Study Of The Ablation Of Injected Light-atom Pellets In Tokamak Edge Plasma

Magnetically controlled nuclear fusion device provides a promising way for man to solve the crisis of energy in the long run.Tokamaks have been the most possible device up to date which can be used to realize peaceful use of nuclear fusion energy.Currently,the highconfinement mode（H-mode）is the primary baseline operation mode for the ITER device since is the most reliable and controllable.However,in company with H-mode operation,instability（so called local edge mode,ELM）periodically breaks out in the pedestal region and bursts out high fluxes of particles and energy from the core into the scrape-off-layer.Subsequently,these fluxes deposit a great amount of heat load on a localized region of the plasma-facing wall（PFW）（especially,the divertor）in a short time.The heat load may damage the wall and produce impurity,shorting lifespan of the plasma-facing components and contaminating plasma confinement.To prevent PFW from damage,one must mitigate or suppress giant-ELMs,reducing the energy of ELMs release.Pellet injection is one of two baseline schemes to actively control ELMs.In addition,the scheme is compatible with future reactors.Consequently,it has a good possibility of being adopted in the future.Therefore,it is necessary to understand the physics involved with pellet injection in depth.To start with,an adequate model is needed to model the ablation of a pellet in the pedestal region.The model should take into account the fact that the pellet moves in the background plasma,i.e.,the pellet experiences rapid changes in plasma density and temperature in the pedestal in a H-mode plasma.For this purpose,the previous work for neutral gas shielding（NGS）model is repeated first,in which the NGS model in fact is valid only for core plasma,where the plasma density and temperature are constant.Based on the NGS,the motion of the pellet is considered in the modified model.Thus,its ablation rate is a function of time,which depends on the velocity of the injected pellet.The distribution of neutrals of the injected pellet is given analytically.Profiles of a typical H-mode plasma are then taken,and a Li pellet is injected respectively at different velocities.With dynamic NGS（DNGS）,its ablation rate is evaluated for each velocity.The tempo-distribution of neutrals is subsequently given explicitly for each value.And then the scan of pellet size is also conducted for different injection velocities.The results show that the deposition of the pellet is more sensitive to the size than the velocity.Further,a specific EAST’s Li pellet experiment is numerically reproduced with edge plasma transport package SOLPS with the initial data given by the DNGS.The distributions of different Li species are given in the real Tokamak configuration.This information can be further provided as input for other advanced codes for specific purposes.In addition,real pellet parameters and plasma conditions from the EAST are selected and the ablations of the injection of D2,Li and B pellets into plasma are studied and are compared in detail.All results imply that these low-Z impurity pellets have their own features to pace ELMs.It’s necessary to conduct further study on how the low-Z impurity and fuel pellets affect the background plasma.