Velocimetry of fast microscopic liquid jets by nanosecond dual-pulse laser illumination for megahertz X-ray free-electron lasers

To conduct x-ray free-electron laser (XFEL) measurements at megahertz (MHz) repetition rates, sample solution must be delivered in a micron-sized liquid free-jet moving at up to 100 m/s. This exceeds by over a factor of two the jet speeds measurable with current high-speed camera techniques. Accordingly, we have developed and describe herein an alternative jet velocimetry based on dual-pulse nanosecond laser illumination. Three separate implementations are described, including a small laser-diode system that is inexpensive and highly portable. We have also developed and describe analysis techniques to automatically and rapidly extract jet speed from dual-pulse images. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement