An Assessment of Limit Cycle Oscillation Dynamics Prior to L-H Transition

Abstract

In this article, experimental observations of limit cycle oscillations (LCO) that precede L-to-H transition are discussed. Issues are: (1) the existence of zonal flows, (2) spatio-temporal evolutions of turbulence intensity, and (3) periodic generations/decays of mean radial electric field and density. The role of Reynolds stress to accelerate the LCO flow is also addressed. The propagation of changes of the density gradient and turbulence amplitude into the core is commented. Varieties in experimental reports on these issues are explained, and possible origins of different interpretations are discussed. Problem definitions for the future research for resolution are presented.

Low-to-High Confinement Transition Mediated by Turbulence Radial Wave Number Spectral Shift in a Fusion Plasma

“…This new finding may shed light on the recent experimental observations concerning the L-H transition process [14], which appear to be inconsistent with the previous model based on turbulence-driven zonal flows.…”

“…Shunting turbulence energy to zonal flows offers one possible explanation. Given the diverse experimental observations, it is, in fact, reasonable to suspect that there might be multiple mechanisms at play [14]. More recently, a different L-H transition mechanism based on a turbulence radial wave number spectral shift has been proposed [15][16][17][18], where the turbulence suppression at the transition is due to breaking down of the ballooning symmetry by velocity shear in a toroidal system, which leads to a shift in the radial wave number spectrum of turbulence, tilts the turbulence eddies, and scatters the spectral energy to higher k ⊥ (perpendicular wave number) region where the energy is damped, since the viscous dissipation scales with k 2 ⊥ [15].…”

Symmetry-Breaking of Turbulence Structure and Position Identification in Toroidal Plasmas

“…The research has been advanced substantially in the last decade [1][2][3]. Among various progresses, one may point out that the essential elements in these developments are the physics understanding of, such as, (i) the multiple-scale nonlinear interactions (including nonlinear instabilities) , (ii) bifurcation of the combined structure of turbulence and global inhomogeneity [26][27][28][29][30][31][32][33][34][35][36][37][38], and (iii) new aspect of the far-nonequilibrium feature of confined plasmas (i.e., the idea of 'heating heat turbulence', or 'fuelling fuels turbulence') [39,40]. Another decisive progress is the advancement of the data-analysis method, which has enabled one to measure the magnitude of nonlinear interactions quantitatively [41][42][43][44][45], with examples of successful identifications of nonlinear excitations [46][47][48][49][50][51] and the identification of hysteresis in gradient-flux relation [52] (see reviews [53,54]).…”

On the Origin of Steep and Localized Radial Electric Field in the Transport Barrier at Plasma Edge

“…The experimental observations of the steep and localized radial electric field have become abundant [8][9][10][11]. Sequential repetition of growth and decay of the transport barrier was observed just before the L-H transition (so-called limit-cycle oscillation (LCO) [12][13][14][15][16][17][18][19][20][21]), and has stimulated the study of physics of H-mode (see a review [22] and references therein). Electrode-biasing experiments [23,24] have also contributed to our understanding of H-mode transition.…”