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

Abstract

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 Å, 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 the local magnetic field direction. In the TFTR experiment, spatial resolution of the measurement is typically ∼7 cm, limited by the requirement that a large number of photons must be collected in order to make the measurement of the polarization angle. Typically, the pitch of the field is measured with an accuracy of ±0.01 rad, limited by the photon statistics. The measurements of the internal field pitch angle, combined with external magnetic measurements, have been used in a code which finds the solution of the Grad–Shafranov equation, yielding the equilibrium which is the best fit to the measured inputs. The q profile constructed from this equilibrium is believed to be accurate to ∼±10% over the region where there are internal magnetic measurements. Internal field measurements and equilibrium reconstructions have been performed for a variety of TFTR discharges, including 1.6 MA ohmic plasmas where the internal field is measured at the beginning of the current flat top (before the onset of sawteeth) and 2 s into the flat top (with sawteeth), and in extremely high βp(Ip=0.3 MA, βp≊4.5) discharges.