Ultrasound Velocimetry in High Current Driven Liquid Metal Flow

The understanding of magnetohydrodynamic phenomena in liquid metals relies heavily on measurement techniques such as ultrasound Doppler velocimetry that can be used to characterize flow patterns within them. The Tayler instability (TI), as well as electro-vortex flow (EVF) in a liquid GaInSn column whose top and bottom are bounded by copper current collectors connected to a switching mode power supply are considered here. The TI is driven by currents above a critical value of 2.7 kA and results in a number of vortices with vertical velocity components of the order of several mm s^−1 , whereas EVF is caused by inhomogeneities in the current density from the current collectors. Ultrasound transducers embedded within the top current collector are in direct contact with the liquid metal. At the high currents required to investigate the TI such a configuration is problematic, because the electromagnetic noise the transducers are exposed to greatly overshadows the signal from ultrasound echoes, if left unchecked. The noise generated by the power supply was significantly damped with the addition of toroidal cores and film capacitors to the current circuit. Moreover, coiling the coaxial cables connecting the transducers to the velocimeter around ring and split cores reduced the noise further. The velocimeter itself drew power through an isolation transformer and was thus prevented from being disturbed by ground loops. Reliable velocity data could be obtained from the experiment after these measures were taken and will be discussed here.