Željko Andreić

Droplet formation during laser sputtering of silicon

Abstract High purity single-crystal silicon was ablated with nitrogen laser radiation wavelength 337 nm, pulse length 6 ns, maximal energy density 1.1 J/cm 2 , nonuniform target energy distribution. Many droplets were observed around the damaged target area, which seem to be ejected out of it and splashed vigorously onto the surrounding target surface. Their diameters are found to be in the range of a micrometer. The droplets were most probably produced within a single laser pulse as a result of hydrodynamical instability of the molten surface layer. Intense splashing occurs as a consequence of the large plume pressure generated by the most intense parts of the laser beam. The irregular power distribution on the target seems to enhance droplet formation significantly, since their abundance is drastically lower or even missing in similar experimental conditions but with uniform power distribution.

Dynamics of aluminum plasma produced by a nitrogen laser

13 mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/cm2 were achieved with a laser pulse duration of 16 ns. The high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 × 1019 cm3 and an electron temperature of about 2eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 500 ns the plasma cloud expands to about 0.5 mm above the target surface, cools down to about 1.2eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2km/s. The experimentally determined plasma parameters were input into numerical models of target heating and plasma expansion. The numerical results outrule the so called outflow model of plasma expansion and show reasonable agreement with an effusion model. The observed discrepancies in observed and calculated plasma parameters are attributed to the fact that the theoretical models describe the plasma expansion in vacuum only.

DYNAMICS OF LASER-PRODUCED CARBON PLASMA

Carbon plasmas produced by radiation from a ruby laser (wavelength 694.3 nm) focussed onto a carbon target in vacuum are studied spectroscopically with a time resolution of 40 ns. Measured line profiles of several ionic species (CI-CIV) were used to infer electron density and temperature at several positions above the target surface as function of time elapsed after the beginning of the laser pulse. The particle density at several positions above the target surface as function of time was judged from corrected line intensities. Experimental data are compared with theoretical predictions made with the effusion model of plasma expansion (Kelly R and Braren B 1991 Appl. Phys. B 53 160). The effusion model provided the relative particle density in the expanding plasma cloud as a function of initial target temperature. By comparing predicted and measured time evolution of particle density, an initial target temperature of about 125eV was inferred. The coupling of the laser beam energy to the plasma itself was inferred from the failure of the model of the direct target surface heating (Andreic Z, Henc-Bartolic V and Kunze H-J 1993 Physica Scripta 48 331) to produce the required target temperature.

In-band EUV radiation of ablative capillary discharges in PVC

A study of the performance of an ablative capillary discharge through a PVC capillary is reported. It is shown that the PVC capillary is capable of producing strong in-band 13.5 nm EUV radiation. This band of radiation is produced by transition arrays of multiply ionised heavy metal ions (Sn, Zn, etc.) that are constituents of various chemical additives in commercial PVC grades. A low concentration of radiating ions, good thermalisation of the capillary plasma and plasma homogeneity along the capillary axis produce well-defined bands with little line radiation outside the band of interest. Two interesting cases are discussed, both producing band radiation with a full-width at half-maximum of around 1 nm and maxima near 10, 13.5 and 15.5 nm, respectively. The plasma spectra are not very sensitive to input discharge voltage, but in some cases the amount of energy in the band of interest can be controlled by it. The most serious problem of such discharges still remains the amount of macroscopic debris produced by the capillary ablation.

Aluminum plasma produced by a nitrogen laser

13 mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/cm2 were achieved with a laser pulse duration of 16ns. This high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 × 1019 cm−3 and an electron temperature of about 2 eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 500 ns the plasma cloud expands to about 0.5 mm above the target surface, cools down to about 1.2 eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2 km/s.

Titanium plasma produced by a nitrogen laser

Titanium plasmas produced in vacuum and in air by radiation from a nitrogen laser focused onto a solid titanium target are studied spectroscopically. The energy deposition is more effective than in other cases since the wavelength of the laser is in the vicinity of Ti resonance lines.

Time resolved diagnostics of plasmas in polyacetal ablative capillary discharges

The dynamics of the plasma in an ablative capillary discharge made of polyacetal has been studied by time resolved XUV pinhole imaging and spectroscopy. Pinching of the plasma column occurring shortly after the current maximum is observed and characterized. A waved structure imprinted into the capillary wall stabilizes ablation and makes pinching more reproducible.

A SIMPLE XUV SOURCE AT 13.5 NM BASED ON ABLATIVE CAPILLARY DISCHARGE

A XUV source that produces a strong emission band at the wavelength of 13.5 nm with a FWHM of 0.6 nm and a duration of about 100 ns is described. In particular this wavelength has attracted the attention of many scientists working in the field by being a good candidate for the development of XUV lithography. The source was generated by using an ablative capillary discharge where the capillary was made of PVC (polyvinyl chloride). A remarkable burst of radiation at the above wavelength was recorded, the intensity of the radiation being higher by a factor of 10 in the spectral region of interest, as compared to usually used capillaries made of POM (polyacetal), or to recently developed capillary discharges in noble gases. Total XUV radiation energy of up to 50 mJ per pulse seems to be possible with such a device. Due to its simplicity, the described capillary discharge is a good candidate for a simple incoherent XUV source at 13.5 nm.

Dynamical modeling validation of parent bodies associated with newly discovered CMN meteor showers

Context. Results from previous searches for new meteor showers in the combined Croatian Meteor Network and SonotaCo meteor databases suggested possible parent bodies for several newly identified showers. Aims. We aim to perform an analysis to validate the connection between the identified showers and candidate parent bodies. Methods. Simulated particles were ejected from candidate parent bodies, a dynamical modeling was performed and the results were compared to the real meteor shower observations. Results. From the 13 analysed cases, three were found to be connected with comets, four with asteroids which are possibly dormant comets, four were inconclusive or negative, and two need more observational data before any conclusions can be drawn.

Evidence of lasing on the Balmer-α line of OVIII in an ablative capillary discharge

In a low-inductance ablative discharge through a capillary made of polyacetal (POM), lasing on the Balmer-α line of OVIII at 10.24 nm is identified. In line with previous studies of lasing on CVI ions, it is argued to be the consequence of charge exchange collisions after a m=0 instability. Lasing in both cases occurred at about the same time after beginning of the discharge, although lasing on the Balmer-α line of OVIII was less frequently observed, i.e., in approximately one out of ten discharges. Lasing on the CVI ion was seen in one out of three discharges. This is probably due to the need of reaching higher electron temperatures to completely strip oxygen ions simultaneously in the hot constrictions (necks) of the plasma instability.