Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

Abstract

Abstract. The experimental observation made on the TCV tokamak of a significant confinement improvement in plasmas with negative triangularity ( δ < 0) compared to those with standard positive triangularity has been interpreted in terms of different degrees of profile stiffness (Sauter et al. Phys. Plasmas 21 055906, 2014) and/or different critical gradients. Employing the Eulerian gyrokinetic code GENE (Jenko et al. Phys. Plasmas 7 1904, 2000, profile stiffness and critical gradients are studied under TCV relevant conditions. For the considered experimental discharges, trapped electron modes (TEMs) and electron temperature gradient (ETG) modes are the dominant microinstabilities, with the latter providing a significant contribution to the non-linear electron heat fluxes near the plasma edge. Two series of simulations with different levels of realism are performed, addressing the question of profile stiffness at various radial locations. Retaining finite collisionality, impurities and electromagnetic effects, as well as the physical electron-to-ion mass ratio are all necessary in order to approach the experimental flux measurements. However, flux-tube simulations are unable to fully reproduce the TCV results, pointing towards the need to carry out radially nonlocal (global) simulations, i.e. retaining finite machine size effects, in a future study. Some conclusions about the effect of triangularity can nevertheless be drawn based on the flux-tube results. In particular, the importance of considering the sensitivity to both temperature and density gradient is shown. The flux tube results show an increase of the critical gradients towards the edge, further enhanced when δ < 0, and they also appear to indicate a reduction of profile stiffness towards plasma edge.

Identifying microturbulence regimes in a TCV discharge making use of physical constraints on particle and heat fluxes

“…Finally, since TCV is a small-sized tokamak, it is expected that finite ρ effects could lead to a significant discrepancy between local flux-tube results and global ones [19] (ρ ∼ 1/100 at ρ tor = 0.6). Previous turbulent transport studies of TCV discharges using flux-tube models, addressing internal transport barriers [21] and plasma shaping effects [20] have pointed out the importance of these effects. This will involve carrying out simulations using the global version of the GENE code.…”

“…The results presented in this paper are limited to local simulations, even though finite machine size effects requiring so-called global simulations are known to be important in smaller size tokamaks like TCV [19][20][21]. It has indeed been shown that important properties of such smaller systems can be understood by performing simpler local simulations [20]. After this first step, we plan in the future to also carry out global GENE simulations.…”

Investigating the radial structure of axisymmetric fluctuations in the TCV tokamak with local and global gyrokinetic GENE simulations

“…The focus of this paper is indeed on qualitatively reproducing the experimental observations regarding the spatial nature of axisymmetric fluctuations. We have in particular investigated the effect of finite collisionaly, a parameter know to have a strong impact on turbulence in TCV relevant conditions [28] and in general affecting the dynamics of zonal flows, and of the safety factor profile. A quantitative investigation, aiming at reproducing both the experimental transport and fluctuation level, will likely require not only to adopt the realistic deuterium-to-electron mass ratio but also to include impurities which are known to potentially strongly affect the turbulence level [28] as well as the linear GAM frequency [7].…”

The effect of triangularity on fluctuations in a tokamak plasma

“…Other non-linear simulations performed using the GENE code [11], on the same experimental proles and equilibria, again found TEM dominated uctuations, reproduced the heat ux dierence at the plasma edge and suggested that, to reproduce the penetration of the heat ux reduction in the plasma core, global simulations would be necessary. Moreover, indications were found that changing triangularity from positive to negative increases the critical electron temperature gradients for the saturated heat ux [7] number radiative temperature uctuations. It exploits cross-correlation analysis on close but not overlapping plasma volumes to detect small uctuations (<1%).…”