Measurement of internal magnetic field pitch using Li pellet injection on TFTR (invited)

A diagnostic technique which measures the direction of the internal magnetic field pitch angle has been used successfully on TFTR. The technique requires the injection of high‐speed Li pellets. The magnetic field direction is measured by observing the polarization direction of the intense visible line emission from Li+ (λ≊5485 A, 1s2p 3P0,1,2→1s2s 3S0) in the pellet ablation cloud. The presence of the large (primarily toroidal) magnetic field causes the line to be split due to the Zeeman effect, and the unshifted π component is polarized with its polarization direction parallel to the local magnetic field. In devices with sufficiently strong fields (B≳4.5 T), the Zeeman splitting of the line is large enough, relative to the linewidth of each Zeeman component, that enough residual polarization remains. Because the pellet moves about 1 cm before the Li+ is ionized (τionization≲10 μs), the time history of the polarization direction (as the pellet penetrates from the outside toward the plasma center) yields t...