Pellet induced transition from low to high confinement in separatrix phase spaces of ASDEX Upgrade

Abstract The Separatrix Operational Space (SepOS) concept [T. Eich and P. Manz Nucl. Fusion 61, 086017 (2021)] describes the operational space of tokamaks with respect to MHD, density limits and the L-H transition in terms of density and temperature at the separatrix. It has been inspired by the phase space developed by Rogers, Drake, and Zeiler [Phys. Rev. Lett. 81, 4396 (1998)] (RDZ). Their idea was contested by pellet-induced L-H transitions [P. Gohil et al. Phys. Rev. Lett. 86, 644 (2001)] evaluated inside the pedestal and subsequently this theory was widely dismissed. Whether the SepOS concept face similar challenges when applied to a pellet-induced L-H transition is studied. On the low-density branch of the L-H transition, the separatrix density is temporarily increased by pellet injection, thereby lowering the L-H power threshold in agreement with the SepOS model. The results will also be discussed in RDZ's phase space. We conclude that the basic idea of the plasma edge phase space proposed by RDZ, more precisely that an increase in plasma beta at low collisionalities has a beneficial effect on confinement and at high collisionalities has a detrimental effect on confinement, was discarded unjustly.