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 <i>et al.</i> 1982 <i>Phys. Rev. Lett.</i> <b>49</b>, 1408-1412 (doi:10.1103/PhysRevLett.49.1408)) to the density limit (DL) (Greenwald <i>et al.</i> 1988 <i>Nucl. Fusion</i> <b>28</b>, 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 <i>Plasma Phys. Control. Fusion</i> <b>42</b>, R1-R74 (doi:10.1088/0741-3335/42/1/201); Wagner 2007 <i>Plasma Phys. Control. Fusion</i> <b>49</b>, B1-B33 (doi:10.1088/0741-3335/49/12b/s01)) are applied to the shear layer collapse paradigm (Hong <i>et al.</i> 2017 <i>Nucl. Fusion</i> <b>58</b>, 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 [Formula: see text] 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 <i>Plasma Phys. Control. Fusion</i> <b>44</b>, R27-R53 (doi:10.1088/0741-3335/44/8/201); Greenwald <i>et al.</i> 2014 <i>Phys. Plasmas</i> <b>21</b>, 110501 (doi:10.1063/1.4901920)), as well as the emerging power scaling (Zanca, Sattin, JET Contributors 2019 <i>Nucl. Fusion</i> <b>59</b>, 126011 (doi:10.1088/1741-4326/ab3b31)). The derivation of the power scaling theory exploits an existing model, originally developed for the L [Formula: see text] H transition (Diamond, Liang, Carreras, Terry 1994 <i>Phys. Rev. Lett.</i> <b>72</b>, 2565-2568 (doi:10.1103/PhysRevLett.72.2565); Kim & Diamond 2003 <i>Phys. Rev. Lett.</i> <b>90</b>, 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'.