The separatrix operational space of ASDEX Upgrade due to interchange-drift-Alfvén turbulence

Abstract

The efficient operation of a tokamak is limited by several constraints, such as the transition to high confinement or the density limits occurring in both confinement regimes. These particular boundaries of operation are derived in terms of a combination of dimensionless parameters describing interchange-drift-Alfvén turbulence without any free adjustable parameter. The derived boundaries describe the operational space at the separatrix of the ASDEX Upgrade tokamak, which is presented in terms of an electron density and temperature existence diagram. The derived density limits are compared against Greenwald scaling. The power threshold and role of ion heat flux for the transition to high confinement are discussed.

Turbulent broadening of electron heat-flux width in electromagnetic gyrokinetic simulations of a helical scrape-off layer model

“…In figure 4 we show midplane profiles of α for each case. The range of maximum values α = 0.3 − 1.5 is similar to the range observed in experiments, although in realistic geometry the ideal limit can be somewhat higher so that experimental α values may not exceed the ballooning limit 24,25,63 . The fact that we observe midplane values of α that exceed the ideal ballooning limit α = 1 for our geometry will be examined in a separate paper 64 .…”

“…The edge/SOL region features steep pressure gradients, especially in the H-mode transport barrier and SOL regions, which can contribute to the importance of electromagnetic effects. Experimental evidence has indicated that the edge plasma state is controlled by electromagnetic drift-interchange dynamics [23][24][25] . In this regime, the parallel electron dynamics is no longer very fast relative to the drift turbulence, so electrons can no longer be treated adiabatically 26 .…”

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

“…The L-H transition (grey line) occurs when the turbulence suppression by the E × B flow shear caused by the background ion pressure gradient, which is consistent with the neoclassical E × B shear, is stronger than the turbulent growth rate. Additionally diamagnetic and β stabilization of turbulence are included [90]. As shown in figure 9, L-H transitions (grey line) only occur at separatrix densities n e,sep 2.5 × 10 19 m −3 .…”

Reduction of transport due to magnetic shear in gyrokinetic simulations of the scrape-off layer

“…Extension of the sheath-modified interchange-ballooning analysis in helical geometry presented in [27] to include magnetic shear is left to future work, but we do plot α = γ 2 int /(k 2 0 v 2 A ) = (L c /π) 2 (β e + β i )/(RL p ) evaluated at the midplane in figure 6, where here k 0 = π/L c itself varies with radius because of the variation of L c . Taking the peak α value from each the case, we have a range α ≈ 0.3 − 3, which is in the range of mea-surements of α in the near SOL of present experiments [39,40,41].…”