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.
密度均匀化过程中的漂移位移被建模，用于注入大型螺旋装置（LHD）的氢弹丸。针对中性束注入加热等离子体，模拟了弹丸烧蚀和沉积剖面，并显示其能很好地再现低场侧和高场侧（HFS）注入弹丸所观测到的漂移位移的主要特征。该模型描述了电离的烧蚀粒子云团（plasmoid）在非轴对称磁位形中的平行膨胀，以及由弹丸长度上的平均磁场梯度所施加的等离子体团漂移加速力的相关演化。结果表明，在烧蚀和早期均匀化阶段，等离子体团被强烈加速，方向与局部磁场梯度方向相反。在LHD的情况下，其方向和大小主要取决于相对于外部螺旋线圈的弹丸发射位置。虽然这种在烧蚀区域附近引发的初始漂移，一旦等离子体团长度达到与环向连接长度相当的量级，就会因等离子体团内部电流而受到有效阻尼，但在整个均匀化过程中仍维持着一个较弱的漂移加速力，其方向取决于约束磁场是具有磁阱还是磁脊结构。模拟显示，在像LHD这样的强磁脊位形中，这种小而长期的漂移变得显著，即使对于从高场侧注入的弹丸，也会导致质量沉积的径向向外位移。