Summary of experimental core turbulence characteristics in ohmic and electron cyclotron resonance heated discharges in T-10 tokamak plasmas

Abstract

This report summarizes the results of experimental turbulence investigations carried out at T-10 for more than 10 years. The turbulence characteristics were investigated using correlation reflectometry, multipin Langmuir probe (MLP) and heavy ion beam probe diagnostics. The reflectometry capabilities were analysed using 2D full-wave simulations and verified by direct comparison using a MLP. The ohmic and electron cyclotron resonance heated discharges show the distinct transition from the core turbulence, having complex spectral structure, to the unstructured one in the scrape-off layer. The core turbulence includes 'broad band, quasi-coherent' features, arising due to the excitation of rational surfaces with high poloidal m-numbers, with a low frequency near zero and specific oscillations at 15-30 kHz. All experimentally measured properties of low frequency and high frequency quasi-coherent oscillations are in good agreement with predictions of linear theory for the ion temperature gradient/dissipative trapped electron mode instabilities. Significant local changes in the turbulence characteristics were observed at the edge velocity shear layer and in the core near q = 1 radius after switching off the electron cyclotron resonance heating (ECRH). The local decrease in the electron heat conductivity and decrease in the turbulence level could be evidence of the formation of an electron internal transport barrier. The dynamic behaviour of the core turbulence was also investigated for the case of fast edge cooling and the beginning phase of ECRH.

Identification of trapped electron modes in frequency fluctuation spectra

“…The error bars of the v E×B provided are of the order of 0.5 to 1 km/s. [28,32,33]. As previously reported in [28], coherence shows clear QC modes wheras they are not/barely observable in the spectra of the complex signal shown in figure 5(b)/(c) respectively.…”

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…As previously introduced, in T-10 [7], TEXTOR [6] and Tore Supra [5], reflectometry fluctuation spectra can show QC oscillations (30 < f < 150 kHz) in different regimes. As shown in figure 2, QC modes (Δ ≈ f f) are in-between narrow peaks associated to coherent modes such as MHD or GAM (Δ ⩽ ) f f and broad-band fluctuations induced by turbulence (Δ ≫ f f).…”

“…Initial investigations suggested ITG as a driving mechanism [6,7] but this interpretation has been re-evaluated since regimes in which these modes were observed contradict this interpretation [5].…”

Simulation of core turbulence measurement in Tore Supra ohmic regimes

“…17 The fluctuation spectra inferred from the reflectometry measurements usually show some distinct features in the ITG-and TEM-dominated regimes. While a broadband spectrum is typically observed in the ITG-dominated regimes, the appearance of quasi-coherent modes in the TEM-dominated regimes has been evidenced in a number of the existing fusion devices: T-10, 18 TEXTOR, 19 Tore Supra, 10,13 JET, 20 ASDEX-Upgrade, 10 etc. The quasi-coherent modes associated with TEM-dominated turbulence (QC-TEM) can be identified in the frequency spectra of the reflectometry signals displaying a carrier frequency on the order of magnitude of 50-150 kHz with a bandwidth of about 10-50 kHz.…”