Quasi-coherent modes and electron-driven turbulence

Abstract

This letter reports on quasi-coherent (QC) modes observed in fluctuation spectra from Tore Supra and TEXTOR reflectometers. QC modes have characteristics in between coherent and broad-band fluctuations as they oscillate around a given frequency but have a wide spectrum. They are ballooned at the LFS midplane and appear usually on a frequency ranging from 30 to 120 kHz. In ohmic plasmas from both tokamaks, QC modes are detected only in linear ohmic confinement (LOC) regime and disappear in saturated ohmic confinement (SOC) regime. Linear simulations from Tore Supra predict that the LOC and SOC regimes are dominated by electron and ion modes respectively. Measurements of the perpendicular velocity of density fluctuations have been made from the top of TEXTOR by poloidal correlation reflectometry. They suggest that QC modes have a phase velocity ∼400 m s−1 higher in the electron diamagnetic direction than lower frequency fluctuations. Additionally, the onset of QC modes during electron cyclotron resonance heating has been observed in a Tore Supra region where turbulence is suspected to be driven by electron modes. These experimental results and instability calculations show a correlation between onsets of QC modes and predictions of trapped electron modes.

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…However, two dedicated studies involving intrinsic rotation measurements and linear gyrokinetics simulations were performed in a similar Tore Supra experiment [5,48].…”

“…Such measurements were performed from the LFS and the top of TEXTOR. They have shown from the top of the torus only, that QC-TEM rotate faster in the electron diamagnetic direction than lower broadband frequency fluctuations [5]. As Ω ⊥ × E B is constant at a given position and time, only a distinct Ω ⊥ phase can explain such velocity difference (400 m s −1 ).…”

“…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

“…The ohmic discharges exhibit two distinct confinement regimes, the so-called linear ohmic confinement (LOC) and saturated ohmic confinement (SOC) regimes, the transition from one to the other being achieved from a scan of either the density or the plasma current. 13 As it is commonly alleged that, in the most plasma scenarios, the LOC and the SOC regimes are generally dominated by TEM and ITG, respectively, [14][15][16] ohmic discharges are of interest to study separately these two turbulence 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.…”