Nonlinear Stabilization of Tokamak Microturbulence by Fast Ions

Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al. [Phys. Rev. Lett. 107, 135004 (2011)], this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.

Figure 1

Ion heat flux versus $R/L_{Ti}$, from JET data presented in Ref. [7] showing a separation between high and low stiffness regimes at $\rho =0.33$. The specific discharges studied in this Letter are circled.

Figure 2

Ion heat flux in nonlinear GENE $R/L_{Ti}$ and ${\gamma }_E$ scans based on discharge 70084 parameters at $\rho =0.33$ ($q/ϵ=11.8$ for circular geometry). $\widehat{s}/q=0.2/1.3$ throughout. Results are shown both including (a) and neglecting (b) PVG destabilization. All runs were electrostatic, collisionless, used circular geometry, and assumed $T_e/T_i=1$. The results are compared with the low stiffness data from Ref. [7].

Figure 3

Linear growth rates (a) and nonlinear ion heat fluxes (b) calculated in ${\beta }_e$ and $R/L_{Ti}$ scans based on discharge 66404 parameters. In the linear scan, $k_y=0.4$ in units of $1/{\rho }_s$. Runs included collisions, experimental geometry, two species, and assumed $T_e/T_i=1$.

Figure 4

The comparative influence of thermal and suprathermal pressure components on $\alpha$ for discharge 66404. Increased $\alpha$ corresponds qualitatively to increased electromagnetic stabilization. The strong influence of suprathermal pressure for $\rho <0.4$ coincides with the measured low stiffness zone.

Figure 5

Comparison of nonlinear GENE simulations and experimental ion heat flux measurements for the five separate discharges at $\rho =0.33$. The importance of the fast ion contribution is underlined by the sensitivity studies carried out for discharges 66404 and 73224. The dashed lines connect the results of the nominal 66404 and 73224 simulations with results obtained at reduced $R/L_{Ti}$.