TJ-II: impact of impurities on turbulence

Description of the activity

Motivation.

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. , 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 ($Z_{\text{eff}}$) on ITG stability was numerically confirmed in , 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 . 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 . 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 , 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, have just become affordable recently and, indeed, have been reported in the stellarator literature for the first time in . 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.

Proposal

The present proposal focuses on studying the impact of impurities on the turbulent transport and measured fluctuations. The central actuator in the proposal will be the controlled injection of impurities with low to moderate charge state (either with TESPEL or Laser Blow Off (LBO)). Possible choices could be the injection of LiF, BN, Fe. In order to induce observable changes in the characteristics of the plasma turbulence and performance, the impurities must be injected at non-tracer amount ($Z^{2}n_{Z}/n_{i}\sim 1$, with $Z$ the charge state of the impurity, $n_{Z}$ the impurity density and $n_{i}$ the density of the main ions). To estimate the amount of impurities introduced and their localization, an estimate of the effective charge as well as the evolution of the tomographic reconstruction of the bolometry and soft X rays (SXR) camera will be of key importance. In that sense, the present proposal is complementary to the proposal Zeff measurement using visible bremsstrahlung (VB) with NBI heating (II).

International or National funding project or entity

Include funding here (grants, national plans)

Description of required resources

In order to assess the impact of the injections in the plasma performance and turbulence monitors, monitoring the time the evolution of the electron and ion temperature ($T_{e}$ and $T_{i}$, respectively), as well as the diagmagnetic energy, will be essential. Ideally, $T_{e}$ and $T_{i}/$ should be measured at a radial position near to the that with largest impurity concentration and strongest impurity density gradient. If that information cannot be experimentally determined, a position from the inner core and mid-plasma radius will be chosen. Doppler Reflectometry fluctuation measurements radial profiles will be necessary in order to assess the changes in the amplitude of the turbulent density fluctuations after the impurity injections. For modeling purposes, Thomson Scattering electron density ($n_{e}$) and temperature profiles shall be measured at a time instant of the discharge. Whenever available, a $T_{i}$ radial profiles will be highly valuable. As the impact on the plasma foreseen after the injection is expected to depend on how the impurities distribute radially, either forming a peaked or a hollow density profile, two plasma scenarios are to be looked at: a plasma scenario with predominantly ion-root ambipolar electric field throughout the hole plasmas, which should lead impurities to peak; and plasma scenario under broader core electron root and transition to ion root in the outer half of the plasma column.

Required resources:

• Number of plasma discharges or days of operation:
• Essential diagnostic systems:
• Type of plasmas (heating configuration):
• Specific requirements on wall conditioning if any:
• External users: need a local computer account for data access: yes/no
• Any external equipment to be integrated? Provide description and integration needs:

Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy)

References

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