Measurements of neutrons and hard X-rays are made with a pair of plastic scintillators during injection of deuterium pellets into deuterium TFTR plasmas. Three cases are investigated. During Ohmic heating in plasmas with few runaway electrons, the neutron emission does not increase when a pellet is injected, indicating that strong acceleration of the pellet ions does not occur. In Ohmic plasmas with low but detectable levels of runaway electrons, an X-ray burst is observed on a detector near the pellet injector as the pellet ablates, while a detector displaced 126° toroidally from the injector does not measure a synchronous burst. Reduced pellet penetration correlates with the presence of X-ray emission, suggesting that the origin of the burst is bremsstrahlung from runaway electrons that strike the solid pellet. In deuterium beam heated discharges, an increase in the d-d neutron emission is observed when the pellet ablates. In this case, the increase is due to fusion reactions between beam ions and the high density neutral and plasma cloud produced by ablation of the pellet; this localized density perturbation equilibrates in about 700 μs. Analysis of the data indicates that the density propagates without forming a sharp shock front with a rapid initial propagation velocity (≳ 2 × 107 cm·s−1) that subsequently decreases to around 3 × 106 cm·s−1. Modelling suggests that the electron heat flux into the pellet cloud is much less than the classical Spitzer value.
Modelling of pellet ablation in additionally heated plasmas