Effects of excitation laser wavelength on Ly-α and He-α line emission from nitrogen plasmas

Investigation of ion dynamics of laser ablated single and colliding carbon plasmas using Faraday cup

We report a comparative study of a single plasma and a colliding laser produced plasma, investigated using a Faraday cup. An enhancement in ion emission and stagnation is observed in colliding plasma plume compared to single plasma plume. We observed that fast ion generation in laser ablated plasma can be achieved at large laser intensity on to the target. As laser intensity increases ionic yield increases for both colliding and single plume and at a fixed laser intensity ionic yield decreases with increase in ambient pressure. The double peak structure is observed in the ion signal at large fluence where the peaks correspond to fast and slow species. A Faraday cup composed of nine collectors is used to measure the spatial/angular distribution of ion of expanding plasma plume. Ionic yield is found to be larger in the colliding plasma plume than the single plasma plume at all spatial/angular positions.

Soft X-ray spectral analysis of laser produced molybdenum plasmas using the fundamental and second harmonics of a Nd:YAG laser

Our measurement of the soft X-ray emission of Mo plasmas produced by picosecond Nd:YAG lasers emitting on the fundamental (1064 nm, 150 ps) and second (532 nm, 130 ps) harmonics is presented. The contrast in intensity between spectral peaks and the intensity outside them is lower for the second harmonic produced plasmas probably due to the presence more intense satellite emission and higher optical thickness. The measured spectra are absolutely calibrated and the observed output photon flux was (7 - 9) × 10¹³ photons/sr in the water-window (2.3 - 4.4 nm) spectral range for a laser energy of 160 mJ independent of laser wavelength. However, in the short wavelength range 1.5 - 2 nm, the emission using the second harmonic is strongly enhanced and is even higher than for the maximum energy of 220 mJ of the fundamental wavelength, so despite inevitable energy losses, laser wavelength conversion may lead to emission enhancement in certain spectral ranges. This enhancement is attributed to higher absorption of short wavelength laser light and higher charge state generation in denser plasmas.

Enhancement of water-window soft x-ray emission from laser-produced Au plasma under low-pressure nitrogen atmosphere

To generate bright water-window (WW) soft x rays (2.3-4.4 nm), gold slab targets were irradiated with laser pulses (1064 nm, 7 ns, 1 J). Emission spectroscopy showed that the introduction of low-pressure nitrogen enhanced the soft x-ray yield emitted from the laser-produced Au plasma. The intensity of the WW x-ray transported in a 400-Pa N₂ atmosphere from the laser-produced plasma increased by 3.8 times over that in vacuum. Considering a strong x-ray absorption, the x-ray yield emitted directly from the Au plasma in the N₂ gas was evaluated to be 13 times higher than that in vacuum. Although similar measurements were made for various gases, only N₂ gas causes an increase in a soft x-ray yield. The processes leading to this enhancement mechanism were revealed by using hydrodynamic simulation and atomic structure codes.

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

The effect of optical thickness in a bismuth water-window soft x-ray source is considered by comparing the emission from laser-produced plasmas of a 7.5% atomic density foam target and a solid-density target. The number of photons recorded in the 4 nm region was comparable for both targets at a plasma-initiating laser pulse duration of 6 ns. From experiments at different pulse durations of 150 ps and 6 ns, self-absorption (opacity) effects were found to be relatively small for bismuth plasmas as compared to those of tin, based on the same emission mechanism and which are used in 13.5 nm sources for extreme ultraviolet lithography.

Plume Dynamics of Laser-Produced Swine Muscle Tissue Plasma

We report on the plume dynamics of the plasma induced by laser ablation of a swine skeletal muscle tissue sample in different vacuum conditions. Pulses from a transversely excited atmospheric CO2 laser were focused onto a target sample and the induced plasma was allowed to expand in different air pressures. The expansion features were studied using fast photography of the overall visible emission by using a gated intensified charged coupled device. Free expansion and plume splitting were observed at different pressure levels. The expansion of the plasma plume front was analyzed using various expansion models and the velocity of the plume front was estimated. The effect of the number of accumulated laser shots on the crater volume at different ambient air pressures and an elemental analysis of the sample were performed using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (EDX) analysis. The surface morphology of the irradiated surface showed that increasing the pressure of the ambient gas decreased the ablated mass, or in other words it reduced significantly the laser-target coupling.

Atomic and optical properties of warm dense copper

The emission of x rays from warm dense matter is of great interest for both spectroscopic diagnostics and development of intense x-ray sources. We report the results from the collisional-radiative steady-state (CRSS) modeling of atomic and optical properties of copper plasmas at near-solid and solid-state density for a range of temperatures. The CRSS model is validated against the available data on the average charge state and shifts of energy levels in aluminum and the opacity and emissivity spectra of carbon and aluminum plasmas. The average charge states, number density of ion species, and free electrons as a function of temperature are investigated for the solid-density copper plasma. Due to the dense plasma environment the four outer electrons are found to be unbounded even in the low-temperature limit ∼1eV. As the temperature changes from 1 to 100 eV, the predominant species vary from fivefold- to twelvefold-ionized copper ions. The opacity and emissivity spectra of dense copper plasmas are studied using the local thermodynamic equilibrium (LTE) and non-LTE approaches. It is found that the non-LTE effects are important in the spectral region of soft x rays emitted from the K shell. The emissivity in spectral lines is completely suppressed, indicating the importance of the energy-dissipating radiative processes in this soft x-ray region. Line broadening and redshifts of the K- and L-shell spectral lines toward higher wavelengths are observed with the increase of plasma density. These results have important implications for understanding the radiative properties of warm dense copper and can be useful for future experimental studies.