TJ-II: impact of impurities on turbulence: Difference between revisions

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 In the context of gyrokinetic theory, the stabilizing role of impurities on the Ion-Temperature-Gradient (ITG) driven instability has been known for decades, see e.g. <ref>R. R. Dominguez and M. N. Rosenbluth, Nuclear Fusion '''29''' 844 (1989).</ref>, where the linear dispersion relation analytically derived shows that the increase of the impurity concentration has a positive impact on the critical gradient of the toroidal ITG branch and its growth rate. Approaching the problem quasi-linearly, the benign impact of increasing the effective charge (<math>Z_{\text{eff}}</math>) on ITG stability was numerically confirmed in <ref>R. R. Dominguez and G. M. Staebler, Nuclear Fusion '''33''' 51 (1993).</ref> , albeit for the simplified slab geometry. In contrast, while the impact is found beneficial for the stability of the ITG mode, it is found deleterious for Trapped Electron Modes (TEMs) in the cited work. And, importantly, the stabilizing role of impurities on ITG vanishes when the impurity density profile is hollow, as found in <ref>J. Q. Dong and W. Horton, Phys. Plasmas '''2''' 3412 (1995)</ref>. These works underline the complexity of microturbulence in plasmas when its full multi-species character is taken into account.
-The interest in these early works and on the question itself about the active role of impurities on the overall turbulence behavior has been brought to the front line of stellarator research by recent W7-X experiments <ref>R. Lunsford ''et al'' Phys. Plasmas '''28''' 082506 (2021) </ref>. In that work, the conclusions highlight the increase of up to a 30% in the central ion temperature that follows after the injection of non-trace amounts of Boron. Given the limitations found in W7-X to achieve high core ion temperature <ref>M. N. A. Beurskens ''et al''., Nuclear Fusion '''61''' 116072 (2021)</ref>, with the exception of scenarios with reduced turbulence, the motivation to systematically study the means to reduce the turbulence ion heat transport and, in particular, the deliberate injection of impurities are clear. In contrast with the afore-mentioned analytical and numerical works, that employ approximations of different kind or consider simplified geometries, the possibility to study the problem numerically in all its complexity is at hand. Multi-species gyrokinetic simulations with the codes stella<ref>M. Barnes ''et al''., J. Comp. Phys '''391''' 365 (2019)</ref>, have just become affordable and, indeed, have been reported in the stellarator literature for the first time only recently <ref>J. M. García-Regaña ''et al''., J. Plasma Phys. '''87'''(1) 855870103 (2021)</ref>. Preliminary stella simulations performed for W7-X, the stabilization of ITG is confirmed for W7-X, see fig. 1 left, and the decrease of the heat flux of the ions is also found, see fig. 1 right.
+The interest in these early works and on the question itself about the active role of impurities on the overall turbulence behavior has been brought to the front line of stellarator research by recent W7-X experiments <ref>R. Lunsford ''et al'' Phys. Plasmas '''28''' 082506 (2021) </ref>. In that work, the conclusions highlight the increase of up to a 30% in the central ion temperature that follows after the injection of non-trace amounts of Boron. Given the limitations found in W7-X to achieve high core ion temperature <ref>M. N. A. Beurskens ''et al''., Nuclear Fusion '''61''' 116072 (2021)</ref>, with the exception of scenarios with reduced turbulence, the motivation to systematically study the means to reduce the turbulence ion heat transport and, in particular, the deliberate injection of impurities are clear. In contrast with the afore-mentioned analytical and numerical works, that employ approximations of different kind or consider simplified geometries, the possibility to study the problem numerically in all its complexity is at hand. Multi-species gyrokinetic simulations with the codes stella<ref>M. Barnes ''et al''., J. Comp. Phys '''391''' 365 (2019)</ref>, have just become affordable and, indeed, have been reported in the stellarator literature for the first time only recently <ref>J. M. García-Regaña ''et al''., J. Plasma Phys. '''87'''(1) 855870103 (2021)</ref>. Preliminary stella simulations performed for W7-X, the stabilization of ITG is confirmed.
 ''' Proposal '''