[Paper Review] Effect of collisions on non-adiabatic electron dynamics in ITG-driven microturbulence

C J Ajay, S. Brunner+1 more|arXiv (Cornell University)|Apr 26, 2021

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TL;DR

This study investigates how electron collisions affect non-adiabatic electron dynamics in ion temperature gradient (ITG) microturbulence, showing that increased collisionality reduces the linear growth rate of ITG eigenmodes by making electron response more adiabatic away from mode rational surfaces (MRSs). Collisions broaden radial mode structures and scale the parallel scale length of eigenmode tails with the electron mean free path, while fine-structures at MRSs and their self-driven zonal flows persist under realistic collisionality.

ABSTRACT

Non-adiabatic electron response leads to significant changes in ion temperature gradient (ITG) eigenmodes, leading, in particular, to fine-structures that are significantly extended along the magnetic field lines at corresponding mode rational surfaces (MRSs). These eigenmodes can nonlinearly interact with themselves to drive zonal flows via the so-called self-interaction mechanism. In this paper, the effect of collisions on these processes are studied. In the presence of non-adiabatic electrons, the linear growth rate of ITG eigenmodes decreases with the increasing collisionality. Detailed velocity space analysis of the distribution function shows that this results from collisions leading to a more adiabatic-like response of electrons away from MRSs. In linear simulations, collisions are furthermore found to broaden the radial width of the fine-structures, which translates to narrower tails of the eigenmode in extended ballooning space. The characteristic parallel scale length associated with these tails is shown to scale with the mean free path of electrons. In nonlinear turbulence simulations accounting for physically relevant values of collisionality, the fine-structures located at MRSs, together with the associated drive of zonal flows via self-interaction, are shown to persist and play a significant role. Published under an exclusive license by AIP Publishing.

Motivation & Objective

• To understand the impact of electron collisions on non-adiabatic electron response in ITG-driven microturbulence.
• To analyze how collisionality alters the linear growth rate and radial structure of ITG eigenmodes.
• To investigate the persistence of fine-structures at mode rational surfaces (MRSs) and their role in zonal flow generation under realistic collisionality.
• To determine the scaling of parallel scale lengths in eigenmode tails with electron mean free path.

Proposed method

• Linear and nonlinear gyrokinetic simulations are used to model ITG-driven microturbulence with varying collisionality.
• Velocity space analysis of the electron distribution function is performed to assess the transition from non-adiabatic to adiabatic-like response.
• Ballooning mode representation is employed to examine radial and parallel structure of eigenmodes.
• The self-interaction mechanism for zonal flow generation is analyzed in the presence of collisions.
• Collisionality is varied to study its effect on mode growth rates, radial width, and parallel scale lengths.
• Physical values of collisionality are used in nonlinear simulations to assess real-world relevance.

Experimental results

Research questions

• RQ1 How does increasing collisionality affect the linear growth rate of ITG eigenmodes with non-adiabatic electrons?
• RQ2 What is the role of collisions in modifying the radial width and parallel scale length of fine-structures in ITG eigenmodes?
• RQ3 How do collisions influence the transition from non-adiabatic to adiabatic electron response away from mode rational surfaces?
• RQ4 To what extent do fine-structures at MRSs and their self-interaction mechanism for zonal flow generation persist under realistic collisionality?
• RQ5 What is the scaling relationship between the parallel scale length of eigenmode tails and the electron mean free path?

Key findings

• Increased collisionality reduces the linear growth rate of ITG eigenmodes due to a more adiabatic-like electron response away from mode rational surfaces (MRSs).
• Collisions broaden the radial width of fine-structures in the eigenmodes, leading to narrower tails in extended ballooning space.
• The characteristic parallel scale length of eigenmode tails scales with the electron mean free path.
• Fine-structures localized at MRSs persist in nonlinear simulations with physically relevant collisionality levels.
• The self-interaction mechanism for zonal flow generation remains active and significant in the presence of collisions.
• The transition to more adiabatic electron response is quantitatively linked to collisional scattering in velocity space, reducing non-adiabatic effects.