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

Linear gyro-kinetic stability calculations predict unstable toroidal ion temperature gradient modes (ITGs) with normalised poloidal wave vectors well above one (k(theta)rho(i) > 1) for standard tokamak parameters with adiabatic electron response. These modes have a maximum amplitude at a poloidal angle theta that is shifted away from the low field side (theta not equal 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 reduces the effective drift frequency which allows for the instability to develop. Numerical tests using the gyro-kinetic model confirm this physical mechanism. Furthermore, it is shown that modes localized away from the low field side can be important also for k(theta)rho(i) < 1 close to the threshold of the ITG. In fact, modes with maximum amplitude at theta not equal 0 can exist for normalised temperature gradient lengths below the threshold of the ITG obtained for the case with the maximum at theta = 0. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789856]