Control of Transport-Barrier Relaxations by Resonant Magnetic Perturbations

Transport-barrier relaxation oscillations in the presence of resonant magnetic perturbations are investigated using three-dimensional global fluid turbulence simulations from first principles at the edge of a tokamak. It is shown that resonant magnetic perturbations have a stabilizing effect on these relaxation oscillations and that this effect is due mainly to a modification of the pressure profile linked to the presence of both residual magnetic island chains and a stochastic layer.

Figure 1

Effects of RMPs on the dynamics of barrier relaxations: time traces of the thermal energy (a, c, e) and the radial heat flux (b, d, f), in the presence of an imposed mean shear flow with shear rate $\Omega =4$, for (a, b) no RMP perturbation, (c, d) $I_D=0.5\mathrm{kA}$, and (e, f) $I_D=1\mathrm{kA}$.

Figure 2

Effects of resonant magnetic perturbations on (a) the pressure gradient and (b, c) radial heat fluxes, for different values of the perturbation current $I_D$. $Q_{\mathrm{conv}}$ denotes the convective heat flux, and $Q_{\mathrm{RMP}}$ is the conductive heat flux induced by the resonant magnetic perturbations. The total heat flux is $Q_{\mathrm{tot}}=10$.

Figure 3

Components of the radial convective flux: (a) turbulent convective flux $Q_{\mathrm{conv}}^{\mathrm{turb}}$, and (b) equilibrium convective flux $Q_{\mathrm{conv}}^{\mathrm{eq}}$, e.g., RMP linked, for different values of the perturbation current $I_D$. The total heat flux is $Q_{\mathrm{tot}}=10$.

Figure 4

Effects of RMPs on the topology of magnetic field lines: Poincaré map for a perturbation current $I_D=0.5\mathrm{kA}$. There is clear evidence of three residual magnetic island chains, and of a stochastic layer in between the central island chain and the one on the right.

Figure 5

Effects of width $d$ of the shear layer on the dynamics of barrier relaxations: time traces of the radial turbulent flux, in the presence of an imposed mean sheared flow with shear rate $\Omega =2$, for (a) $d=20\%$ and (b) $d=10\%$.