Measurement of the internal toroidal magnetic field on the helicity injected tokamak using the transient internal probe

Abstract

The Transient Internal Probe is a novel diagnostic technique specifically developed to measure internal magnetic field profiles in hot plasmas. The concept involves shooting a magneto-optic probe through a plasma at high velocity so that field measurements are obtained before ablation of the probe begins. The magnetic field profiles are obtained by illuminating the probe with a linearly polarized laser and measuring the amount of Faraday rotation induced in the beam as it passes through the probe and is reflected back to the detection system. Coupling between the gas gun and the high vacuum regime of the plasma chamber is achieved with a highly efficient gas interface system such that less than 0.4 torr-I (0.003%) of the muzzle gas enters the plasma chamber. Determination of the polarization angle of the reflected light is accomplished with a high speed, high resolution polarimeter.The TIP concept was demonstrated by measuring the internal toroidal magnetic field profile on the Helicity Injected Tokamak. Measurements were conducted with the probe traveling more than 1.9 km/s. Field measurement accuracy was better than 2.5% with spatial resolution of better than 1 centimeter. Qualitative results concerning the time required for ablation of the probe to begin were obtained and these results supported the surface boiling model used to predict probe behavior. These results proved the diagnostic to be a valuable new tool for investigating the magnetic structure of hot plasmas.