Imaging of lithium pellet ablation trails and measurement of q profiles in TFTR

Video images with 2 μs exposures of the Li⁺ emission in Li pellet ablation clouds have been obtained in a variety of Tokamak Fusion Test Reactor tokamak discharges. The pellet clouds are viewed from behind the pellet, which is injected from the outside midplane. In this view, the emission forms an elongated cigar shape with the long dimension of the cigar aligned with the local magnetic field. In some cases, two distinct parallel cigars can be seen simultaneously, displaced vertically from one another by ∼5 cm. Measurements using a ten channel array of position sensitive photodiodes show that the mean position of the ablation cloud emission can oscillate vertically by ∼4 cm with periods in the 60-100 μs range, and that these oscillations are highly correlated with "bursts" in the cloud emission. The tilt of the cloud is also measured as a function of time as the pellet traverses the plasma, and in this way the poloidal field profile is obtained. (The total transit time of the pellet is ∼1 ms.) Magnetohydrodynamic equilibrium reconstructions of q profiles have been determined using these measurements.