TJ-II:Er and turbulence asymmetries in low ripple configurations measured by Doppler reflectometry

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Experimental campaign

2018 Autumn

Proposal title

Er and turbulence asymmetries in low ripple configurations measured by Doppler reflectometry

Name and affiliation of proponent

T. Estrada, J. M. García-Regaña, E. Sánchez, D. Carralero, C. Hidalgo, J.L. Velasco

CIEMAT

Details of contact person at LNF (if applicable)

N/A

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

Motivation

Experimental studies have been performed in TJ-II aiming at the verification of the spatial localization of instabilities predicted by the Gyrokinetic simulations in stellarators [1][2][3] and the verification of the electrostatic potential variation on the flux surface, φ1, as calculated by Neoclassical codes and its possible impact on the radial electric field [4]. The experimental technique used to measure these quantities, Doppler reflectometry, allows the measurement of the density turbulence and its perpendicular rotation velocity at different turbulence scales and with good spatial and temporal resolution [5]. It can cover the radial region from ρ ≈ 0.6 to 0.9, at different perpendicular wave-numbers of the turbulence in the range k⊥ ≈ 1-14 cm-1, and at two plasma regions poloidally separated. The main results, discussed in [6], can be summarized as follows:

  • Er profiles measured at poloidally separated positions in the same flux-surfaces show pronounced differences in low density plasmas, i.e. plasmas in neoclassical electron root confinement. At higher plasma densities the Er asymmetry gradually decreases and almost disappears in ion root plasmas. The asymmetry in the Er profile can be explained to be due to the radial dependence of electrostatic potential varying over the flux surface, φ1 [7].
  • Differences in the turbulence intensity have been found when comparing the k⊥ spectra measured at poloidally separated positions in the same flux-surface. The results are in good qualitative agreement with the spatial localization of instabilities as calculated using the global gyrokinetic code EUTERPE [8].
  • Experiments performed in a magnetic configuration with high rotational transform show a less pronounced and reversed poloidal asymmetry.

Proposal

We propose to explore the influence of the magnetic ripple on the poloidal asymmetry of Er and compare with the Neoclassical expectations. To that end, we propose to explore configurations with different plasma volume, and therefore different magnetic ripple, while keeping the rotational transform profile fixed (as in the standard magnetic configuration). The main properties of the proposed magnetic configurations are summarized in the table. We propose to measure the asymmetry properties of the Er profiles in the four magnetic configurations in low density (0.5 1019 m-3), ECH heated plasmas at maximum power (≈ 500 kW) on-axis. Besides, the asymmetry properties of the k⊥ spectra will be measured in the lowest ripple configuration for comparison with the asymmetry found in the standard one.

Config. Rax (0º) -Zlim(m) ι0 ιa Well(%) ripp_axis ripp_edge a(cm) V(m3)
100_44_64 1.739 0.362 1.551 1.650 2.390 1.900 37.600 20.64 1.0976
071_44_52 1.722 0.319 1.549 1.649 3.200 1.500 29.400 17.37 0.7575
054_43_45 1.709 0.288 1.543 1.648 3.200 ------- ------- 14.63 0.5315
039_42_38 1.699 0.261 1.549 1.674 3.000 2.900 20.000 12.35 0.3741

If applicable, International or National funding project or entity

Proyecto del Plan Nacional, referencia: FIS2017-88892-P

EUROfusion WP.S1

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation:

The characterization of the Er profiles will required four reproducible discharges in each magnetic configuration (4 shots x 4 configurations: 16 discharges); and to properly measured the k⊥ spectra at the two poloidally separated positions a series of about 20 similar discharges is needed in in the lowest ripple configuration.

  • Essential diagnostic systems:

Doppler reflectometer, microwave interferometer, Thomson scattering, ECE, Hα detectors, diamagnetic loop, Rogosky and Mirnov coils, SXR, bolometry, etc.

  • Type of plasmas (heating configuration):

ECH on-axis, full power

  • 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

December

References

  1. M. Nadeem, et al., Phys. Plasmas 8 (2001) 4375
  2. P. Xanthopoulos, et al., Phys. Rev. X 6 (2016) 021033
  3. E. Sánchez, et al., 21st ISHW (2017) Kyoto, Japan
  4. J.M. García-Regaña, et al., Nucl. Fusion 57 (2017) 056004
  5. T. Happel, et al., Rev. Sci. Instrum. 80 (2009) 073502
  6. T. Estrada, et al., IAEA FEC (2018)
  7. J.M. García-Regaña, et al., PPCF 60 (2018) 10402
  8. E. Sánchez, et al., IAEA FEC (2018)

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