Plasma Physics (physics.plasm-ph)

Abstract: A novel channel for fuel ions heating in tokamak core plasma is proposed and analyzed using nonlinear gyrokinetic theory. The channel is achieved via spontaneous decay of reversed shear Alfv\'en eigenmode (RSAE) into low frequency Alfv\'en modes (LFAM), which then heat fuel ions via collisionless ion Landau damping. The conditions for RSAE spontaneous decay are investigated, and the saturation level and the consequent fuel ion heating rate are also derived. The channel is expected to be crucial for future reactors operating under reversed shear configurations, where fusion alpha particles are generated in the tokamak core where the magnetic shear is typically reversed, and there is a dense RSAE spectrum due to the small alpha particle characteristic dimensionless orbits.

Abstract: The first full-F and turbulent simulations based on the Gyro-Moment (GM) are presented by considering a linear device configuration with open and straight field lines. The simulations are based on a simplified version of the gyrokinetic (GK) model proposed by B. J. Frei et al. [J. Plasma Phys. 86, 905860205 (2020)]. By focusing on the electrostatic and long-wavelength limit, a full-F GM hierarchy equation is derived to evolve the ion dynamics, which includes a nonlinear Dougherty collision operator, localized sources, and Bohm sheath boundary conditions. An electron fluid Braginskii model is used to evolve the electron dynamics, coupled to the full-F ion GM hierarchy equation via a vorticity equation. A set of full-F turbulent simulations is performed using the parameters of the LAPD experiments with different numbers of GMs and regimes of collisionality. The GM results (time-averaged profiles and turbulent properties) are compared with those from two-fluid Braginskii simulations, finding good qualitative agreement. Furthermore, the ion distribution function is analyzed, showing the good convergence properties of the GM approach.

Abstract: In decaying magnetohydrodynamic (MHD) turbulence with a strong magnetic field, the spectral magnetic energy density increases with time at small wavenumbers $k$, provided the spectrum at low $k$ is sufficiently steep. This is inverse cascading and occurs for an initial Batchelor spectrum, where the magnetic energy per linear wavenumber interval increases like $k^4$. For an initial Saffman spectrum that is proportional to $k^2$, however, inverse cascading is known not to occur. We study here the case of an intermediate $k^3$ spectrum, which may be relevant for magnetogenesis in the early Universe during the electroweak epoch. This case is not well understood in view of the standard Taylor expansion of the magnetic energy spectrum for small $k$. Using high resolution MHD simulations, we show that also in this case there is inverse cascading with a strength just as expected from the conservation of the Hosking integral, which governs the decay of an initial Batchelor spectrum.