The feasibility of re-fuelling a fusion reactor by injecting pellets of frozen hydrogen isotopes is reviewed. First a general look is taken of the dominant energy fluxes received by the pellet, the re-fuelling rate required and the relation between pellet size, injection speed and frequency. Current available theories of pellet ablation are then discussed. For a given penetration depth inside the reactor, the necessary pellet injection speed is examined in terms of the ablation theory adopted and the temperature and density profiles of the reactor plasma. The interaction between the injected pellet and the background plasma is described with reference to some of the avaialbe tokamak transport codes; its relation to the ignition requirement is mentioned. Various types of pellet sources and different approaches to injection are described and assessed. Past experimental efforts on pellet ablation are summarized and compared with theories. Subjects requiring further investigation are pointed out.

No abstract

The magnetic shielding effect of a refuelling pellet is considered by first briefly reviewing the existing balloon model. The limitation of the model is pointed out and discussed. Since solid deuterium is an insulator and the ablated plasma is expected to be cold and dense, it is felt that the existence of a field-free balloon is questionable; rather some field will be trapped by the pellet. On the assumption that the flow of the ablated plasma is governed by a flexible magnetic nozzle, it is shown that the reduction of the ablation rate of the pellet is not due to the exclusion of the field by a balloon, but to the reduction of the ablated plasma pressure through the trapping of the field inside the nozzle. The model also indicates that the penetration of a mm-size pellet into the centre of a low-β reactor might be possible, provided an injection speed around 104 m ·s−1 can be attained and no appreciable deceleration process is present.

No abstract

By incorporating a simplified collisional-radiative model into a self-limited ablation model, the correlation between the emission rate of the H a -line and the ablation rate of a hydrogen pellet in tokamak discharges is studied. In normal Ohmically heated discharges, the fuel deposition profile is reproduced accurately by the H a -line emission profile, except for a slight delay of the maximum ablation rate with respect to the maximum emission rate.