TJ-II: impact of impurities on turbulence

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Experimental campaign

Spring 2022

Proposal title

TJ-II: Impact of impurities on turbulence transport

Name and affiliation of proponent

García-Regaña, J. M., Calvo, I., López-Miranda, B., Baciero, A., Estrada T., Carralero D., Ochando, M. A., Medina, F., McCarthy K. J., Velasco, J. L. and the TJ-II team. Laboratorio Nacional de Fusion, CIEMAT (Spain)


Details of contact person at LNF

jose.regana@ciemat.es

Description of the activity

Motivation.

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. [1], 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 ($ Z_{\text{eff}} $) on ITG stability was numerically confirmed in [2] , 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 [3]. 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 [4]. 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 [5], 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[6], have just become affordable and, indeed, have been reported in the stellarator literature for the first time only recently [7]. Preliminary stella simulations performed for W7-X, the stabilization of ITG is confirmed.

Proposal

The present proposal focuses on studying the impact of impurities on the properties of turbulence and the transport it drives. In particular, the controlled injection of impurities with low to moderate charge state values (either with TESPEL or Laser Blow Off (LBO)) is desired. In order to provoke observable changes in the characteristics of the plasma turbulent behavior and its performance, the impurities must be injected at non-tracer concentration, i.e. $ 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 its localization, an estimate of the effective charge as well as the tomographic reconstruction of the radiation monitor signals will be of key importance. In that sense, the present proposal can greatly benefit from the work carried out in the proposal [1]


International or National funding project or entity

Description of required resources

In order to assess the impact of the impurity injections in the plasma performance and turbulence, monitoring the time the evolution of the electron and ion temperature ($ T_e $ and $ T_i $, respectively), as well as the diamagnetic energy, will be essential. Ideally, the time evolution of $ T_e $ and $ T_i/ $ shall 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, two positions, one at the inner core and another at the mid-plasma radius will be chosen. Doppler Reflectometry (DR) fluctuation measurements along the radial coordinate will be necessary in order to assess the changes in the amplitude of the turbulent density fluctuations. For modeling purposes, Thomson Scattering (TS) electron density ($ n_e $) and temperature profiles shall be measured at two time instants during the discharge, one previous to the injection of impurities and another shortly after the injection. If that were not possible, pairs of discharges with and without impurity injection would be necessary. If available, the $ T_i $ radial profile will be highly valuable. As the impact on the plasma foreseen after the injection is expected to depend on how 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 develop peaked profiles; and a plasma scenario under broader core electron root and transition to ion root in the outer half of the plasma column, which would lead to hollow density profiles. Whether those two scenarios are accessed by scanning the ECRH power and plasma density or by adding NBI to the heating scenario is left for discussion. The decision will depend on how the different heating schemes constraint the quality and availability of the diagnostics data.

  • Number of plasma discharges or days of operation: 40, approximately.
  • Essential diagnostic systems: spectroscopic system, Bolometric arrays, X-Ray detectors, Doppler reflectometer, HIBP, VUV spectrometer, NPA system, ECE, Thomson Scattering.
  • Type of plasmas: standard configuration (100_44_64), ECRH (possibly NBI too). Density and ECRH power scan on a shot-to-shot basis so that the plasma transit from CERC to predominantly ion root conditions. In each shot, the injection of a different impurity source will be carry out in ideally stationary conditions.
  • Specific requirements on wall conditioning if any: to be discussed.
  • External users need a local computer account for data access: no.
  • Any external equipment to be integrated? Provide description and integration needs: None.

Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy)

References

  1. R. R. Dominguez and M. N. Rosenbluth, Nuclear Fusion 29 844 (1989).
  2. R. R. Dominguez and G. M. Staebler, Nuclear Fusion 33 51 (1993).
  3. J. Q. Dong and W. Horton, Phys. Plasmas 2 3412 (1995)
  4. R. Lunsford et al Phys. Plasmas 28 082506 (2021)
  5. M. N. A. Beurskens et al., Nuclear Fusion 61 116072 (2021)
  6. M. Barnes et al., J. Comp. Phys 391 365 (2019)
  7. J. M. García-Regaña et al., J. Plasma Phys. 87(1) 855870103 (2021)

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