Characterization of 3D filament dynamics in a MAST SOL flux tube geometry

Abstract

Abstract. Non-linear simulations of filament propagation in a realistic MAST SOL flux tube geometry using the BOUT++ fluid modelling framework show an isolation of the dynamics of the filament in the divertor region from the midplane region due to three features of the magnetic geometry; the variation of magnetic curvature along the field line, the expansion of the flux tube and strong magnetic shear. Of the three effects, the latter two lead to a midplane ballooning feature of the filament, whilst the former leads to a ballooning around the X-points. In simulations containing all three effects the filament is observed to balloon at the midplane, suggesting that the role of curvature variation is sub-dominant to the flux expansion and magnetic shear. The magnitudes of these effects are all strongest near the X-point which leads to the formation of parallel density gradients. The filaments simulated, which represent filaments in MAST, are identified as resistive ballooning, meaning that their motion is inertially limited, not sheath limited. Parallel density gradients can drive the filament towards a Boltzmann response when the collisionalityof the plasma is low. The results here show that the formation of parallel density gradients is a natural and inevitable consequence of a realistic magnetic geometry and therefore the transition to the Boltzmann response is a consequence of the use of realistic magnetic geometry and does not require initializing specifically varying background profiles as in slab simulations. The filaments studied here are stable to the linear resistive drift wave instability but are subject to the non-linear effects associated with the Boltzmann response, particularly Boltzmann spinning. The Boltzmann response causes the filament to self-organise and spin on an axis. In later stages of its evolution a non-linear turbulent state develops where the vorticity evolves into a turbulent eddy field on the same length scale as the parallel current. The transition from interchange motion to the Boltzmann response occurs with increasing temperature through a decrease in collisionality. This is confirmed by measuring the correlation between density and potential perturbations within the filament, which is low in the anti-symmetric state associated with the interchange mechanism, but high in the Boltzmann regime. In the Boltzmann regime net radial transport is drastically reduced whilst a small net toroidal transport is observed. This suggests that only a subset of filaments, those driven by the interchange mechanism at the separatrix, can propagate into the far SOL. Filaments in the Boltzmann regime will be confined to the near separatrix region and quickly disperse. It is plausible that filaments in both regimes can contribute to the SOL transport observed in experiment; the former by propagating the filament into the far SOL and the latter by dispersion of the density within the filament.

Modeling of large amplitude plasma blobs in three-dimensions

“…Although 3D effects are more important than full inertia effects in large tokamaks, plasma blobs in smaller tokamaks and other experimental devices may not suffer strongly from the drift wave instability, and the full inertia term may be more important. 21 The numerical technique described in this paper can be useful to relax the Boussinesq approximation in plasma blobs simulations and could also be useful for other edge phenomena such as edge localized modes (ELMs) and fully turbulent profiles with many blobs present. However, care should be taken when applying the technique outlined in this paper for optimizing the convergence by choosing a relaxation constant.…”

“…Further discussion of the physics of plasma blobs and the derivation of Eqs. (20) and (21) can be found in the aforementioned references in the Introduction.…”

Aspect ratio effects on limited scrape-off layer plasma turbulence

“…12 Although the importance of including a realistic magnetic equilibrium to study turbulence has been known since the early 1980s, 13 only a small number of studies have been carried out in the SOL with an advanced geometry. The mirror force has been found to play an important role in the SOL of MAST, 14,15 while the aspect ratio dependence of the parallel flow profile in the SOL has been investigated in a 2D particle code. 16 Aspect ratio effects are thought to play an important role in determining the characteristic crosssectional area of the SOL and explain in part the improvement of confinement when moving the toroidal limiter from the low-field-side to the high-field-side of the tokamak.…”

X-point modelling in linear configurations using BOUT++

“…An isothermal cold-ion fluid model initially constructed for plasma blob studies [15,16] is extended here to perform initial studies of X-point dynamics. The model is electrostatic, inviscid, and employs the Boussinesque approximation [17,18].…”

“…This model differs from that used in reference [16] in that it incorporates parallel ion free streaming, u , as parallel flows are vital when determining the effects of X-points. Additionally, energy conservation required the restriction that N is considered constant (N 0 ) in terms where it is not differentiated.…”