DETERMINATION OF THE SAFETY FACTOR IN SAWTOOTHING DISCHARGES IN DIII-D

Local magnetic field pitch angle is determined in DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] by measuring the direction of polarization of Stark components of the D-alpha (deuterium Balmer-alpha) line emitted by the heating neutral beam. The measurement result is incorporated into the magnetic equilibrium code to provide a constraint on the calculated profiles of current density and safety factor (inverse rotational transform). The safety factor profiles obtained with the use of the polarimetry data indicate that the safety factor on axis q0 is close to 1.0 at the onset of the sawtooth instability but may be substantially smaller than 1.0 in later stages of a discharge. The magnetic field pitch angle variations associated with the sawtooth instability were observed in some discharges with a time resolution of about 10 msec. The increase of q0 during a sawtooth crash is found to be not larger than about 0.1 +/- 0.05.