Lithium pellet deposition and penetration in TFTR

The electron deposition resulting from the injection of Li pellets into Tokamak Fusion Test Reactor, measured by a multichannel (10) infrared interferometer, is compared with that deduced from the pellet ablation cloud emission, measured by a filtered diode array which views the pellet from behind. By assuming that the ablation rate Ṅ(r) is proportional to the pellet cloud emissivity, which is dominated by Li⁺ line emission in the 548.5±5 nm bandpass of the interference filter, the post-pellet, line averaged density perturbations along the interferometer chords were calculated and compared with those measured. Good agreement is observed. The experimental ablation rate profiles obtained using the emissivity have also been compared with predictions of the theoretical models. There is an agreement between the time history of the emissivity and the predicted ablation rate at the plasma edge where the electron temperature values are less than 1-1.5 keV. When the pellet penetrates more deeply, the experimental Ṅ(r) values are systematically smaller than those predicted. This points out the necessity of taking into account plasma shielding and/or precooling of the target plasma during pellet injection in the ablation model.