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

In the context of analytical theory, the stabilizing role of impurities on Ion-Temperature-Gradient (ITG) driven instability has been known for decades, see e.g. <ref>R. R. Domínguez and M. N. Rosenbluth, Nuclear Fusion '''29''' 844 (1989).</ref>, where the derived linear dispersion relation shows that the increase of the impurity concentration has a positive impact on the critical gradient of the toroidal ITG mode 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. Domínguez 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 work just cited. And, importantly, the stabilizing role 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>. Works like the ones just mentioned point out to a more complex description 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 where W7-X, the motivation to study systematically the means to reduce the turbulence ion heat transport is strongly motivated. 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 recently and, indeed, have been reported in the stellarator literature for the first time in <ref>J. M. García-Regaña ''et al''., J. Plasma Phys. '''87'''(1) 855870103 (2021)</ref>. In 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 present proposal focuses on studying the impact of impurities on the turbulent transport and measured fluctuations. In particular, the controlled injection of impurities with low to moderate charge state (either with TESPEL or Laser Blow Off (LBO)), is desired. In order to provoke observable changes in the characteristics of the turbulence of the plasma, the impurities must be injected at non-tracer amount (<math>Z^2 n_Z/n_i\sim 1</math>, with <math>Z</math> the charge state of the impurity, $n_Z$ the impurity density and $n_i$ the density of the main ions).