Pellet ablation flow near the stagnation plane at low magnetic Reynolds number

The two-dimensional flow pattern of the ablation cloud surrounding a solid pellet embedded in a magnetized plasma is analysed. In the reference frame of the pellet, the magnetic field confines the ablation cloud in the transverse direction. Transverse force balance then determines the width of the parallel to flow channel. The ablation flow is assumed to have spatially uniform temperature and electrical conductivity σ, with magnitudes prescribed by their values at the sonic radius r*, near the pellet where the flow is essentially radial. It is found that the transverse half-width of the cloud — the stagnation radius rb — depends on two key dimensionless parameters: N* = σ*, r* B∞2 /ρ* ν* and p∞/p*, where B∞ is the magnetic field strength in the background plasma. For most representative cases, the cloud width is 10–20 pellet diameters. The implications of these results for pellet ablation studies and studies oriented towards the goal of using (impurity) pellets for fusion plasma diagnostic purposes are discussed.