Spectral analysis and kinetic modeling of radioluminescence in air and nitrogen.

In this article we present a quantitative analysis of the second positive system of molecular nitrogen and the first negative system of the molecular nitrogen cation excited in the presence of ionizing radiation. Optical emission spectra of atmospheric air and nitrogen surrounding 210Po sources were measured from 250 to 400 nm. Multi-Boltzmann and non-Boltzmann vibrational distribution spectral models were used to determine the vibrational temperature and vibrational distribution function of the emitting N2(C3Πu) and N2+(B2Σ+u) states. A zero-dimensional kinetic model, based on the electron energy distribution function (EEDF) and steady-state excitation and de-excitation of N2(X1Σ+g), N2+(B2Σ+u), N2+(X2Σ+g), N4+, O2+, and N2(C3Πu, v), was developed for the prediction of the relative spectral intensity of both the N2+(B2Σ+u → X2Σ+g) emission band and the vibrational bands of N2(C3Πu → B3Πg) for comparison with the experimental data.

Hydrogen isotopic analysis of nuclear reactor materials using ultrafast laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy is a promising method for rapidly measuring hydrogen and its isotopes, critical to a wide range of disciplines (e.g. nuclear energy, hydrogen storage). However, line broadening can hinder the ability to detect finely spaced isotopic shifts. Here, the effects of varying plasma generation conditions (nanosecond versus femtosecond laser ablation) and ambient environments (argon versus helium gas) on spectral features generated from Zircaloy-4 targets with varying hydrogen isotopic compositions were studied. Time-resolved 2D spectral imaging was employed to detail the spatial distribution of species throughout plasma evolution. Results highlight that hydrogen and deuterium isotopic shifts can be measured with minimal spectral broadening in a ∼ 10 Torr helium gas environment using ultrafast laser-produced plasmas.

Time-resolved absorption spectroscopic characterization of ultrafast laser-produced plasmas under varying background pressures.

Time-resolved tunable laser absorption spectroscopy is used to characterize the physical properties of ultrafast laser-produced plasmas. The plasmas were produced from an Inconel target, with ≤0.4wt% Al, using ∼35fs, ∼800nm, ∼5mJ laser pulses at varying Ar background pressures from 1 to 100 Torr. The absorption spectrum of atomic Al is measured with high spectral and temporal resolution when the probe laser is stepped across the selected Al transition at 394.4 nm. Spectral fitting is used to infer linewidths, kinetic temperature, Al column density, and pressure broadening coefficient. The late time physical properties of plasmas are compared for various pressure levels. Our studies highlight that a significant lower state population exists even at early times of ultrafast laser-produced plasma evolution, and lower state population persistence decreases with increasing ambient pressure. We also show that the fundamental optical properties, such as pressure broadening, can be measured using ultrafast laser-produced plasmas combined with laser absorption spectroscopy.

Physical conditions for UO formation in laser-produced uranium plumes.

We investigate the oxidation of uranium (U) species, the physical conditions leading to uranium monoxide (UO) formation and the interplay between plume hydrodynamics and plasma chemistry in a laser-produced U plasma. Plasmas are produced by ablation of metallic U using nanosecond laser pulses. An ambient gas environment with varying oxygen partial pressures in 100 Torr inert Ar gas is used for controlling the plasma oxidation chemistry. Optical emission spectroscopic analysis of U atomic and monoxide species shows a reduction in the emission intensity and persistence with increasing oxygen partial pressure. Spectral modelling is used for identifying the physical conditions in the plasma that favor UO formation. The optimal temperature for UO formation is found to be in the temperature range of ∼1500-5000 K. The spectrally integrated and spectrally filtered (monochromatic) imaging of U atomic and molecular species reveals the evolutionary paths of various species in the plasma. Our results also highlight that oxidation in U plasmas predominantly occurs at the cooler periphery and is delayed with respect to plasma formation, and the dissipation of molecular species strongly depends on oxygen partial pressure.

Tracking of oxide formation in laser-produced uranium plasmas.

