Laser-produced Lithium Plasma Research Articles

Calculation of high-order harmonic generation in laser-produced lithium plasma.

We report on detailed calculations of high-order harmonic generation (HHG) spectra from Li, Li+, and He using the Lewenstein model. Our model well describes the HHG experiment in Li plasma [J. Phys. B45, 065601 (2012)JPAPEH0953-407510.1088/0953-4075/45/6/065601]. The cutoff position, in the case of neutral lithium atoms (43th harmonic of 800nm radiation), corresponds to 3.5×1014 W/cm2 laser intensity, as numerically-simulated in the current work. The difference from the experimental value can be well explained by the measurement errors and uncertainty in determining the laser intensity, which is usually around 25%. We found that Li+ ions do not contribute to HHG plateau region of the spectra and that those ions start to contribute only at very high intensities. Our calculations show that the application of materials possessing higher ionization potential does not necessarily leads to the extension of HHG cutoff.

Neutral and ion composition of laser produced lithium plasma plume in front and back ablation of thin film

We report detailed comprehensive compositional analysis of the evolution of lithium plasma in back and front ablation geometries.

Magnetic field selective enhancement of Li I lines comparing Li II line in laser ablated lithium plasma at 10− 2 mbar air ambient gas

In this paper, the effect of magnetic field (1.1T) on the atomic and ionic spectral emission of a laser produced lithium plasma at low pressure has been investigated. The experimental results indicate that magnetic field enhances the intensities of Li I spectral lines but reduces the Li II spectral lines intensities. In this study, two narrowband filters were placed before the ICCD camera to observe the evolution feature of Li II spectral line (548.39nm, 2p3P2,1,0→2s3S1) and Li I spectral line (610.30nm, 3d2P3/2, 5/2→2p2P1/2, 3/2), respectively. The plasma dynamic images show that with the magnetic field, the number density of luminous Li atoms is higher, while the number density of luminous Li ions is lower in comparison to the field-free case. The reduced Li II spectral intensities indicate that the quenching rate of Li ions in the excited state is greater than that without the magnetic field. The enhanced impact frequency of recombination indicates that magnetic field increases the recombination process of electron and Li ions. All of these observations strongly suggest that magnetic confinement increases the recombination process of the electrons with Li ions in the plasma, which results in the decrease in the intensity of Li II line. The results are useful for applying laser-induced breakdown spectroscopy (LIBS) to in-situ diagnose the processes of lithium wall conditioning in EAST tokamak.

Image analysis of expanding laser-produced lithium plasma plume in variable transverse magnetic field

AbstractUsing fast imaging technique, the effect of transverse magnetic field on the shape and dynamics of the lithium plasma plume has been studied. Enhancement in the overall emission intensity as well as appearance of distinct structures (lobes) in the plasma plume in the presence of magnetic field has been observed. By introducing a variable magnetic field, the influence of J × B force in expanding plasma plume across the transverse magnetic field has been explored. It appears that J × B force does not has a substantial role in the observed structures in the present case rather different atomic processes, which get affected due to change in plasma parameters, are responsible for appearance of these structures.

Effect of a transverse magnetic field on the plume emission in laser-produced plasma: An atomic analysis

We have investigated the effect of varying transverse magnetic field on the plasma plume emission of laser-produced lithium plasma. Two atomic transitions for lithium neutral Li (I) and two for Li ion LI (II) are taken for the study. It has been found that for Li (I), the emission from 670.8 nm transition (2s 2S 1/2←2p 2P 3/2 ′ 1/2) shows initial enhancement and then subsequent decrease for higher fields. Of course, the overall intensity is increased for all the fields when compared to the case of without field. On the other hand, for 610.3 nm (2p 2P 1/2←3d 2P 3/2 ′ 5/2), there is continuous decrease in intensity. Interestingly, for Li (II) transitions also, after an initial increase in intensity up to 0.08 Ta decrease is observed. From the atomic analysis, we find that for 670.8 nm line, the cause of initial enhancement is increase in electron impact excitation whereas for decreased intensity, increased field-induced ionization appears to be responsible mechanism. However, for 610.3 nm line, decrease in intensity appears to be due to decreased recombination. For Li (II), 478.8 nm (3p 1P 1←4d 1D 2) and 548.4 nm (2s 3S 1←2p 3P 2,1,0) transitions, initial increase appears to be due to increased confinement (increase in plasma density) and subsequent decrease in intensity with increase in field due to decreased recombination.

Emission analysis of expanding laser produced lithium plasma plume in presence of ambient gas

Atomic analysis of the emission from laser-produced plasma from bulk lithium (Li) block has been made. The observed changes in the emission from lithium neutral and ionic species have been explained by considering various atomic processes. We demonstrate that the excitation process in case of neutral lithium Li (I) is electron impact excitation whereas for singly ionized lithium Li (II), radiative recombination is the dominant mechanism. We also show that the ionized species are dominant in the LPP plume.

Space-resolved soft X-ray emission from laser produced lithium plasma

Space-resolved emission spectroscopy was used to study the evolution of 13.5 nm line intensity and electron temperature of the plasma generated by laser ablation of lithium target. Two emitting regions were observed, their intensities depending on laser fluency. Plasma image is discussed in the frame of a Gaussian model of particle expansion.

