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

• Published in: arXiv.org 2012-11
• Main Authors: Migliano, Pierluigi, Camenen, Yann, Casson, Francis, Hornsby, William, Peeters, Arthur
• Format: Article
• Language: English
• Subjects: Ion temperature; Larmor radius; Mathematical models; Temperature gradients
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1211.2692

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 \(\theta = 0\).