Poloidal field measurements during sawteeth and disruptions on MTX

A fifteen chord far infrared polarimeter has been used to measure internal poloidal field profiles in the Microwave Tokamak Experiment (MTX) during sawteeth, disruptions and electron cyclotron heating (ECH). A brief overview is presented of the instrument and the techniques used to analyse the profile data, followed by experimental results. Polarimeter current density (J) measurements are shown to be in good agreement with the expected resistivity profile based on Te measurements. Measurements of safety factor (q) profiles during sawtoothing ohmic discharges are presented and are compared with various sawtooth models. It is found that q0 approximately 0.7-0.85 during sawteeth and q0>1 without sawteeth. The change in q0 during the sawtooth ramp is small, at delta q0 approximately 0.01. Finally, data are presented for ohmic discharges with higher core impurity radiation levels. The large radiated power produces hollow Te and J profiles, and often leads to a disruption. This type of disruption has been stabilized on MTX by peaking the J profile through ECH in the core