How the birth and death of shear layers determine confinement evolution: from the L → H transition to the density limit

Abstract

Electric field profile structure—especially its shear—is a natural order parameter for the edge plasma, and characterizes confinement regimes ranging from the H-mode (Wagner et al. 1982 Phys. Rev. Lett. 49 , 1408–1412 ( doi:10.1103/PhysRevLett.49.1408 )) to the density limit (DL) (Greenwald et al. 1988 Nucl. Fusion 28 , 2199–2207 ( doi:10.1088/0029-5515/28/12/009 )). The theoretical developments and lessons learned during 40 years of H-mode studies (Connor & Wilson 1999 Plasma Phys. Control. Fusion 42 , R1–R74 ( doi:10.1088/0741-3335/42/1/201 ); Wagner 2007 Plasma Phys. Control. Fusion 49 , B1–B33 ( doi:10.1088/0741-3335/49/12b/s01 )) are applied to the shear layer collapse paradigm (Hong et al. 2017 Nucl. Fusion 58 , 016041 ( doi:10.1088/1741-4326/aa9626 )) for the onset of DL phenomena. Results from recent experiments on edge shear layers and DL phenomenology are summarized and discussed in the light of L → H transition physics. The theory of shear layer collapse is then developed. We demonstrate that shear layer physics captures both the well known current (Greenwald) scaling of the DL (Greenwald 2002 Plasma Phys. Control. Fusion 44 , R27–R53 ( doi:10.1088/0741-3335/44/8/201 ); Greenwald et al. 2014 Phys. Plasmas 21 , 110501 ( doi:10.1063/1.4901920 )), as well as the emerging power scaling (Zanca, Sattin, JET Contributors 2019 Nucl. Fusion 59 , 126011 ( doi:10.1088/1741-4326/ab3b31 )). The derivation of the power scaling theory exploits an existing model, originally developed for the L → H transition (Diamond, Liang, Carreras, Terry 1994 Phys. Rev. Lett. 72 , 2565–2568 ( doi:10.1103/PhysRevLett.72.2565 ); Kim & Diamond 2003 Phys. Rev. Lett. 90 , 185006 ( doi:10.1103/PhysRevLett.90.185006 )). We describe the enhanced particle transport events that occur following shear layer collapse. Open problems and future directions are discussed. This article is part of a discussion meeting issue ‘H-mode transition and pedestal studies in fusion plasmas’.

The power dependence of the maximum achievable H-mode and (disruptive) L-mode separatrix density in ASDEX Upgrade

“…We can call the high density phase, the DL phase. In analogy to phase coexistence triple points in thermodynamics, one would expect the existence of a DL-L-H triple point [26]. Only the ES-EM transition in SepOS shows such a triple point [16].…”

“…The Giacomin-Ricci model n ∝ P 0.48 SOL is slightly outside the regression result, but here the density at ρ = 0.9 should be used. The Singh-Diamond scaling is for the edge ion heat flux [26], where in [11] it is the external input total power.…”

“…The existence of the DL-H gap directly contradicts the existence of a DL-L-H triple point in the (n e ,T e ) phase diagram. The SepOS concept, the Giacomin-Ricci and the work by Diamond [26], all assume the existence of three phases in the confinement; the L-mode with high turbulent transport levels, the H-mode with suppressed turbulent transport, and the high-density phase with strongly enhanced nearcatastrophic turbulent transport. We can call the high density phase, the DL phase.…”

“…The radiation collapse is believed to occur at the end of the events that lead to the disruption. Let us move on to the initiation phase which is though to be related to enhanced transport [6,26,[50][51][52]. Shear layers are known to suppress turbulent transport.…”

H-mode transition and pedestal studies

“…Thyagaraja et al [ 4 ] report CUTIE simulation results on edge localized mode (ELM) control by resonant magnetic perturbations (RMP) fields, highlighting the bifurcation phenomenon in the RMP amplitude in ELM suppression; Andrew et al [ 5 ] present the first experimental results of transition from L-mode to dithering H-modes on the compact, spherical tokamak ST40 and extend the L-H transition predator–prey model to include the contribution from MHD instabilities and pedestal pressure gradient. A comprehensive review of the role of sheared plasma flow on the evolution of confinement from the L-H transition to the H-mode density limit is provided by Diamond et al in [ 6 ]. Yan et al [ 7 ] provide detailed observations of edge plasma turbulence flow and analyse their effect on the transition to H-mode on the DIII-D experiment.…”