We use a spatially and temporally resolved emission tracking technique based on optical emission spectroscopy to map the evolution of emission features from uranium and its compounds in a plasma produced by a nanosecond laser. We observe quenching of the emission from neutral uranium (591.538 nm) and uranium monoxide (593.55 nm) species with increasing oxygen concentration and discuss possible reaction pathways for dissociation or formation of higher uranium oxides (UxOy). We further identify spectral features between 320 nm and 380 nm and between 520 nm and 640 nm, which we attribute to UxOy.

Standoff analysis of laser-produced plasmas using laser-induced fluorescence.

We report the use of laser-induced fluorescence (LIF) of laser ablation (LA) plumes for standoff applications. The standoff analysis of Al, as major and minor species in samples, is performed in a nanosecond laser-produced plasma created at a distance of ∼10 m. The LIF of LA plumes is carried out by resonantly exciting an Al transition at 394.4 nm (S1/22-P1/22) using a continuous wave (cw) tunable laser and by collecting the direct-line fluorescence signal at 396.15 nm. The spectral resolution of LIF is obtained by scanning the cw tunable LIF laser across the selected Al transition. Our results highlight that LIF provides enhanced signal intensity, emission persistence, and spectral resolution when compared to thermally excited emission.

High-resolution spectroscopy of laser ablation plumes using laser-induced fluorescence.

We report laser-induced fluorescence spectroscopy (LIF) of laser-produced plasmas under varying nitrogen pressure levels up to atmospheric pressure. The plasmas were generated on a glass target containing minor amounts of U and Al using 1064 nm, 6 ns pulses from a Nd:YAG laser. A frequency-doubled continuous-wave Ti:Sapphire laser was used as an ultra-narrowband tunable LIF excitation source to increase the magnitude and persistence of emission from selected U and Al atomic transitions in a laser-produced plasma. 2D-fluorescence spectroscopy (2D-FS) absorption/emission images were recorded at various nitrogen pressure levels, showing both excitation and emission spectral features. At lower pressure levels (⪝100 Torr), fluorescence emission was found to be well separated in time from thermally-excited emission. However, as the ambient pressure increased, the thermally-excited emission persisted for longer times along with a reduction of LIF emission persistence and intensity. The excitation spectral features showed the inherent linewidths of various transitions in the plasma, which have significantly narrower spectral linewidths than observed in emission spectra. We evaluated two nearby transitions separated by only 18 pm to demonstrate the effectiveness of fluorescence spectra over thermally-excited spectra for high-resolution studies. The present results highlight the importance of LIF as a diagnostic tool employing continuous-wave laser re-excitation, addressing some of the limitations of traditional emission and absorption spectroscopic methods.

Consequences of femtosecond laser filament generation conditions in standoff laser induced breakdown spectroscopy.

The combination of femtosecond laser filament ablation and emission spectroscopy is a potential analytical tool for standoff characterization of samples of interest. We compare the emission features and physical conditions of plasmas generated from metal targets using either by loosely focused femtosecond filaments or by lens-free filaments. Our results show that the filament generation conditions influence the plasma properties appreciably which include the atomic and molecular emission features, persistence and plasma fundamentals (temperature and density). The loosely focused fs pulse filaments are found to generate ablation plumes with higher temperature and density along with increased persistence compared to plumes generated by lens-free filaments.

Two-dimensional fluorescence spectroscopy of laser-produced plasmas.

We use a two-dimensional laser-induced fluorescence spectroscopy technique to measure the coupled absorption and emission properties of atomic species in plasmas produced via laser ablation of a solid aluminum target at atmospheric pressure. Emission spectra from the Al I 394.4 nm and Al I 396.15 nm transitions are measured while a frequency-doubled, continuous wave (cw) Ti:sapphire laser is tuned across the Al I 396.15 nm transition. The resulting two-dimensional spectra show the energy coupling between the two transitions via increased emission intensity for both transitions during resonant absorption of the cw laser at one transition. Time-delayed, gated detection of the emission spectrum is used to isolate resonantly excited fluorescence emission from thermally excited emission from the plasma. In addition, the tunable cw laser measures the absorption spectrum of the Al transition with ultrahigh resolution after the plasma has cooled, resulting in narrower spectral linewidths than observed in emission spectra. Our results highlight that fluorescence spectroscopy employing cw laser re-excitation after pulsed laser ablation combines benefits of both traditional emission and absorption spectroscopic methods.

