Grad-B-driven transport of ionized pellet cloudlets in a tokamak device

The radial transport of ionized pellet cloudlets driven by the toroidal magnetic field gradient is investigated. Such transport occurs on a fast timescale compared to the pellet injection velocity and results in a mass deposition profile that is shifted towards the low field side (LFS) of the tokamak. The pellet cloudlets are formed by periodic disruptions, which are caused via rotational shift instability within the shielding cloud that is attached to the pellet. Owing to the observed transport towards the LFS, the investigation is directed towards cloudlet transport for high field side (BFS) injection scenarios. The assumption of a finite conductivity within the cloudlet along the toroidal field is made, thus allowing the momentum equation to be separated into components along and perpendicular to the field. The one-dimensional gas dynamic equations in lagrangian form are utilized for the pressure relaxation in the direction parallel to the field. These equations are then coupled to the transverse momentum equation via a toroidal drive integral, which is a function of the pressure difference between the cloudlet and the ambient plasma. The initial conditions of the cloudlet are assumed to be those from the attached cloud channel flow just before separation. Such are modeled as a one-dimensional duct flow. The boundary condition for the ambient pressure is specified, and drift distances are then calculated for this fixed pressure condition, which is specified at local peak ablation rate values. The fixed pressure condition is then relaxed, and a variable profile shape for the ambient plasma pressure is employed. The results are compared with experimental shots from the International Pellet Ablation Database (IPAD). The model shows promise when compared to the standard pellet ablation model and experimentally inferred post injection density profiles. The increase in drift distance of a cloudlet for increasing ambient plasma pressure, n_e∞T_e∞, may result in a solution to the fuelling problem on current and future tokamak devices.