CURRENT-DENSITY PROFILE MEASUREMENT ON THE TEXAS EXPERIMENTAL TOKAMAKUSING A HEAVY-ION BEAM PROBE

For the first time, a heavy ion beam probe has been used to measure th e poloidal magnetic flux in a tokamak. In this measurement, first prop osed over 20 years ago, the toroidal displacement of the heavy ion bea m probe particles is caused by the force of the poloidal magnetic fiel d on the ion beam probe particles. In a nearly toroidally symmetric de vice such as the Texas Experimental Tokamak, the toroidal position of the ions at the detector consists of a part proportional to the poloid al magnetic flux at the sample volume and an integrated contribution o f the poloidal magnetic flux along the trajectory. The local part in t he relation between beam position and magnetic flux is used as a corre ction term in an iterative algorithm that calculates the poloidal magn etic flux. The q profile and the current density profile can be derive d from the measured poloidal flux. Errors of the measurement can be du e 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 mea surement. It is essential to know the magnetic field outside the plasm a. The calculation of the outside field is difficult due to the presen ce 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 measu red for discharges with on axis and off axis electron cyclotron resona nce heating (ECRH), and with impurity injection. As expected, all curr ent profiles peak in the center. With central ECRH, the measured curre nt profiles are consistent with a prediction based on Spitzer resistiv ity and the measured electron temperature profile. In these plasmas, t he safety factor has a central value of q(c) = 0.95 +/- 0.1 and near t he center of the plasma, the poloidal flux is observed to increase wit h time. The current profiles for off axis ECRH exhibit a larger curren t density in the wings of the distribution than comparable centrally h eated ECRH plasmas. Results from plasmas with impurity injection yield q(c) = 0.6 which is significantly lower than expected since these pla smas have m = 2 activity. (C) 1998 American Institute of Physics. [S00 34-6748(98)02611-2].