Advanced electron cyclotron heating and current drive experiments on the stellarator Wendelstein 7-X

During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3·1019 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2·1020 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization

Accessory titanomagnetite typomorphism of devonian granitoids in North-Western area of Rudni Altai

Research relevance consists in determining the demand in mineral geochemical investigation of potential granitoid ore-bearing com plexes in Rudni Altai, embracing significant reserves of non-ferrous, noble and rare metals, which, in its turn, would expand the existing Russian mineral resources. Research target is to identify the distinctive accessory titanomagnetite as typomorphism characteristics of potential ore-bearing polyphase Aleysky-Zmeinogorsk granitoid complexes. Research methods in determining mineral chemical composition are: micro-sonde CAMEBAX-MICRO; electronic scanning microscope JSM-6510LV (Joel Ltd.) with energy-dispersive spectroscopy INCAEnergy350+. Research results include the following: initial detailed investigation of accessory magnetite of Aleysky-Zmeinogorsk granitoids and identification of two generations - early-magnetic titanomagnetite and post-magnetic titanomagnetite. The first of which is the decomposition-oxidation of magnetite-ulvospinel structure system. The following siderophile inclusion elements are predominate - Mn, V, Mg, Cr, Ge, while lithophylic elements - Al, Si, Y are less and chalcophylic elements are absent. The post-magmatic magnetite genesis was determined, being the result of titanomagnetite leaching via magmatogene hydrothermal solutions and further crystallization of magnetite, which, in its turn, derives the chemical behavior of titanomagnetite itself. In this case it characterizes low content or absence of Ti and other inclusion elements. The characteristic of V behavior was noted - in the final leucogranite phases magnetite contains more V than in early-magnetic titanomagnetite. Conclusion. Specific decomposition structure and titanomagnetite inclusion elements are typomorphic features of the basaltoid nature of granitoids. Specific potassic features of alkalinity, increased silica acidity and volatile activity during the final phase inhibit the possible V accumulation in titanomagnetite. Early-magnetic titanomagnetite is the mineral-concentrator V. This fact reflects the syngenetic enrichment of these melted elements and geochemical differentiation of Aleysky-Zmeinogorsk granitoids into Fe, Ti and V, as well as, Y and REE

Impurity transport and plasma flow in a mixed collisionality stellarator plasma

Neoclassical accumulation of impurities in the core of hot stellarator plasmas is a known problem. The complexity of neoclassical transport in stellarators means that few analytic studies are available to support numerical modelling efforts, and a robust understanding of the parameter dependence of the impurity flux is still lacking.Therefore we present an extension of the existing analytic treatment for highly collisional plasmas, into the experimentally relevant mixed collisionality regime -- where a dominant heavy, collisional, impurity is present in a collisionless bulk plasma, taken here to be in the 1/\nu regime.We find that temperature screening of the impurity flux by the bulk ion temperature gradient will arise.We also determine the bulk ion flow in the flux surface, and thus the effect of the impurity on the bulk ion contribution to the bootstrap current

Impurity transport and plasma flow in a mixed collisionality stellarator plasma

Neoclassical accumulation of impurities in the core of hot stellarator plasmas is a known problem. The complexity of neoclassical transport in stellarators means that few analytic studies are available to support numerical modelling efforts, and a robust understanding of the parameter dependence of the impurity flux is still lacking.Therefore we present an extension of the existing analytic treatment for highly collisional plasmas, into the experimentally relevant mixed collisionality regime -- where a dominant heavy, collisional, impurity is present in a collisionless bulk plasma, taken here to be in the 1/\nu regime.We find that temperature screening of the impurity flux by the bulk ion temperature gradient will arise.We also determine the bulk ion flow in the flux surface, and thus the effect of the impurity on the bulk ion contribution to the bootstrap current

Direct measurement of refracted trajectory of transmitting electron cyclotron beam through plasma on the Large Helical Device

The electron-cyclotron (EC) -beam refraction due to the presence of plasma was investigated in the Large Helical Device. The transmitted-EC-beam measurement system was constructed and the beam pattern on the opposite side of the irradiated surface was measured using an IR camera. Clear dependence of the EC-beam refraction on the electron density was observed and the beam shift in the toroidal direction showed good agreement with the ray-trace calculation of TRAVIS. The influence of the peripheral density profile and the thermal effect on the beam refraction were discussed

Direct measurement of refracted trajectory of transmitting electron cyclotron beam through plasma on the Large Helical Device

The electron-cyclotron (EC) -beam refraction due to the presence of plasma was investigated in the Large Helical Device. The transmitted-EC-beam measurement system was constructed and the beam pattern on the opposite side of the irradiated surface was measured using an IR camera. Clear dependence of the EC-beam refraction on the electron density was observed and the beam shift in the toroidal direction showed good agreement with the ray-trace calculation of TRAVIS. The influence of the peripheral density profile and the thermal effect on the beam refraction were discussed