Turbulence and energy confinement in TORE SUPRA Ohmic discharges

Abstract

Results on confinement and turbulence from a set of Ohmic discharges in TORE SUPRA are discussed. Attention is focused on the saturation of the energy confinement time and it is emphasized that this saturation can be explained by a saturation of the electron heat diffusivity. The ion behaviour is indeed governed by dilution and equipartition effects. Although the ion heat transport is never neoclassical, there is no enhanced degradation at saturation. This behaviour is confirmed by turbulence measurements using CO2 laser coherent scattering. The level of density fluctuations follows the electron heat diffusivity variations with the average density. Waves propagating in the ion diamagnetic direction are always present in turbulence frequency spectra. Thus, the saturation cannot be explained by the onset of an ion turbulence. The existence of ion turbulence in the edge at all densities cannot be excluded. However, this ion feature in scattering spectra could be explained by a Doppler shift associated with an inversion point of the radial electric field at the edge

Interchange Turbulence Model for the Edge Plasma in SOLEDGE2D‐EIRENE

“…For ideal collisionless interchange Θ = 5(1 + T i /T e ) [15]. A reasonable critical gradient for interchange can be given by R/λ p > Θ = 10.…”

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…If the onset of an ion mode in the SOC regime was proposed to explain the energy confinement time saturation [39,40], it is now widely believed that the electrons play a major role, as suggested in [41,42]. During the last decade, the conventional understanding of turbulence in Ohmic plasma is that the LOC regime is dominated by TEM and the SOC regime by ITG [3,5,14,[43][44][45][46].…”

Correlation ECE diagnostic in Alcator C-Mod

“…The components in the RF section and the first amplifier for 2-18 GHz signals are installed in front of the diagnostic port as shown in Figure 2(b). The current CECE IF section has 8 channels, two of which are tunable (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). More details about Correlation ECE diagnostic in C-Mod can be found in [5] and upgrades in the IF section will be published in the future [6].…”

“…One hypothesis is that a change of the dominant type of turbulence is responsible for the ohmic confinement transition: Trapped Electron mode (TEM) is dominant in the LOC regime and Ion Temperature Gradient (ITG) mode is dominant in the SOC regime. Although the LOC/SOC transition has been studied extensively in the past [8][9][10][11][12][13], the transition still remains a mystery in fusion transport research. In previous studies, only line integrated n e fluctuations were measured across the LOC/SOC transition [9,13], and local turbulence changes and other fluctuating quantities, such as potential and temperature fluctuations, were not available.…”

“…Although the LOC/SOC transition has been studied extensively in the past [8][9][10][11][12][13], the transition still remains a mystery in fusion transport research. In previous studies, only line integrated n e fluctuations were measured across the LOC/SOC transition [9,13], and local turbulence changes and other fluctuating quantities, such as potential and temperature fluctuations, were not available. Thus, changes in local T e fluctuations across the LOC/SOC transition at C-Mod are new observations, and may be helpful to explain the physics of the transition.…”