Ion temperature gradient instability at sub-Larmor radius scales with non-zero ballooning angle

TL;DR

This paper investigates ion temperature gradient instabilities at sub-Larmor radius scales with non-zero ballooning angles, revealing that modes shifted away from the low field side can develop and influence plasma stability.

Contribution

The study demonstrates that ITG modes with non-zero ballooning angles are significant at small scales and near the instability threshold, expanding understanding of plasma turbulence mechanisms.

Findings

Modes with $ heta e 0$ exist for $k_ heta ho_i > 1$ and influence stability.

Shifted modes reduce effective drift frequency, enabling instability.

Non-zero ballooning angle modes can occur below traditional ITG thresholds.

Abstract

Linear gyro-kinetic stability calculations predict unstable toroidal Ion Temperature Gradient modes with normalised poloidal wave vectors well above one ($k_\theta \rho_i > 1$) for standard parameters and with adiabatic electrons. These modes have a maximum amplitude at a poloidal angle $\theta$ that is shifted away from the low field side ($\theta \ne 0$). The physical mechanism is clarified through the use of a fluid model. It is shown that the shift of the mode away from the low field side ($\theta \ne 0$) reduces the effective drift frequency, and allows for the instability to develop. Numerical tests using the gyro-kinetic model confirm this physical mechanism. It is furthermore shown that modes with $\theta \ne 0$ can be important also for $k_\theta \rho_i < 1$ close to the threshold of the ITG. In fact, modes with $\theta \ne 0$ can exist for normalised temperature gradient lengths below the threshold of the ITG obtained for $\theta = 0$.