Study of low-frequency core-edge coupling in a tokamak: III. Core-localized MHD continuum pulsations \& distant forced reconnection

Abstract

Slow magnetoacoustic pulsations (SMAPs) are found in MHD simulations of a tokamak plasma whose safety factor $q$ near the center is flat and slightly above unity ($q \gtrsim 1$). SMAPs are located on the central plateau of the slow magnetoacoustic continuum $ω_{\rm S} = k_\parallel c_{\rm S}$, where $c_{\rm S}$ is the speed of sound and $k_\parallel$ the wavenumber parallel to the magnetic field. In our model, SMAPs exist when the ion viscosity and thermal diffusivity are sufficiently low. They can be driven unstable by a pressure gradient in the $q \sim 1$ region when the electric resistivity is sufficiently high. Exponentially growing SMAPs consist of standing slow waves on magnetic surfaces that are radially synchronized into a quasi-interchange structure with poloidal/toroidal mode numbers $m/n=1/1$. After free energy depletion, saturated quasi-linear pulsations (alternating $n=1$ and $0$) in the central $q\sim 1$ region couple to distant $q\geq 2$ rational surfaces that undergo "reversible" magnetic reconnection: as the magnetic islands wax and wane with period $2π/ω_{\rm S}$, their X- and O-points alternate. These results show how the MHD model facilitates non-local coupling of slow waves, pressure-driven resistive interchange and tearing. This motivates further study in kinetic models, where collisionless mechanisms for fast reversible reconnection exist and proper treatment of parallel dynamics will allow to assess the role of Landau damping as well as the question whether the ${\mathbf B}$ field's weak ergodicity in the $q\sim 1$ region allows the waves to outpace the ion's parallel streaming to maintain the thermal misbalance underlying SMAPs. Also of interest are pulsations closer to the Alfvénic branch, satisfying $ω\approx k_\parallel v_{\rm A}$ with Alfvén speed $v_{\rm A}$, which require no resistivity and are less dependent on thermal misbalance.