Finite β effects on short wavelength ion temperature gradient modes

Abstract

The electromagnetic effect is studied on the short wavelength branch of the ion temperature gradient mode in the linear regime for the first time using a global gyrokinetic model. The short wavelength ion temperature gradient mode growth rate is found to be reduced in the presence of electromagnetic perturbations at finite plasma β. The effect on real frequency is found to be weak. The threshold value of ηi is found to increase for the mode as the magnitude of β is increased. The global mode structure of the short wavelength branch of the ion temperature gradient mode is compared with the conventional branch. The magnetic character of the mode, measured as the ratio of mode average square values of electromagnetic potential to electrostatic potential, is found to increase with increasing values of the plasma β. The mixing length estimate for flux shows that the maximum contribution still comes from the long wavelengths modes. The magnitude of the flux decreases with increasing β.

Gyrokinetic simulation of short wavelength ion temperature gradient instabilities in the ADITYA-U tokamak

“…Here Mα k , k ′ is the convolution matrix [6] in Fourier space and it has been given in the appendix B. The radial and poloidal wave numbers κ and m, respectively, are represented by the wave vector k = (κ, m) when the toroidal mode number n for an axisymmetric system is fixed.…”

“…The ADITYA-U tokamak [12,25,26] which has recently been upgraded to divertor configuration [26], is small in size and well suited for the investigation of micro-instabilities in the presence of density and temperature gradients. The gradient in the plasma profile can drive several temperature and density gradient instabilities, such as ITG [6,[27][28][29][30], trapped electron mode (TEM) [31,32], universal drift modes [33,34] etc which are electrostatic in nature. Similar profile gradients may also produce electromagnetic instabilities such as kinetic ballooning mode (KBM) [35][36][37][38][39][40] and micro tearing mode [41][42][43][44][45] if the plasma β is high [46,47].…”

“…The nature of the conventional instabilities may vary when strong gradients are present. Instabilities at low frequency as compared to the ion gyro-motion frequency and scale length comparable to the ion Larmor radius instabilities [3][4][5][6][7][8] are thought to be the cause of confinement's degradation, which results in anomalous transport of energy and particles. The density and temperature inhomogeneities that are present in a magnetically confined plasma are the source of free energy for these modes.…”

“…For example, it is observed that even at wave lengths k θ ρ s > 1.0, the ion temperature gradient (ITG) mode, which is driven by the temperature gradient of ions, becomes unstable, when the background gradients (density and temperature) are extremely sharp [3,4]. These background gradients tend to drive short scale ITG modes (SWITGs) unstable [3][4][5][6]. Similarly, trapped electron modes can also exhibit a shorter wavelength branch when significant gradients are present [9,10].…”

Low n electromagnetic modes in spherical tokamaks

“…The unstable kinetic ballooning modes (KBMs) [3][4][5][6][7][8][9][10][11][12] are interchange modes due to bad curvature, and thus closely related to ideal MHD ballooning modes; KBMs and ideal ballooning modes have similar β thresholds and are modelled to generate very strong transport in the threshold β regime. Note the contrast with the essentially electrostatic ion-scale and shorter wavelength drift modes such as ion temperature gradient (ITG) modes or trapped electron modes (TEMs), that tend to lead to a softer limit to plasma gradients [13][14][15][16].…”