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

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 (<math>Z^2 n_Z/n_i\sim 1</math>, with <math>Z</math> the charge state of the impurity, <math>n_Z</math> the impurity density and <math>n_i</math> 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 [http://fusionwiki.ciemat.es/wiki/TJ-II:_Zeff_measurement_using_visible_bremsstrahlung_(VB)_with_NBI_heating_(II) ''Zeff measurement using visible bremsstrahlung (VB) with NBI heating (II)''].

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 (<math>T_e</math> and <math>T_i</math>, respectively), as well as the diagmagnetic energy, will be essential. Ideally, <math>T_e</math> and <math>T_i/</math> 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 (<math>n_e</math>) and temperature profiles shall be measured at a time instant of the discharge. Whenever available, a <math>T_i</math> 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.