Emission of water-window soft x-rays under optically thin conditions using low-density foam targets

Water-window x-ray emission from laser-produced Au plasma under optimal target thickness and focus conditions

Optimal laser irradiation conditions for water-window (WW) x-ray emission (2.3-4.4 nm) from an Au plasma are investigated to develop a laboratory-scale WW x-ray source. A minimum Au target thickness of 1 µm is obtained for a laser intensity of ∼10^{13} W/cm^{2} by observing the intensity drop in the WW spectra. Au targets produced by thermal evaporation are found to have a higher conversion efficiency than commercial foil targets for WW x-ray radiation. In addition, optimal laser spots for fixed laser energies (240 and 650 mJ) are found for an Au target ∼1 mm in front of the focal point, where suitable conditions for plasma temperature and plume volume coupling are achieved. The mechanism of the optimal target thickness and spot size can be well explained using a radiation hydrodynamic simulation code.

Dynamics of laser produced plasma from foam targets for future nanolithography devices and X-ray sources

Foam targets are expected to be more efficient candidates than solid targets for laser produced plasma (LPP) for extreme ultraviolet (EUV) and X-ray radiation sources due to the expected plasma conditions that can be optimized regarding plasma opacities, volumetrics heating effects, and the produced ions debris characteristics. In this paper, a comparison of ion dynamics between low-density foam and solid Ni plasma was systematically investigated at CMUXE. The foam Ni target (density 0.6 g/cm³) and solid Ni target (density 8.9 g/cm³) were irradiated with 1064 nm Nd:YAG laser in vacuum. A Faraday cup (FC) was used to record the ion flux and time-of-flight (TOF) signals. A lower and wider TOF signal was observed for foam Ni plasma on the time scale. The average ion energy and peak of the TOF signal of solid Ni plasma were much higher than that of the foam Ni plasma. However, the total charge values between foam and solid Ni plasma were comparable indicating a more volumetric absorption of laser energy for foam Ni. The average ion energy and peak of the TOF signal of solid Ni showed a stronger angular and laser energy dependence than that of foam Ni. The plume shape of the solid Ni plasma appeared as an oblong ellipse at each time, while that of foam Ni plasma tended to be more circular, especially at early times. The results of mass ablation rate were consistent with the FC signals and showed a more intense plasma shielding for solid Ni.

High-efficiency water-window x-ray generation from nanowire array targets irradiated with femtosecond laser pulses

We demonstrate the high-efficiency generation of water-window soft x-ray emissions from polyethylene nanowire array targets irradiated by femtosecond laser pulses at the intensity of 4×10¹⁹ W/cm². The experimental results indicate more than one order of magnitude enhancement of the water-window x-ray emissions from the nanowire array targets compared to the planar targets. The highest energy conversion efficiency from laser to water-window x-rays is measured as 0.5%/sr, which comes from the targets with the longest nanowires. Supported by particle-in-cell simulations and atomic kinetic codes, the physics that leads to the high conversion efficiency is discussed.

Easy-handling minimum mass laser target scaffold based on sub-millimeter air bubble -An example of laser plasma extreme ultraviolet generation

Low density materials can control plasma properties of laser absorption, which can enhance quantum beam generation. The recent practical extreme ultraviolet light (EUV) is the first industrial example of laser plasma source with low density targets. Here we propose an easy-handling target source based on a hollow sub-millimeter microcapsule fabricated from polyelectrolyte cationic and anionic surfactant on air bubbles. The lightweight microcapsules acted as a scaffold for surface coating by tin (IV) oxide nanoparticles (22-48%), and then dried. As a proof of concept study, the microcapsules were ablated with a Nd:YAG laser (7.1 × 1010 W/cm2, 1 ns) to generate 13.5 nm EUV relatively directed to laser incidence. The laser conversion efficiency (CE) at 13.5 nm 2% bandwidth from the tin-coated microcapsule (0.8%) was competitive compared with bulk tin (1%). We propose that microcapsule aggregates could be utilized as a potential small scale/compact EUV source, and future quantum beam sources by changing the coating to other elements.