High Repetition-Rate Target Focusing Using White-Light Interferometry

When high repetition-rate lasers (10 Hz) are fielded for laser-driven dynamic compression experiments at x-ray free electron laser (XFEL) and other facilities the issue of how to field the experiments becomes a primary concern. Chief among these is the alignment of the target and the various pumps and probes. Specifically, the target needs to be simultaneously aligned with the XFEL beam, the drive laser beams and the VISAR probe laser at the single point in space where all three beams cross. In many experiments, the beams cross at about 45° so that if the target position is off by more than ~25% of the drive-laser spot size, the experiment will be irretrievably compromised. In an attempt to address this problem we have designed and assembled a ride-along VISAR in-line white-light interferometric objective that allows us to measure the target focal position with an adjustable precision ranging from 1 – 20 um in less than 10 ms. We will present proof of principle data demonstrating the feasibility of this system.