Plasma temperature clamping in filamentation laser induced breakdown spectroscopy.

Ultrafast laser filament induced breakdown spectroscopy is a very promising method for remote material detection. We present characteristics of plasmas generated in a metal target by laser filaments in air. Our measurements show that the temperature of the ablation plasma is clamped along the filament channel due to intensity clamping in a filament. Nevertheless, significant changes in radiation intensity are noticeable, and this is essentially due to variation in the number density of emitting atoms. The present results also explain the near absence of ion emission but strong atomic neutral emission from plumes produced during fs LIBS in air.

Morphological changes in ultrafast laser ablation plumes with varying spot size.

We investigated the role of spot size on plume morphology during ultrafast laser ablation of metal targets. Our results show that the spatial features of fs LA plumes are strongly dependent on the focal spot size. Two-dimensional self-emission images showed that the shape of the ultrafast laser ablation plumes changes from spherical to cylindrical with an increasing spot size from 100 to 600 µm. The changes in plume morphology and internal structures are related to ion emission dynamics from the plasma, where broader angular ion distribution and faster ions are noticed for the smallest spot size used. The present results clearly show that the morphological changes in the plume with spot size are independent of laser pulse width.

Recrystallization and grain growth induced by ELMs-like transient heat loads in deformed tungsten samples.

Tungsten has been chosen as the main candidate for plasma facing components (PFCs) due to its superior properties under extreme operating conditions in future nuclear fusion reactors such as ITER. One of the serious issues for PFCs is the high heat load during transient events such as ELMs and disruption in the reactor. Recrystallization and grain size growth in PFC materials caused by transients are undesirable changes in the material, since the isotropic microstructure developed after recrystallization exhibits a higher ductile-to-brittle transition temperature which increases with the grain size, a lower thermal shock fatigue resistance, a lower mechanical strength, and an increased surface roughening. The current work was focused on careful determination of the threshold parameters for surface recrystallization, grain growth rate, and thermal shock fatigue resistance under ELM-like transient heat events. Transient heat loads were simulated using long pulse laser beams for two different grades of ultrafine-grained tungsten. It was observed that cold rolled tungsten demonstrated better power handling capabilities and higher thermal stress fatigue resistance compared to severely deformed tungsten. Higher recrystallization threshold, slower grain growth, and lower degree of surface roughening were observed in the cold rolled tungsten.

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

Laser-produced nitrogen plasmas emitting radiation at 2.48 nm (Ly-α) and 2.88 nm (He-α) are considered potential efficient sources for water-window (WW) microscopy. The atomic and optical properties of nitrogen plasma and influence of the laser wavelength on the line emission in the WW range are investigated. It is found that the optimal temperatures for maximum emission from Ly-α and He-α spectral lines are 40-60 eV and 80-100 eV, respectively. The WW line emission and the conversion efficiency (CE) are estimated for three distinct Nd:YAG laser wavelengths (1064 nm, 532 nm, and 266 nm). The calculated CEs are compared with experimentally observed CE values. It is found that 1064 nm wavelength provides the highest CE from laser to Ly-α and He-α radiation.

The effect of ultrafast laser wavelength on ablation properties and implications on sample introduction in inductively coupled plasma mass spectrometry.

We investigated the role of femtosecond (fs) laser wavelength on laser ablation (LA) and its relation to laser generated aerosol counts and particle distribution, inductively coupled plasma-mass spectrometry (ICP-MS) signal intensity, detection limits, and elemental fractionation. Four different NIST standard reference materials (610, 613, 615, and 616) were ablated using 400 nm and 800 nm fs laser pulses to study the effect of wavelength on laser ablation rate, accuracy, precision, and fractionation. Our results show that the detection limits are lower for 400 nm laser excitation than 800 nm laser excitation at lower laser energies but approximately equal at higher energies. Ablation threshold was also found to be lower for 400 nm than 800 nm laser excitation. Particle size distributions are very similar for 400 nm and 800 nm wavelengths; however, they differ significantly in counts at similar laser fluence levels. This study concludes that 400 nm LA is more beneficial for sample introduction in ICP-MS, particularly when lower laser energies are to be used for ablation.

