Current density profile measurement on the Texas Experimental Tokamak using a heavy ion beam probe

Abstract

For the first time, a heavy ion beam probe has been used to measure the poloidal magnetic flux in a tokamak. In this measurement, first proposed over 20 years ago, the toroidal displacement of the heavy ion beam probe particles is caused by the force of the poloidal magnetic field on the ion beam probe particles. In a nearly toroidally symmetric device such as the Texas Experimental Tokamak, the toroidal position of the ions at the detector consists of a part proportional to the poloidal magnetic flux at the sample volume and an integrated contribution of the poloidal magnetic flux along the trajectory. The local part in the relation between beam position and magnetic flux is used as a correction term in an iterative algorithm that calculates the poloidal magnetic flux. The q profile and the current density profile can be derived from the measured poloidal flux. Errors of the measurement can be due to uncertainties in the analyzer position and also in the knowledge of the location of the sample volume. Also uncertainties in the plasma current and position contribute to the error of the poloidal flux measurement. It is essential to know the magnetic field outside the plasma. The calculation of the outside field is difficult due to the presence of the iron core. The error in the measurement of the poloidal flux is on the order of 1%. The current profiles and q profiles were measured for discharges with on axis and off axis electron cyclotron resonance heating (ECRH), and with impurity injection. As expected, all current profiles peak in the center. With central ECRH, the measured current profiles are consistent with a prediction based on Spitzer resistivity and the measured electron temperature profile. In these plasmas, the safety factor has a central value of qc=0.95±0.1 and near the center of the plasma, the poloidal flux is observed to increase with time. The current profiles for off axis ECRH exhibit a larger current density in the wings of the distribution than comparable centrally heated ECRH plasmas. Results from plasmas with impurity injection yield qc=0.6 which is significantly lower than expected since these plasmas have m=2 activity.