8 research outputs found
Turbulent transport of impurities in 3D devices
A large diffusive turbulent contribution to the radial impurity transport in
Wendelstein 7-X (W7-X) plasmas has been experimentally inferred during the
first campaigns and numerically confirmed by means of gyrokinetic simulations
with the code stella. In general, the absence of strong impurity accumulation
during the initial W7-X campaigns is attributed to this diffusive term. In the
present work the diffusive contribution is also calculated in other stellarator
plasmas. In particular, the diffusion (D) and convection (V) coefficients of
carbon and iron impurities produced by ion-temperature-gradient (ITG)
turbulence are obtained for W7-X, LHD, TJ-II and NCSX. The results show that,
although the size of D and V can differ across the four devices, inward
convection is found for all of them. For W7-X, TJ-II and NCSX the two
coefficients are comparable and the turbulent peaking factor is surprisingly
similar. In LHD, appreciably weaker diffusive and convective impurity transport
and significantly larger turbulent peaking factor are predicted. All this
suggests that ITG turbulence, although not strongly, would lead to negative
impurity density gradients in stellarators. Then, considering mixed ITG/Trapped
Electron Mode (TEM) turbulence for the specific case of W7-X, it has been
quantitatively assessed to what degree pellet fueled reduced turbulence
scenarios feature reduced turbulent transport of impurities as well. The
results for trace iron impurities show that, although their turbulent transport
is not entirely suppressed, a significant reduction of V and a stronger
decrease of D are found. Although the diffusion is still above neoclassical
levels, the neoclassical convection would gain under such conditions a greater
specific weight on the dynamics of impurities in comparison with standard ECRH
scenarios.Comment: 16 pages, 8 figure
Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X
We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.EC/H2020/633053/EU/Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortium/ EUROfusio
Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X
| openaire: EC/H2020/633053/EU//EUROfusionWe present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.Peer reviewe