Plasma flow and magnetic mode rotation in the MST reversed-field pinch

Summary form only given, as follows. We measure the bulk fluid velocity with 100 kHz resolution in the MST reversed-field pinch (RFP). This diagnostic capability provides the ability to study the fast flow dynamics associated with locked modes. A fast duo-spectrometer simultaneously measures the red and blue Doppler shifts of spectral line radiation emitted by impurity ions in the hydrogen plasma. The phase velocity of the magnetic fluctuations of the tearing modes is measured with a toroidal array of magnetic pickup coils. Both velocities track each other very closely during the spinup phase between sawtooth events, typically reaching a maximum speed of about 20 km/s. Both slow down at the sawtooth crash within a few tens of microseconds. Locked modes in MST occur coincident with the large impulsive vertical field associated with a sawtooth event; they can also be controllably induced by a negatively biased electrode inserted into the plasma. After the onset of a locked mode the phase velocity of the magnetic fluctuations is zero, while the bulk fluid continues to flow at a velocity of few km/s.