The processes involving edge plasma and plasma-material interactions in magnetic fusion devices are very multifaceted and include a wide spectrum of phenomena ranging from plasma turbulence and meso-scale stability, recycling and transport processes of hydrogen species in the wall material, to the modification of wall material properties. In many cases these processes are strongly coupled and exhibit synergistic effects. Here we present the results of our studies of a wide range of edge plasma related issues: Our numerical simulations solve a long standing dispute on the roles of impurity radiation loss, plasma volumetric recombination, and ion-neutral friction in the rollover of the plasma flux to the target, which is the manifestation of detachment. We show that the rollover is caused by the increase of the impurity radiation loss and volumetric plasma recombination while the ion-neutral friction, although important for establishing necessary edge plasma conditions, does not contribute per se. With numerical modelling and theoretical analysis we consider stability of detachment and show that the absorption and desorption of hydrogen and impurity species from the wall can be crucial for a global stability of detached plasma. We also identify different mechanisms of meso-scale thermal instabilities driven by impurity radiation and resulting in a self-sustained oscillations of edge plasma parameters. We consider a trapping of He, which is an intrinsic impurity of fusion plasmas, in the first wall tungsten material. Our newly developed model, accounting for the generation of additional He traps caused by He bubble growth, fits all available experimental data on the layer of nano-bubbles observed in W under irradiation of low energy He plasma. Finally, we report on an impact of sheared magnetic field on the dynamics of blobs and ELM filaments playing an important role in edge and SOL plasma transport.
Blobs, high plasma density coherent filamentary structures, propagating in tokamak edge toward outer wall with speed ~1 km/s, play an important role in the scrape-off layer plasma transport in both L-mode and H-mode in between ELMs [1]. Although the blobs are studied for about 15 years, the mechanism(-s) of blob formation is still under debates. Meanwhile
The dynamics of relativistic electrons in the intense laser radiation and quasi-static electromagnetic fields both along and across to the laser propagating direction are studied in the 3/2 dimensional Hamiltonian framework. It is shown that the unperturbed oscillations of the relativistic electron in these electric fields could exhibit a long tail of harmonics which makes an onset of stochastic electron motion be a primary candidate for electron heating. The Poincaré mappings describing the electron motions in the laser and electric fields only are derived from which the criterions for instability are obtained. It follows that for both transverse and longitudinal electric fields, there exist upper limits of the stochastic electron energy depending on the laser intensity and electric field strength. Specifically, these maximum stochastic energies are enhanced by a strong laser intensity but weak electric field. Such stochastic heating would be reduced by the superluminal phase velocity in both cases. The impacts of the magnetic fields on the electron dynamics are different for these two cases and discussed qualitatively. These analytic results are confirmed by the numerical simulations of solving the 3/2D Hamiltonian equations directly.
A 2D slab approximation of the interactions of electrons with intense linearly polarized laser radiation and static electric and magnetic fields is widely used for both numerical simulations and simplified semi-analytical models. It is shown that in this case, electron dynamics can be conveniently described in the framework of the 3/2 dimensional Hamiltonian approach. The electron acceleration beyond a standard ponderomotive scaling, caused by the synergistic effects of the laser and static electro-magnetic fields, is due to an onset of stochastic electron motion.
The generations of zonal flow (ZF) and density (ZD) and their feedback on the resistive drift wave turbulent transport are investigated within the modified Hasegawa-Wakatani model. With proper normalization, the system is only controlled by an effective adiabatic parameter, α, where the ZF dominates the collisional drift wave (DW) turbulence in the adiabatic limit α>1. By conducting direct numerical simulations, we found that the ZF can significantly reduce the transport by trapping the DWs in the vicinities of its extrema for α>1, whereas the ZD itself has little impact on the turbulence but can only assist ZF to further decrease the transport by flattening the local plasma density gradient.
The impact of neutrals on anomalous edge plasma transport and zonal flow (ZF) is considered.As an example, it is assumed that edge plasma turbulence is driven by the resistive drift wave (RDW) instability. It is found that the actual effect of neutrals is not related to a suppression of the instability per se, but due to an impact on the ZF. Particularly, it is shown that, whereas the neutrals make very little impact on the linear growth rate of the RDW instability, they can largely reduce the zonal flow generation in the non-linear stage, which results in an enhancement of the overall anomalous plasma transport. Even though only RDW instability is considered, it seems that such an impact of neutrals on anomalous edge plasma transport, i.e. neutrals enhance the transport via the reduction of ZF, has a very generic feature. It is conceivable that such neutral induced enhancement of anomalous plasma transport is observed experimentally in a detached divertor regime, which is accompanied by a strong increase of neutral density.
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