Ultrasound Doppler Velocimetry Measurements in the Princeton Magnetorotational Instability Experiment

The standard magnetorotational instability (SMRI) is widely believed to be responsible for the fast angular momentum transport in accretion discs. Nonetheless, its existence has never been identified unambiguously in astronomical observations or in laboratory experiments. By measuring the radial magnetic field, a recent study at Princeton Plasma Physics Laboratory shows evidence of SMRI in a modified Taylor-Couette device that uses liquid metal as the working fluid. This study is aimed at supporting these findings using velocity data measured by ultrasonic doppler velocimetry on the outer cylinder. Through the analysis of the average velocity field measured at different azimuthal angles, the radial velocity field is obtained. Further, the azimuthal structure of SMRI in the velocity field is obtained using modal analysis, the results of which are in good agreement with results obtained from magnetic field diagnostics. The growth of SMRI, as well as the time delay of its onset, are studied to understand the underlying mechanisms.