Dynamics of ion temperature gradient modes in burning plasma conditions in the presence of energetic particles

TL;DR

Using the ORB5 gyrokinetic code, the authors investigate ITG stability in burning-plasma conditions across Maxwellian and slowing-down EP distributions and in both electrostatic and electromagnetic regimes. They confirm two stabilization mechanisms, direct dispersion relation modification (DDRM) active at intermediate EP temperatures and the dilution effect (DE) dominating at high EP temperatures, while slowing-down distributions show no DDRM. Electromagnetic stabilization can overwhelm EP stabilization in ITER-like scenarios, underscoring the importance of beta effects. Overall, the work broadens the parameter space for EP-ITG interactions and informs turbulence and confinement predictions for ITER and future burning plasmas.

Abstract

The interaction between energetic particles (EPs) and ion temperature gradient (ITG) modes is studied using the global particle in cell ORB5 code. In this work, we extend previous studies to a broader range of EP temperatures, including the burning plasma regime and to wider variety of EP distribution functions. Two main stabilization mechanisms are found to be effective in ITG stabilization confirming previous studies: direct dispersion relation modification (DDRM) effective only at intermediate EP temperatures and dilution effect (DE) which is independent of EP temperature and becomes dominant in burning plasma regime ($T_f > 50T_i$). The study is further extended to slowing-down EP distributions which in contrast exhibit no DDRM-related stabilization. The findings are further validated in an ITER pre-fusion operation scenario and additionally compared with electromagnetic effects. In this scenario EP stabilization is found to be weaker than $β$-stabilization. Overall, these results provide better understanding of EP-ITG interactions over a wider range of EP parameters relevant to burning plasma regime which is important for predicting turbulence and confinement in future devices such as ITER.

Figures (7)

• Figure 3.1: a) - growth rate $\gamma(n)$ scan in single toroidal mode number for two cases: without EPs and with EPs of concentration $n_f/n_i=0.1$ and temperature $T_f/T_i=10$. Presence of EPs only suppresses $\gamma (n)$ without changing the spectrum shape; b) - growth rate $\gamma(n)$ scan in EP temperature for constant EP density in stabilizing regime $\eta_f^{-1}=0.15$ (blue) and destabilizing $\eta_f^{-1}=4$ (green), constant EP beta (yellow). DDRM effect is observed at $T_f\approx10\,T_i$ for a blue curve, while the steeper drop is seen for the yellow curve for lower temperatures due to the higher concentration of stabilizing EPs, for the green curve there is no DDRM as the condition on zero of the denominator is not fulfilled
• Figure 3.2: Power exchange between the Maxwellian EPs and ITG from the MPR diagnostic, at different temperatures: a) - $T_f=10\,T_i$. Power exchange is effective in the parabola region signifying an effective DDRM; b) - $T_f=2\,T_i$, power exchange is effective in the region where the most particles are, without effective interaction in the parabola region, DDRM is ineffective. Black parabola corresponds to zero of the denominator of eq. \ref{['eq:F1_DiSiena']}.
• Figure 3.3: a) - growth rate scan in EP temperature from the scalar potential evolution for constant EP density for different EP distributions: Maxwellian, isotropic SD, anisotropic SD with $\xi=-0.66$ and $\sigma=0.22$; b) - EP growth rate scan from the MPR diagnostic DDRM is observed only in the Maxwellian case, while in SD the behavior is monotonous with the EP destabilization as the drive term is always negative
• Figure 3.4: Power exchange between EPs and ITG from the MPR diagnostic in the adhoc case with anisotropic SD EPs with $\xi=-0.66$ and $\sigma=0.22$: DDRM is ineffective, as power exchange occurs not at DDRM condition (black)
• Figure 3.5: PFPO ITER case Hayward2022: a) - Safety factor $q$ profile; b) - temperature and density profiles