Overview
Research into burning plasma presents computational challenges due to complex multi-scale interactions. These involve energetic particles (EPs), Alfven eigenmodes (AEs), and microinstabilities that drive turbulence. Historically, many investigations simplify this complexity by concentrating on individual instabilities, which may lead to an incomplete or inaccurate global instability spectrum and generate inconsistent results.
This work systematically evaluates the influence of commonly adopted assumptions in burning plasma simulations. It aims to establish the minimum necessary set of assumptions for self-consistent modeling, specifically concerning the modes and species present in the system.
Research Context
Burning plasma simulation is computationally demanding. A primary difficulty arises from the multi-scale interactions between energetic particles, Alfven eigenmodes, and various microinstabilities, all of which contribute to turbulence. Past approaches frequently involved isolating a single instability for study, employing corresponding simplifying assumptions. However, these simplifications have been observed to potentially fail in preserving the global instability spectrum, resulting in conflicting findings and inconsistencies across different studies.
Approach
The study utilized the global gyrokinetic code ORB5. This computational tool was employed to systematically assess the impact of various assumptions frequently made in burning plasma simulations on the plasma's response. The methodology focused on identifying the minimal set of assumptions required to achieve a self-consistent model of a burning plasma, considering the full complement of modes and species within the system.
Findings
- It is essential to include both Shafranov shift and finite $\beta$ effects originating from all magnetically confined species in burning plasma simulations.
- Excluding these effects leads to the appearance of unphysical electromagnetic modes, such as internal kinks and kinetic ballooning modes (KBMs), which then falsely dominate the instability spectrum.
- The contribution of energetic particles (EPs) to the Shafranov shift holds particular importance. This EP contribution stabilizes both the toroidal ion temperature gradient (ITG) and toroidal Alfven eigenmode (TAE) instabilities at longer wavelengths (specifically, at low toroidal mode numbers).
- For TAEs, these identified effects significantly reduce the linear growth rate. Instead of increasing proportionally with the EP fraction, the growth rate saturates.
- In the nonlinear regime, the Shafranov shift does not affect ITG-driven heat and particle fluxes when self-consistent magnetic equilibria are employed.
- The nonlinear saturation level of the TAE remains unchanged across all evaluated cases.
- Unlike the ITG case, the Shafranov shift does reduce TAE-driven energetic particle fluxes.
Why This Matters
The identification of essential assumptions for self-consistent burning plasma modeling, particularly regarding the inclusion of Shafranov shift and finite $\beta$ effects from all species, is crucial. Without these inclusions, simulations risk producing unphysical electromagnetic modes that distort the instability spectrum. The observed stabilization of ITG and TAE by energetic particle contribution to the Shafranov shift, alongside its impact on linear growth rates and EP fluxes, refines the understanding of plasma behavior and stability under burning plasma conditions.