Gyrokinetic simulation of the effect of transient fueling on plasma turbulence in ADITYA-U tokamak

Abstract

The gradient-driven microturbulence in ADITYA-U tokamak plasmas has been suppressed by injecting short gas puffs. The suppression of microturbulence increases the core temperature and subsequently the energy confinement time following the gas puff. The gas injection modifies the radial density profile, making it relatively flatter near the mid-radius. Global electrostatic gyrokinetic simulations show that this modification to the radial density profile due to gas injection suppresses the existing trapped electron mode (TEM). Simulation results show that the TEM-dominated turbulence suppression reduces the turbulence-driven heat transport, leading to an increase in core temperature. Applying multiple periodic gas-puffs leads to multiple periodic events of TEM suppression, improving the overall energy confinement time, and is used as an active control mechanism to influence microturbulence in ADITYA-U tokamak.

Figures (3)

• Figure 1: (a) Temporal evolution of plasma parameters (Shot $\#36136$); $I_P$ (black), loop voltage $V_{loop}$ (blue), central chord averaged electron density $\langle n_e\rangle$; central chord averaged SXR intensity and core temperature $\langle T_e \rangle$. (b) The time span $133-151$ ms during the plasma current flat-top (yellow shaded area in (a)) is expanded for all the parameters. The experimental plasma density profiles are shown in (c) and the electron temperature data with a fit is given in (d); here $T_i$ profile is assumed to be one third of $T_e$; The gradients are given in (e) and (f), where $\kappa_X := - d(\log X)/dr$. In (c), the core density is flattened after gas puffing. Correspondingly there is a dip in the gradients shown in (e). The region between the vertical dashed lines indicate the simulation domain, with inner boundary at $\psi/\psi_X=0.1$ and outer boundary at $\psi/\psi_X=0.95$.
• Figure 2: (a) The poloidal cross-section of ADITYA-U flux surface and the corresponding mode structure in the electric potentials are shown for both before and after cases in the linear (TEM) and turbulent regimes. The electrostatic potential is normalized by $T_e/e$; (b) Comparing the energy transport and particle transport for the Before- and After-cases we find all diffusivities have similar growth rates. In particular, The saturation level for After case is reduced by $\approx 84\%$ in $D_e$ and $\approx 94\%$ in $D_i$. The saturation values $\chi_e$ is an order of magnitude higher than $\chi_i$.
• Figure 3: Evolution plots of the radial profiles of turbulent transport fluxes show that in general the radial spread are lower in the after case compared to the before case. The fluctuations start to grow at location where the TEM is strong and spreads radially. The inward propagation of the fluctuations are curtailed after gas puffing.