Refueling is one of essential methods in order to control plasma density and sustain steady state plasmas. A gas puffing has been successful for building and sustaining a plasma density in an experimental system of past generation. However, in a large scale experimental system, e.g., LHD, the plasma sources induced by the gas puffing are strongly localized near the plasma surface. Then, a pellet injection is placed as a fundamental tool and has been mainly used to obtain a high density plasma and control a density profile.
When a pellet consisting of solid deuterium is injected into torus plasma, it is heated by an energy flux in the plasma, and it changes to vapor though phase transition and subsequently to plasma through atomic processes. A two-dimensional hydrodynamic simulation code CAP has been developed in order to investigate the dynamics of hydrogenic pellet ablation in magnetized plasmas throughout their temporal evolution. One of the properties of the code is that is treats the solid-to-gas phase change at the pellet surface without imposing artificial boundary conditions there, as done in previous ablation models. The simulation includes multi-species atomic processes, mainly molecular dissociation and thermal ionization in the ablation flow beyond the pellet, with a kinetic heat flux model.
In result, a shock wave is found to be driven by ionization in the ablation cloud in the supersonic region of the ablation flow because the ionization reduces expansion of the cloud. Though dissociation dose not drive a shock wave, it induces a plateau region in the ablation cloud. These atomic processes have much effect on spatial profiles of the ablation cloud, but little effect on the ablation rate and pellet life time. Anisotropic heating, due to the directionality of the magnetic field, contributes to a nonuniform ablation (recoil) pressure distribution over the pellet surface. Since the shear stress can exceed the yield strength of the solid for a sufficiently high heat flux, the solid pellet can be fluidized and flattened into a ``pancake'' shape while the pellet is ablating and losing mass. The pellet deformation makes the ablation rate increase, reach a peak, and then fall to zero later on. It can shorten the pellet lifetime almost 3 times from that assuming the pellet remains rigid and stationary during ablation. Increase of the ablation rate induces expansion of the subsonic region around the pellet, and then eliminates the shock. However, the shock appears again when the ablation rate decreases later on. The ablation rate and pellet life time will be evaluated with those effects in the present work.