Experimental investigation of the Kerr effect in water under intense and fast transient electric fields

An essential requirement arising in the research programs at both Loughborough University (UK) and the University of Pau (France) into a non-invasive pulsed electric field (PEF) food processing technique is the reliable measurement of the intense fast transient electric fields occurring in the near field of an antenna immersed in water. Field measurements are conventionally performed using either D-dot probes or special purpose antennae in the far field, but such methods are clearly unsuitable in the present application. Both Loughborough and Pau are therefore developing fast electro-optic sensors based on the Kerr effect in water. Once successfully developed, this technique is expected to offer several advantages over conventional sensors, such as a very large bandwidth, miniaturization and complete immunity to any perturbing electromagnetic influence from nearby high-voltage generators, making the sensors extremely suitable for a non-invasive PEF application. The most difficult issue in designing and calibrating the future sensors operating in water is that the literature data for the so-called ´Kerr´ constant for water is inconsistent, with the values provided differing by substantial amounts. This fact triggered the decision to initiate the investigation that the paper will describe. Two different water cells were designed and constructed in the two research centers, each with its own high voltage pulsed power generator. The Kerr constant for water was determined as 2.45 · 10^-14 m/V² ± 10% with water samples probed by 658 nm lasers under homogeneous transient electric field strength exceeding 300 kV/cm. The results obtained at both Universities are analyzed in detail and the way ahead is briefly presented.