Fast Low-to-High Confinement Mode Bifurcation Dynamics in a Tokamak Edge Plasma Gyrokinetic Simulation

Abstract

Transport barrier formation and its relation to sheared flows in fluids and plasmas are of fundamental interest in various natural and laboratory observations and of critical importance in achieving an economical energy production in a magnetic fusion device. Here we report the first observation of an edge transport barrier formation event in electrostatic gyrokinetic simulation carried out in a realistic diverted tokamak edge geometry under strong forcing by a high rate of heat deposition. The results show that turbulent Reynoldsstress driven sheared ExB flows act in concert with neoclassical orbit loss to quench turbulent transport and form a transport barrier just inside the last closed magnetic flux surface.

A fast low-to-high confinement mode bifurcation dynamics in the boundary-plasma gyrokinetic code XGC1

“…The scheme allows ambipolar electron and ion loss to the material wall, which results in neutral atomic particle recycling back into the plasma, but still preserving the phasespace volume of the charged particles. The scheme has been parallelized and optimized to reduce data communications in extreme-scale parallel computers, and applied to study the present L-H bifurcation study, 31 the divertor heat-flux footprint study, 62 and other boundary physics. A fast, forced bifurcation of turbulence and transport has been observed for the first time in an electrostatic nonlinear gyrokinetic simulation.…”

“…IV, we describe the simulation setup and present the dynamics of the edge turbulence suppression in more detail than the abbreviated report in the Physical Review Letter article. 31 The conclusions and discussions are summarized in Sec. V. XGCa is the axisymmetric version of XGC1 for gyrokinetic neoclassical physics, uses the same equations and the hybrid-Lagrangian scheme, and will not be specifically discussed in the present report.…”

“…However, the existing models are based upon simplified ad-hoc equations and the turbulence simulations assume specific instability mechanisms, ignore possible important kinetic effects, and are not carried out in a realistic geometry. This paper presents a more detailed report on the first study of edge transport barrier formation dynamics in the Physical Review Letter article, 31 using a first-principles based electrostatic gyrokinetic simulation implemented in XGC1 in realistic edge geometry. 32,33 In the gyrokinetic equations, the fast gyro-motion is analytically treated, thus removing the gyrophase angle variable, while preserving the most basic plasma physics element at first principles level; i.e., the motions of individual particles and their parallel Landau resonance with waves.…”

A hybrid discontinuous Galerkin method for tokamak edge plasma simulations in global realistic geometry

“…A proper understanding of the edge would require full-f gyrokinetics simulations. Pioneering full-f gyrokinetic simulations of the edge start appearing in the fusion community, addressing physical phenomena of fundamental interest for fusion operation like transport barrier formation [6,7]. However, despite the exponential growth of computer speed along with significant improvements in computer technology, they remain extremly costly from the computational point of view.…”

Effect of enhanced dissipation by shear flows on transient relaxation and probability density function in two dimensions

“…For example, stable shear flows can dramatically quench turbulent transport by shear-induced-enhanced-dissipation (see, e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]). This occurs as a shear flow distorts fluid eddies, accelerates the formation of small scales, and dissipates them when a molecular diffusion becomes effective on small scales.…”