Characterisation of a 200 nm Thick Liquid Jet Sheet for Ion Acceleration

Recent development of ultrafast lasers has resulted in kHz repetition rate laser systems [1], whose stability opens a way to applications of laser-generated secondary sources as soft X-rays, electrons, and particles. Nowadays the bottleneck of their scientific and industrial applications is the availability and reliability of target systems matching the repetition rate of the lasers. Hard x-ray generation and ion acceleration require high-density targets. One of the most favorable approaches has been the liquid jet sheet [2] with thicknesses of a few $\mu \mathrm{m}$. Recent studies have shown, however, that interaction of femtosecond pulses with few tens of nm thick solid targets may result in higher cut-off energy as well as proton yield [3]. The so far developed characterization methods are suitable for measurement of liquid jets in vacuum down to a $\mu \mathrm{m}$ level [4], [5], while industrial solutions with nm resolution works with 10 mm working distance and in air only.