New High-Confinement Regime with Fast Ions in the Core of Fusion Plasmas

The key result of the present work is the theoretical prediction and observation of the formation of a new type of transport barrier in fusion plasmas, called F-ATB (fast ion-induced anomalous transport barrier). As demonstrated through state-of-the-art global electrostatic and electromagnetic simulations, the F-ATB is characterized by a full suppression of the turbulent transport—caused by strongly sheared, axisymmetric $E\times B$ flows—and an increase of the neoclassical counterpart, albeit keeping the overall fluxes at significantly reduced levels. The trigger mechanism is shown to be a mainly electrostatic resonant interaction between suprathermal particles, generated via ion-cyclotron-resonance heating, and plasma microturbulence. These findings are obtained by realistic simulations of the ASDEX Upgrade discharge No. 36637—properly designed to maximized the beneficial role of the wave-particle resonance interaction—which exhibits the expected properties of improved confinement produced by energetic particles.

Figure 1

Time history of (a) injected power and (b) confinement time for the ASDEX Upgrade discharge No. 36637; (c) Power repartition ($\mathrm{NBI}+\text{ICRF}$) among the different plasma species at $t=1.4$ and $t=4.1\mathrm{s}$ as computed by TORIC/SSFPQL by retaining the effect of collisions; (d) Ion and electron thermal conductivities (${\chi }_i$ and ${\chi }_e$, expressed in SI units) computed by ASTRA at $t=1.4$ and $t=4.1\mathrm{s}$; (e) Ratio between ${\chi }_i/{\chi }_e$; (f) Main ion temperature profiles $(T_i)$ at $t=1.4$ (blue) and $t=4.1\mathrm{s}$ (red) and equivalent fast-ion temperature of distribution function of the hydrogen minority ($T_h$ rescaled by a factor of 50), computed by TORIC-SSFPQL at $t=4.1\mathrm{s}$ (black). The vertical black lines denote the region where the logarithmic temperature gradients deviate the most.

Figure 2

Time evolution of the radial profile of the total ion heat flux (thermal $\mathrm{ions}+\text{hydrogen}$) in gyroBohm units. The same magnetic equilibrium and kinetic bulk profiles are employed in the simulations with (a) and without (b) the suprathermal ions. Radial profile of the (c) different species and (d) total (thermal $\mathrm{ions}+\text{electrons}$ + hydrogen) heat flux in MW averaged over $t[c_s/a]=[400-600]$. The red line in (d) represents the volume integral of the injected sources computed by ASTRA. The vertical black lines denote the radial position of the F-ATB, while the shaded-gray areas, the buffer zones.

Figure 3

Time evolution of the radial profile of $v_{E\times B}$ obtained with (a) or without (b) hydrogen minority. The vertical black lines denote the radial position of the F-ATB.

Figure 4

Radial profile of the (a) electrostatic neoclassical heat flux contribution of each species in MW averaged over $t[c_s/a]=[1250-1350]$ and (b) total heat flux (thermal $\mathrm{ions}+\text{electrons}+\mathrm{hydrogen}$) in MW averaged over $t[c_s/a]=[400-600]$. The blue lines in (b) represent the electromagnetic results while the red ones the electrostatic ones with (continuous) and without (dotted) the hydrogen minority. The vertical black lines denote the radial position of the F-ATB, while the shaded-gray areas, the buffer zones.

Figure 5

(a) Linear growth rates at the toroidal-mode number $n=21$—one of the dominant modes in the nonlinear simulations—with (red) or without (blue) hydrogen minority. The black line denotes the radial profile of the energetic particle heat flux averaged over $t[c_s/a]=[400-600]$. Velocity space structure of the saturated nonlinear heat flux (in gyroBohm units) averaged over each direction at (b) ${\rho }_{\mathrm{tor}}=0.225$ and (c) ${\rho }_{\mathrm{tor}}=0.3$. The black contour lines indicate the phase-space resonance positions for $n=4$ and $n=37$ at $z=0$. Electromagnetic fluctuations are neglected in (b) and (c).