Excitation wavelength dependence of water-window line emissions from boron-nitride laser-produced plasmas.

We investigated the effects of laser excitation wavelength on water-window emission lines of laser-produced boron-nitride plasmas. Plasmas are produced by focusing 1064 nm and harmonically generated 532 and 266 nm radiation from a Nd:YAG laser on BN target in vacuum. Soft x-ray emission lines in the water-window region are recorded using a grazing-incidence spectrograph. Filtered photodiodes are used to obtain complementary data for water-window emission intensity and angular dependence. Spectral emission intensity changes in nitrogen Ly-α and He-α are used to show how laser wavelength affects emission. Our results show that the relative intensity of spectral lines is laser wavelength dependent, with the ratio of Ly-α to He-α emission intensity decreasing as laser wavelength is shortened. Filtered photodiode measurements of angular dependence showed that 266 and 532 nm laser wavelengths produce uniform emission.

Spatial and temporal variations of electron temperatures and densities from EUV-emitting lithium plasmas.

Planar slabs of pure Li were irradiated with 1.064 nm, 6 ns Nd:YAG laser pulses. Determination of plasma densities at both the earliest times of plasma formation and near the target surface was performed using Nomarski interferometry. The plasma parameters at later times were evaluated using optical emission spectroscopy. The space- and time-dependent electron densities and temperatures of the plasma were determined from their Stark broadening and the relative intensities of the spectral lines, respectively. The advantages and disadvantages of both of these techniques are evaluated and discussed.

Pulse shaping of transversely excited atmospheric CO2 laser using a simple plasma shutter.

The pulse from a transversely excited atmospheric CO(2) laser consists of a sharp spike followed by a long, drawn out tail region spanning about 2-5 micros caused by the nitrogen gas in the laser cavity. The nitrogen tail is undesirable in many applications because it decreases the average power of the laser pulse. We employ a pinhole plasma shutter for eliminating the nitrogen tail and shortening the pulse width. The pinhole shutter optically triggers plasma at a certain point in time with respect to the temporal profile of the laser pulse. This way, a good portion of the sharp spike is transmitted, while the energy stored in the nitrogen tail is consumed in heating the plasma. This simplistic plasma shutter is easy to build and inexpensive compared to other existing plasma shutter designs.

IMPACT: a facility to study the interaction of low-energy intense particle beams with dynamic heterogeneous surfaces.

The Interaction of Materials with Particles and Components Testing (IMPACT) experimental facility is furnished with multiple ion sources and in situ diagnostics to study the modification of surfaces undergoing physical, chemical, and electronic changes during exposure to energetic particle beams. Ion beams with energies in the range between 20 and 5000 eV can bombard samples at flux levels in the range of 10(10)-10(15) cm(-2) s(-1); parameters such as ion angle of incidence and exposed area are also controllable during the experiment. IMPACT has diagnostics that allow full characterization of the beam, including a Faraday cup, a beam imaging system, and a retarding field energy analyzer. IMPACT is equipped with multiple diagnostics, such as electron (Auger, photoelectron) and ion scattering spectroscopies that allow different probing depths of the sample to monitor compositional changes in multicomponent and/or layered targets. A unique real-time erosion diagnostic based on a dual quartz crystal microbalance measures deposition from an eroding surface with rates smaller than 0.01 nm/s, which can be converted to a sputter yield measurement. The monitoring crystal can be rotated and placed in the target position so that the deposited material on the quartz crystal oscillator surface can be characterized without transfer outside of the vacuum chamber.

Mass-limited Sn target irradiated by dual laser pulses for an extreme ultraviolet lithography source.

A thin Sn film was investigated as a mass-limited target for an extreme ultraviolet (EUV) lithography source. It was found that those energetic ions that are intrinsic with the mass-limited Sn target could be efficiently mitigated by introducing a low-energy prepulse. High in-band conversion efficiency from a laser to 13.5 nm EUV light could be obtained using an Sn film with a thickness down to 30 nm when irradiated by dual laser pulses. It was shown that the combination of dual pulse and inert Ar gas could fully mitigate ions with a low ambient pressure nearly without the penalty of the absorption of the EUV light.