The role of fast ions in stabilising the ion temperature gradient mode

Abstract: Turbulence driven by small scale instabilities causes large heat and particle transport and is a major limiting factor of current fusion devices. Above a critical value, the ion temperature gradient leads to the growth of a microinstability -- the ion temperature gradient mode -- that often dominates the ion energy transport. It has recently been discovered that energetic ions generated by auxiliary heating may reduce the growth of this instability. By applying the gyrokinetic formalism and performing linear simulations using the local continuum gyrokinetic code GS2, we explore the linear physics of this stabilising effect. In order to isolate important effects due to the presence of fast ions, we make use of the flexibility of GS2 to change the plasma and magnetic geometry parameters independently. We assess the possibility to neglect magnetic geometry changes to simplify the analysis, by investigating its contribution to the stabilising effect. For the cases studied we find that the Shafranov shift and safety factor profile might have to be taken into account. For fixed fast ion density and temperature a destabilising influence of their density gradient is found, while the high fast ion temperature gradient is stabilising, both as predicted by theoretical models. A large part of the observed stabilisation comes from the fast ion contribution to the plasma $\beta$. In addition, the effect of $\beta$ is enhanced because of the large density and temperature gradients of the fast ions. We investigate the role of hot ion mass and charge in order to evaluate the stabilisation of different types of hot ions. Also the possibility of adjusting the electron and ion profiles to account for the presence of fast ions without including them as a kinetic species, is considered. Finally, quasi-linear theory is invoked for linking linear results to saturated values of the nonlinear heat fluxes.