Soft x-ray spectra from laser heated lithium targets

The extreme ultraviolet emission characteristics of laser-produced lithium plasma are experimentally and theoretically investigated. Taking into account the target oxidation, the ion fractional populations are analyzed and the averaged plasma parameters are obtained. Theoretical results show an insignificant influence of oxygen on the temperature dependence of the lithium ion fractional number. A comparison between the theoretical spectrum for stationary and homogenous plasma and experiments shows a reasonable agreement.

Target fabrication of low-density and nanoporous materials to generate extreme ultraviolet (EUV)

Our institute has been investigating laser-produced tin, lithium and xenon plasma for extreme ultraviolet (EUV) light source of EUV lithography system, and found the highest conversion efficiency of 3% at 13.5 nm in 2% bandwidth. In the present paper, we introduce fabrication methods of density-controlled tin targets to generate relatively monochromatic EUV, and higher conversion efficiency than bulk tin. The first method is the nano-template method, where liquid tin solution was immersed into a polymer film with monodispersed size nanoparticle. The density can be controlled by tin concentration of the solution. The target can be classified into mass-limited target. We have shown a tendency of monochromatic EUV emission around 13.5 nm with decreasing of tin density. The intensity was higher than tin foil with bulk density. The tendency has a merit to mitigate heat effect of the first EUV mirror. The fabrication method has another merit to control not only density but also the poresize of tin oxide to be 100 nm ∼ 10 μm. The second is liquid crystalline template method to obtain porous tin oxide. The precursor with tin oxide and cellulose provides mechanically stable and transparent film. The film has wavy sub-microstructure derived from microscopic liquid crystal domain structure. The method is simple and short duration for the hydrolysis reaction to solidify tin compound. This material has a merit of feasibility of fabrication, and has been applied for rotation target for 10 Hz and 5 kHz laser repetition.

Effects of resonant pumping on the temporal and spatial evolution of a laser produced lithium plasma

The spatial and temporal distribution of neutral and singly ionized lithium atoms has been investigated when slab targets are irradiated by ∼1 μs pulses of both resonant and nonresonant radiation from a flashlamp pumped dye laser. The ion population is enhanced when the laser is tuned to resonance with the 2s–2p line of neutral lithium and there is evidence for efficient ion production at distances up to 7.5 mm from the target surface. In particular, the rate of ionization is increased and the maximum ion population is attained between 120 and 200 ns earlier in the resonant case depending on distance above the target surface. However, the on-resonance ion population close to the surface is found to decrease significantly some 200 ns into the laser pulse. Ion and neutral drift velocities were measured by direct observation of the surface of the expanding plasma plume at a laser flux close to 8×108 W cm−2 and were consistent with the value predicted by a dynamic source model for accelerated plasma expansion, as was the value for the electron temperature derived from line intensity ratios. Power density thresholds for ion production have been studied and in particular, the threshold for Li2+ production is established.

Diagnostic of an expanding laser-produced lithium plasma using ICCD frame photography and shadowgraphy

The expansion of a laser-produced lithium plasma is characterized using two different high-speed imaging techniques. Firstly, a sequence of frames of the luminous plume is recorded using an interference filter/gated ICCD camera combination. Expansion velocities are estimated from these images. The conditions, in which the radial distributions of emitters could be recovered using Abel inversion, are discussed. Secondly, shadowgraphs obtained with a synchronized tunable dye laser light source are recorded at different probe wavelengths in the vicinity of the Li 0 670.7-nm resonance. The fringe patterns observed in these images are interpreted in terms of strong refractive index gradients within the plasma. The effect of anomalous dispersion is observed and strongly modifies the appearance of the shadowgraphs.

Absorption spectroscopy of an expanding laser produced lithium plasma in the extreme ultraviolet using the Dual Laser Plasma technique

We describe the essential features of the Dual Laser Plasma (DLP) vacuum ultraviolet photoabsorption spectroscopy technique and the characteristics of our DLP apparatus. We show that the time- and space-resolved capabilities of this technique are suited to the monitoring of the dynamics of expanding plasma plumes in the regime used for pulsed laser deposition of materials. Examples of spectra showing the spatial and temporal evolution of a lithium plasma expanding in vacuum are presented. A model based on a self-similar expansion for the plume is developed and used to analyse the shape of absorption lines. Measurements in the photoionisation continuum of Li + are also presented.

Laser-produced lithium plasma as a narrow-band extended ultraviolet radiation source for photoelectron spectroscopy

Extended ultraviolet (EUV) emission characteristics of a laser-produced lithium plasma are determined with regard to the requirements of x-ray photoelectron spectroscopy. The main features of interest are spectral distribution, photon flux, bandwidth, source size, and emission duration. Laser-produced lithium plasmas are characterized as emitters of intense narrow-band EUV radiation. It can be estimated that the lithium Lyman-alpha line emission in combination with an ellipsoidal silicon/molybdenum multilayer mirror is a suitable EUV source for an x-ray photoelectron spectroscopy microscope with a 50-meV energy resolution and a 10-mum lateral resolution.

Application of Microchemical Analysis to Diagnostics of Lithium Plasma

A method for measuring diffusion of a plasma produced from a metal of low vapour pressure with the aid of microchemical analysis is presented. A pinholed cavity of a suitable material can be used as a perfect sink of such a plasma stream. The trace amounts of the plasma deposited in the cavity are determined by flame photometry, the sensitivity of which is exceedingly high for alkali metals. An example is shown of the application of this method to the electric drift motion of a laser-produced lithium plasma in a uniform magnetic field. The number of lithium atoms colliding with the wall of a vacuum chamber, determined by flame photometry, shows a fairly good agreement with the density of plasma obtained by a double probe.