Modelling of the pellet deposition profile and ∇B-induced drift displacement in non-axisymmetric configurations

Abstract

Drift displacement during density homogenization is modelled for hydrogen pellets injected into the Large Helical Device (LHD). The pellet ablation and deposition profiles are simulated for neutral-beam injection heated plasmas and are shown to reproduce well the main characteristics of the observed drift displacement for both low-field side and high-field side (HFS) injected pellets. The model describes the parallel expansion of ionized ablated pellet particle cloudlets (plasmoid) in non-axisymmetric magnetic configurations and the associated evolution of the plasmoid drift acceleration force exerted by the average magnetic field gradient over the plasmoid length. It is shown that, during the ablation and early homogenization phases, plasmoids are strongly accelerated towards the inverse direction of the local magnetic field gradient. In the case of the LHD, its direction and magnitude depend mainly on the pellet launching location with respect to the external helical coils. While such an initial drift—induced near the ablation region—is efficiently damped by plasmoid internal currents as soon as the plasmoid length becomes comparable to a toroidal connection length, a weak drift acceleration force is maintained over the whole homogenization time, whose direction depends on whether the confining magnetic field possesses a magnetic well or hill structure. Simulations show that, in a strong magnetic hill configuration like the LHD, this small but long-term drift becomes significant and results in a radially outward displacement of the mass deposition even for pellets injected from the HFS.

Pellet Core Fueling in Tokamaks, Stellarators and Reversed Field Pinches

“…7, [24]). Conversely, in stellarators, if the time τ EC is of the same order of magnitude as in tokamaks, the length Z c beyond which the IC becomes efficient is only half the toroidal period of the configuration, Z C ∼ πR m period , where m period is the number of periods [22], Fig. 6b.…”

“…Locally, the damping of the drift is very efficient and the material deposited at this place takes the shape of an overdense poloidal ring that expands in the toroidal direction more slowly than the matter deposited deeper in the discharge [25]. The model described above was implemented in different codes adapted to the tokamak configuration 2 or to the stellarators [22,28] and checked against experiments over a significant number of devices ([2, 3] and references therein, [22,[29][30][31][32]).…”

Using rational surfaces to improve pellet fuelling in stellarators

“…However, close to the integer rational surfaces, it decreases down to 2πqR, where q is the safety factor, which explains the aforementioned major role of the iota profile in stopping the drift (Geulin & Pegourie 2022). The drift-damping produced by external currents is the dominant effect in tokamaks, whereas internal currents are dominant in helical configurations due to their shorter plasmoid-internal charge reconnection lengths (Matsuyama et al 2012a). However, a previous study in TJ-II revealed an interaction between outward-drifting plasmoids and low-order rational surfaces that can be attributed to the ECC effect (McCarthy et al 2021).…”

“…The resultant ∇B-drift arises from a modification of the potential distribution inside a plasmoid (Rozhansky et al 2004;Parks & Baylor 2005;Pégourié et al 2007). For arbitrary magnetic configurations, the main term of cloud drift acceleration is given by (Köchl et al 2012;Matsuyama et al 2012a)…”

“…2012; Matsuyama et al. 2012 a ) However, several mechanisms can either enhance (Parks & Baylor 2005) or reduce (Rozhansky et al. 2004; Senichenkov et al.…”