DETERMINATION OF FIELD LINE LOCATION AND SAFETY FACTOR IN TEXT-U

Biasing a Langmuir probe to collect electrons creates a magnetic pertu rbation by drawing current along a field line. This perturbation propa gates along the magnetic field as a shear-Alfven mode and marks the af fected field Lines with a potential perturbation at the probe sweep fr equency. This technique has been used in TEXT-U edge plasmas to locate a connection between two sensor probes separated by either 12 or 28 m along a field line, and this connection has been verified using magne tic field line tracing codes. The driver probe creates a shadow which spreads in the poloidal direction with increasing distance parallel to the magnetic field due to both the change of safety factor with minor radius and collisional effects. Determination of the radial and poloi dal structure of the perturbation down the magnetic field therefore pr ovides a measurement of the safety factor q as a function of minor rad ius in the edge. In addition, the technique for locating field lines h as been used to establish a purely parallel connection between two flo ating probes and therefore provides a measurement of parallel wave num ber k(parallel to)less than or equal to 0.1 m(-1) and parallel correla tion length l(parallel to)greater than or equal to 12 m. (C) 1997 Amer ican Institute of Physics.