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

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 an 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 Reynolds-stress-driven sheared $E\times B$ 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.

Figure 1

Ion temperature ($T_i$) profile in the greater edge region of the modeled C-Mod plasma at three different times. The effect of the heating on $T_i$ can be seen to be significant in the edge layer.

Figure 2

Time behavior at ${\mathrm{\Psi }}_N=0.975$ of (a) ${(\delta n/n)}^2$, $E\times B$ shearing rate, and $E\times B$ flow; (b) electron heat flux; (c) $E\times B$ shearing rate in radius; (d) Reynolds force at ${\mathrm{\Psi }}_N=0.972$ and 0.984; and (e) orbit-loss force.

Figure 3

Normalized consumption rate $⟨{\tilde{v}}_r{\tilde{v}}_{\theta }⟩V_{\perp }^{\prime }/({\gamma }_{\text{eff}}{\tilde{v}}_{\perp }^{\prime }/2)$ of the turbulence kinetic energy ${\tilde{v}}_{\perp }^2/2$ by the shearing $V_E^{\prime }$ at ${\mathrm{\Psi }}_N=0.972$ around the bifurcation time, using the critical shearing rate 300 kHz as the effective source rate (${\gamma }_{\text{eff}}$). The turbulence intensity dynamics is also shown.