Abstract
We report on the design, construction, and characterisation of a new class of in-vacuo optical levitation trap optimised for use in high-intensity, high-energy laser interaction experiments. The system uses a focused, vertically propagating continuous wave laser beam to capture and manipulate micro-targets by photon momentum transfer at much longer working distances than commonly used by optical tweezer systems. A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with ±5 μm spatial resolution. Optical trapping of 10 ± 4 μm oil droplets in vacuum was demonstrated, over timescales of >1 h at extended distances of ∼40 mm from the final focusing optic. The stability of the levitated droplet was such that it would stay in alignment with a ∼7 μm irradiating beam focal spot for up to 5 min without the need for re-adjustment. The performance of the trap was assessed in a series of high-intensity (10(17) W cm(-2)) laser experiments that measured the X-ray source size and inferred free-electron temperature of a single isolated droplet target, along with a measurement of the emitted radio-frequency pulse. These initial tests demonstrated the use of optically levitated microdroplets as a robust target platform for further high-intensity laser interaction and point source studies.
Highlights
The interaction of an intense laser pulse with micronsized, mass-limited targets has the potential to create low debris, low electromagnetic pulse (EMP), high-brightness Xray or fast particle sources.1–3 A mass-limited target is defined as an object not in direct contact with any surrounding matter and where energy transport mechanisms, such as thermal and hot electron currents are spatially confined
We report on the design, construction, and characterisation of a new class of in-vacuo optical levitation trap optimised for use in high-intensity, high-energy laser interaction experiments
A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with ±5 μm spatial resolution
Summary
The interaction of an intense laser pulse with micronsized, mass-limited targets has the potential to create low debris, low electromagnetic pulse (EMP), high-brightness Xray or fast particle sources. A mass-limited target is defined as an object not in direct contact with any surrounding matter (e.g., a mounting pin) and where energy transport mechanisms, such as thermal and hot electron currents are spatially confined. Direct spatial control of the target is crucial, as is the ability to fully characterise or exploit the laser-target interaction with a diagnostic suite occupying multiple viewing angles These requirements generate a set of constraints that motivated the development of the new in-vacuo optical levitation trap we present here. We report on the development of a new class of optical levitation trap optimised to confine few micron objects in vacuum over extended timescales and at large (⇠40 mm) working distances with a position accuracy of a few microns This will, for the first time, allow optically levitated, electrically neutral, isolated micro-targets to be irradiated at very high intensity using large, low repetition rate national facility scale laser systems
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