Dynamics of seeded blobs under the influence of inelastic neutral interactions

Abstract

Field-aligned filaments, the so-called blobs, born at the edge of the magnetically confined region of tokamaks propagate radially outward into the scrape-off layer (SOL) region that allows for a substantial population of neutral particles compared to the region of confinement. The electrons and ions constituting the blob undergo both elastic and inelastic collisions with the neutral particles, and the latter leads to sources and sinks of the blob density, momentum, and heat. The influence of the inelastic collisions with neutrals on the evolution of seeded blobs is investigated numerically by the nHESEL drift-fluid model through a series of discrete scans in interactions, active source terms, and blob plasma parameters. In light of the results, the potential influence of local inelastic collisions on the SOL density shoulder formation is discussed. It is found that density sources increase the blob compactness, which delays the blob dispersion and decreases the dispersion rate. Density sources or momentum sinks also influence the blob dynamics by increasing the vorticity layer around the perturbation, whereas the pressure sources/sinks only affect the blob dynamics marginally. The change to the vorticity structure leads, in most cases, to a decrease in the radial velocity of the blob center of mass, although, at high source rates, a radial acceleration of the blob center of mass is also observed. Density sources may, thus, contribute to shoulder formation not only by increasing the density locally but also by changing the filament dynamics.

On the interpretation of kinetic effects from ionization in fluid models and its impact on filamentary transport

“…In Figure 3 an instance of the blob density is shown at various time instances for all four simulation cases together with the absolute difference between cases 1 and 2, and cases 3 and 4. For all cases the blob develops asymmetrically due to a finite ion temperature and the warmer blob, with a higher ionization rate, is also observed to remain more coherent, which is consistent with the findings in [35][36][37]. The difference between the rightmost frames in Fig.…”

Physics-informed machine learning techniques for edge plasma turbulence modelling in computational theory and experiment

“…collisionality, 𝛽, closed flux surfaces with sheared magnetic field lines) using electromagnetic fluid theory is the subject of future work. Also, since plasma fluctuations can now be directly compared across models, as gyrokinetic codes begin including fully kinetic neutrals, this optimization technique can help validate reduced source models to accurately account for atomic and molecular interactions with plasma turbulence in the edge of fusion reactors [210,211] since these processes (e.g. ionization, recombination) affect the local electric field.…”

“…Model approximations are thus inevitable, but once approximations are introduced into edge turbulence models, their effects on nonlinear dynamics need to be quantitatively evaluated-against higher fidelity simulations and/or experiment, if possiblefor model predictions to be meaningful. An oft-applied set of equations to study the plasma edge is drift-reduced Braginskii fluid theory, which has been studied for decades starting from Braginskii's original formulation [14], and remains a highly active research area today [47,207,85,200,236,211,66,30]. But the underlying model approximations tend to only be marginally satisfied in fusion plasmas and there is a scarcity of clear-cut validation tests of the theory against higher-fidelity simulations or experimental measurements.…”

“…Transport is primarily along blobby field-aligned structures with increased pressure propagating due to perpendicular drifts which polarize the blob and yield outward E × B drift of the filament. This is related to the Poynting vector representing the directional energy flux density of the electromagnetic field [211,190] the synthetic plasma being analyzed. The lower limit of the Bohm criterion, a necessary condition for the formation of a stationary Debye sheath [183]-the transition from a plasma to a solid surface-is imposed as a parallel boundary condition,…”

“…For example, the analytic form of reduced source models in fluid theories [210,211] can be inserted in the physics-informed deep learning framework to account for local turbulent ionization and inelastic collisions with kinetic neutrals by observing such measurements of 𝑛 𝑒 , 𝑇 𝑒 , and 𝜑 in global simulations [223] and experiments [142].…”