The possibility of using low atomic number (Za) impurity pellets such as carbon, lithium and beryllium as diagnostic probes for fusion grade plasmas is investigated using an ablation model based on the vapour shielding phenomenon. It is shown that pellet injection velocities of approximately six kilometres per second would be required for penetration of a 1 mm sized carbon (diamond) pellet half-way into a CIT-like plasma. Properties of the high density ablation cloud surrounding a low Za pellet are discussed in relation to a diagnostic scheme that uses double charge-exchange interactions of fusion-born alpha particles with the ablation cloud to determine the local alpha energy distribution function. Pellet lifetime results are given and compared with experimental ones.

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