TJ-II: Radiation asymmetries and potential variations

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Experimental campaign

2018 Spring

Proposal title

Impurity dynamics, radiation asymmetries and potential variations

Name and affiliation of proponents

José M García Regaña1, D. Carralero1, M. A. Ochando1, T. Estrada1, M. Ezzat1, F. Medina1, José Luis Velasco1, J. A. Alonso 1, B. López-Miranda, D. Tafalla and TJ-II team.

Details of contact person at LNF (if applicable)

Enter contact person here or N/A

Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)

Neoclassical theory predicts a non-constant portion of the electrostatic potential over the flux surfaces [1], usually denoted by $ \Phi_1=\Phi_1(\theta,\phi) $, with $ \theta $ and $ \phi $ the poloidal and toroidal angular coordinates. When this is taken into account the equilibrium density of the different species a present in the plasma varies accordingly since $ n_{a0}=n_a0(r)\exp\left(-Z_{a}e\Phi_1/T_{a}\right) $. This can introduce a strong density variation of impurities due to their high charge state. In TJ-II plasmas experiments and simulations [2] [3] have shown that $ e\Phi_1/T_{a} $ can take values from $ O(0.01) $ to $ O(0.1) $. Variations are predicted to be larger at the outer radii than at the inner core, and stronger in ECRH plasmas than in NBI plasmas. Under conditions with large $ \Phi_1 $ the impurities of moderate to high $ Z $ should experience strong variations of their densities over the flux surfaces, increasing with $ Z $. These, in turn, should result in an anisotropic radiation over each flux surface and consequently a radially asymmetric radiation pattern should follow.

In the present experiment the analysis of the radial profiles and time evolution of the plasma emissivity using the TJ-II bolometry system [4] after the inyection of some selected impurities by gas puffing is proposed. The experiment aims at studying the relation between the radially asymmetric emissivity and the measured and predicted $ \Phi_1 $ and compare this with the case where $ \Phi_1 $ is neglected [5]. The measurement and evolution of $ \Phi_1 $ will be tracked during the discharges using the duplicated Langmuir probe system plasma floating potential measurements. Numerical calculations of $ \Phi_1 $ will be carried out with the neoclassical version of the code EUTERPE at different radial locations.

Description of required resources

The main experiment conditioning requirements are:

  • ECH is the main and only heating system that will be employed. Accessing the different plasmas regimes will be carried out changing the ECH power and the orientation of the heating beams if necessary (on/off-axis injection).
  • A total of 30 discharges are estimated to be the minimum required.
  • The injection of impurities by means of LBO or puffing is needed, in order to track the impact and evolution of the bolometry radiation signals and correlate this with the measurement and predictions of the potential variations, radial electric field and, importantly, impurity density.
  • Accessing different absolute values and regimes $ E_{r} $ is essential. These regimes can be roughly referred to as "high ion root $ E_{r} $", "low ion root $ E_{r} $" and the same for electron root conditions. 2 discharges for each regime is necessitated in order to characterize $ E_{r} $ at different positions over the same flux surface.
  • Reproducing some of these regimes in two different configurations (high-iota and standard), where changes in the the sign of $ E_{r}^{Left}-E_{r}^{Right} $ have been observed, is planned.
  • Good stationarity of plasma parameters at the instant where the impurities are injected is required in order to extract the stationary background emissivity from that produced by the injected impurity.


The required signals to perform the analysis are:

  • The time evolution of the plasma emissivity along the available lines of sight of the tomography camera systems.
  • The radial electric field profile using Doppler reflectometry (DR), measured on the right and left side (respect to the normal incidence angle) of the DR measurement plane.
  • The time evolution of the line-averaged density $ \left<n_e(t)\right> $ with interferometry.
  • The radial profiles of electron density $ n_{e}(r, t_0) $ and temperature at one time instant $ t_0 $ using Thomson Scattering (TS).
  • The time evolution of the electron temperature profile $ T_{e}(r,t) $ with Electron Cyclotron Emission (ECE), when available, calibrated with TS.
  • When possible the time evolution of the main ion temperature profile $ T_{i}(r,t) $ with the CNPA are four positions that are representative of the inner and mid-core and mid- and far edge.

Preferred dates and degree of flexibility

Preferred dates: due to the availability on-site of the proponents the experimental days should fall out of the calender weeks 14, 17, 18 and 19.

References

  1. H. Mynick Calculation of the poloidal ambipolar field in a stellarator and its effect on transport Phys. Fluids 27(8) 2086 (1984)
  2. M. A. Pedrosa et al., Electrostatic potential variations along flux surfaces in stellarators Nucl. Fusion 55 052001 (2015)
  3. J. M. Garcı́a-Regaña et al. Electrostatic potential variation on the flux surface and its impact on impurity transport Nuclear Fusion 57 056004 (2017)
  4. M. A. Ochando et al. Up-down and in-out asymmetry monitoring based on broadband radiation detectors Fusion Sci. and Technol. 50 313 (2006)
  5. J. A. Alonso et al. Parallel impurity dynamics in the TJ-II stellarator Plasma Phys. Control. Fusion. 58(7) 074009 (2016)

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