Heavy impurity transport in tokamaks subject to plasma rotation, NTV and the influence of saturated ideal MHD perturbations

Strongly peaked tungsten accumulation is a common feature of high performance plasma scenarios in JET with the ITER-like wall, particularly during MHD activity induced by m⁄n = 1⁄1 continuous modes. This study investigates the effect of 1⁄1 long living internal kink modes on heavy impurity transport in the presence of strong flows and NTV ambipolar electric field. A novel formulation which includes these effects is presented and applied in the VENUS-LEVIS code in order to follow tungsten ions in a saturated JET-like 1⁄1 internal kinked toroidally rotating plasma configuration. The synergy between 3D magnetic fields, strong flows and NTV is seen to cause tungsten accumulation in contrast to what is observed in similar axisymmetric configurations. Rapid inward transport of impurities in JET plasmas following the triggering of continuous 1⁄1 modes is explained by the work presented here, and we use the same theory to postulate why outward transport can occur in kinked ASDEX-U plasmas.

Ideal magnetohydrodynamic stability in stellarators with subsonic equilibrium flow

The effect of a subsonic flow, inherent to most stellarators because of a radial electric field, on their ideal magnetohydrodynamic (MHD) stability properties is studied employing the quasi-Lagrangian picture developed by Frieman and Rotenberg [1960 Rev. Mod. Phys. 32, 898]. The Mach number of the perpendicular ExB flow in stellarators is of order 0.01 and, therefore, admits the usage of a subsonic approximation in form of a static equilibrium. A mathematical formulation of the weak form of the stability equation with flow has been implemented in the ideal-MHD stability code CAS3D. This formulation uses magnetic coordinates and does not involve any derivatives across magnetic surfaces. In addition to the expected Doppler shift of frequencies, properties of the spectrum of the ideal MHD force operator, which are already known for tokamaks, but now also shown in the stellarator case, are: firstly, the appearance of unstable flow-induced continua stemming from the coupling of sound and Alfven continuum branches with equal mode numbers; and, secondly, the existence of flow-induced, global, stable modes near extrema of sound continuum branches, the extrema, in turn, being generated by the influence of a sheared flow on the static sound continua.

Can Multi-threaded Flux Tubes in Coronal Arcades Support a Magnetohydrodynamic Avalanche?

Magnetohydrodynamic (MHD) instabilities allow energy to be released from stressed magnetic fields, commonly modelled in cylindrical flux tubes linking parallel planes, but, more recently, also in curved arcades containing flux tubes with both footpoints in the same photospheric plane. Uncurved cylindrical flux tubes containing multiple individual threads have been shown to be capable of sustaining an MHD avalanche, whereby a single unstable thread can destabilise many. We examine the properties of multi-threaded coronal loops, wherein each thread is created by photospheric driving in a realistic, curved coronal arcade structure (with both footpoints of each thread in the same plane). We use three-dimensional MHD simulations to study the evolution of single- and multi-threaded coronal loops, which become unstable and reconnect, while varying the driving velocity of individual threads. Experiments containing a single thread destabilise in a manner indicative of an ideal MHD instability and consistent with previous examples in the literature. The introduction of additional threads modifies this picture, with aspects of the model geometry and relative driving speeds of individual threads affecting the ability of any thread to destabilise others. In both single- and multi-threaded cases, continuous driving of the remnants of disrupted threads produces secondary, aperiodic bursts of energetic release.

Finding the mechanism of wave energy flux damping in solar pores using numerical simulations

Context. Solar magnetic pores are, due to their concentrated magnetic fields, suitable guides for magnetoacoustic waves. Recent observations have shown that propagating energy flux in pores is subject to strong damping with height; however, the reason is still unclear. Aims. We investigate possible damping mechanisms numerically to explain the observations. Methods. We performed 2D numerical magnetohydrodynamic (MHD) simulations, starting from an equilibrium model of a single pore inspired by the observed properties. Energy was inserted into the bottom of the domain via different vertical drivers with a period of 30 s. Simulations were performed with both ideal MHD and non-ideal effects. Results. While the analysis of the energy flux for ideal and non-ideal MHD simulations with a plane driver cannot reproduce the observed damping, the numerically predicted damping for a localized driver closely corresponds with the observations. The strong damping in simulations with localized driver was caused by two geometric effects, geometric spreading due to diverging field lines and lateral wave leakage.

Modeling the formation and eruption of coronal structures by linking data-driven magnetofrictional and MHD simulations

<p>The time-dependent magnetofrictional model (TMFM) is a prevalent approach that has proven to be a very useful tool in the study of the formation of unstable structures in the solar corona. In particular, it is capable of incorporating observational data as initial and boundary conditions and requires shorter computational time compared to MHD simulations. To leverage the efficiency of data-driven TMFM and also to simulate eruptive events in the MHD framework, one can apply TMFM up to a certain time before the expected eruption(s) and then go on with simulation in the full or ideal MHD regime in order to more accurately capture the eruption process. However, due to the different evolution processes in these two models, using TMFM snapshots in an MHD simulation is non-trivial with several issues that need to be addressed, both physically and numerically.</p><p> </p><p>In this study, we showcase our progress in using magnetofrictional model results as input to dynamical MHD simulations. In particular, we discuss the incompatibility of the TMFM output to serve as the initial condition in MHD, and show our methods of mitigating this.</p><p>As our benchmark test-case, we study the evolution of NOAA active region 12673, which was previously studied using data-driven TMFM by Price et al. (2019).</p>