher Correction: Magnetic configuration effects on the Wendelstein 7-X stellarator

In the version of this Article originally published, A. Mollén’s affiliation was incorrectly denoted as number 10; it should have been 1. Throughout the Article, the tilde symbol above b was too high to be visible. In the second column of page five, the superscript 2 in κ≡((ϵt/b~10)2) was also too high to be visible. Finally, the citation to ref. 35 on page one of the Supplementary Information was incorrect; it should have been to ref. 36. These issues have now been corrected

Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy