Effect of static magnetic island on ITG of ADITYA-U tokamak

TL;DR

This work tackles ITG-driven turbulence in tokamak plasmas perturbed by static magnetic islands in the ADITYA-U configuration. It employs the Global Gyrokinetic Code in Cylindrical Coordinates (G2C3) with a neural-network-assisted field-line projection to insert islands at resonant surfaces $q=2$ and $q=3$, adopting a two-phase strategy that first relaxes the island to a flattened density equilibrium and then performs linear ITG analysis on the modified background. The results show island-induced profile flattening and altered field-line topology that reduce the ITG drive and localize potential fluctuations near island boundaries, with toroidal ITG growth rates converging as island width increases and higher-$q$ islands exhibiting more extended mode structures due to longer connection lengths $L_c \\sim qR$. These findings reveal a stabilizing mechanism from island physics and provide a foundation for island-control concepts and confinement optimization in ADITYA-U, guiding future nonlinear investigations of island evolution, zonal flows, and control schemes such as ECCD or RMPs.

Abstract

Magnetic islands play a crucial role in regulating plasma confinement in tokamaks by interacting with micro-instabilities, such as the ion temperature gradient (ITG) mode. This work presents a detailed investigation of the effects of static magnetic islands on ITG instability, relevant to the ADITYA-U tokamak, using the Global Gyrokinetic Code in Cylindrical Coordinates (G2C3), a particle-in-cell (PIC) framework that employs a neural-network-assisted projection scheme. A two-phase simulation strategy is adopted. In the first phase, static magnetic islands with mode numbers (m, n) = (2, 1) and (3, 1) are introduced by perturbing the equilibrium magnetic flux functions. Particle dynamics within these modified topologies result in the flattening of plasma density profiles in the island regions, confirming island formation and its impact on the equilibrium profiles. In the second phase, the flattened profiles serve as new equilibria for linear electrostatic gyrokinetic simulations with adiabatic electrons, enabling the study of the modified ITG behavior. Magnetic islands significantly restructure the ITG mode, producing a spatial redistribution of potential fluctuations within and around the island region. Moreover, as the island width increases, the growth rates of different toroidal ITG modes converge, suggesting a universal stabilization trend. A comparison between the (2,1) and (3,1) islands indicates that higher-q islands lead to a more spatially extended ITG mode structure, reflecting the longer magnetic connection lengths and weaker curvature drive at outer flux surfaces. These results demonstrate the pivotal role of island-induced equilibrium modifications in determining ITG stability and mode structure in tokamak plasmas.

Figures (6)

• Figure 1: (a) Perturbed helical magnetic flux surfaces $\tilde{\psi}_{he}$ showing the formation of a static $(m = 2,\; n = 1)$ magnetic island introduced at the $q = 2$ resonant surface in the ADITYA-U discharge (#36628), plotted on the poloidal plane at $\zeta = 0$. (b) Perturbed helical flux surfaces for a static $(m = 3,\; n = 1)$ magnetic island at the $q = 3$ surface for the discharge(#32802).
• Figure 2: (a) Schematic illustrating the magnetic-island width in flux-space using the perturbed helical flux function $\tilde{\psi}_{he}$. (b) Safety-factor profile $q(\psi)$ showing the resonant surface at which the island forms ( $q=2$ for the $(m,n)=(2,1)$ case).
• Figure 3: (a) Poloidal cross-section showing the perturbed ion density from particle dynamics in the presence of a static $(m=2)$ magnetic island. Black contours represent the island boundaries. Density flattening is visible within the island region due to mirror-like trapping effects. (b) Radial ion density profiles at $\theta = 0$ (low-field side) and $\theta = \pi$ (high-field side), compared with the unperturbed equilibrium profile. Flattening is more prominent on the high-field side, indicating mirror-induced confinement for island width $w = 0.29a$.
• Figure 4: Linear spectrum of the ion temperature gradient (ITG) mode obtained from global gyrokinetic simulations using the G2C3 code. (a) Spectrum for the equilibrium with gradients localized near the q=2 surface. (b) Corresponding spectrum for the equilibrium with gradients localized near the q=3 surface.
• Figure 5: (a) Radial profile of the ion density gradient as a function of normalized poloidal flux $\psi/\psi_w$ for various island widths ($w/a=0$ to $0.4$). Increasing island width leads to localized flattening of the gradient near the resonant surface, suppressing the ITG drive. (b) Corresponding linear growth rates of toroidal ITG modes for different island widths.