Liquid Metal Ultrasound Velocimetry in a High Current Environment

Ultrasound Doppler velocimetry is an important tool for the characterization of liquid metal flows, including those caused by magnetohydrodynamic instabilities. Here we consider the Tayler Instability (TI), which is driven by a high current flowing through the system. This can entail a great deal of electromagnetic noise, which must be decoupled from the signal produced by ultrasound scatterers within the liquid metal. In the experiment described herein, two ultrasound transducers encased within a copper electrode are in contact with a cylindrical liquid GaInSn column. Currents in the order of several kA produced by a switching-mode power supply flow through the electrode and the liquid GaInSn, resulting in the TI, which causes vortices with vertical velocity components in the range of several mm/s to appear. Noise produced by the power supply was significantly reduced by adding film capacitors and toroidal cores, including a common-mode choke to the circuit. Electromagnetic interference was further reduced by adding toroidal and split cores to the coaxial cables connecting the ultrasound transducers to the velocimeter, with the latter drawing power through an isolating transformer. These precautions permitted the retrieval of reliable velocity data, which generally agree with previously conducted B_z-measurements.