Steady progress has been made worldwide in the application and development of hydrogen pellet injection as a method for fuelling magnetically confined plasmas. The theoretical, experimental and technological aspects of this field of research are reviewed, emphasizing developments over the past decade
The observation of Hα-radiation extending along the magnetic field lines during pellet injection in ASDEX suggests a renewed discussion of ablation models and, in particular, an investigation of the additional shielding of the pellet by the high-density, medium-temperature plasma generated by the ablated material. In the present paper the authors have simulated the frozen-in gas flowing along the field lines with a 1-D hydrodynamic code, coupled to the neutral-gas shielding model describing the ablation. To take into account long-mean-free-path effects on the electron heat transport in the plasma, it was necessary to replace the classical Spitzer-Harm conductivity by a non-local transport model. – A plasma hose with a density of up to several hundred times that of the background plasma and extending over several metres was found to form along the field lines. As the temperature of this plasma is much lower than that of the background one (in a computation for ASDEX it dropped from 500 to approximately 40 eV), this effect should lead to a distinct decrease in the resulting ablation rate.
The penetration depths of different impurity pellets, such as carbon and neon, injected into different thermonuclear devices were reproduced by means of a single numerical code with the same set of assumptions, only the atom physical data being changed. All major characteristics of the ablation process were calculated: the spatial variation of the ablation rate, the depositon of ablated particles at a succession of magnetic flux surfaces, the expansion of deposited particles in the directions both parallel and perpendicular to the magnetic field lines, and the temporal and spatial variations of the radiant power emitted by the expanding impurity cloud. The calculations were done by means of a time dependent quasi-three-dimensional code consisting of three modules accounting for the B⊥ and B|| expansions of the cloud and the traversing motion of the pellet, operated interactively and, when needed, iteratively. The radiation characteristics were computed by a collisional-radiative loss model, developed for low temperature light impurities, without the usual equilibrium assumptions. With some modifications, the code is adaptable to predictive pre-disruptive `killer pellet' scenario calculations for future large scale machines, such as ITER.
Shock wave analyses and shielding models pertaining to pellet ablation phenomena are reviewed. Some inconsistencies are shown to exist in some of the studies available. Models are developed and estimates are given for pellet ablation rates in hot plasmas with and without magnetic fields present. The possible existence of a self-regulating ablation mechanism is indicated.
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.
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