Valeria G. Grishina

Interaction of laser ablation plasma plume with grid screens

Optical emission spectroscopy and charge collector time‐of‐flight measurements have been used to study interaction of laser ablation carbon plasma with grid screens in vacuum under conditions typical for pulsed laser deposition of thin diamondlike films. The effect of velocity distribution transformation of the ion flow has been observed and studied in a variety of experimental conditions. Our results indicate that the observed phenomenon is due to interaction of two plasma flows, the initial expanding one and the fraction of that flow scattered by the substrate or the screen. Three typical modes of velocity distribution function transformation have been observed depending on the plasma density: linear attenuation of the flow density, nonlinear attenuation of the slow ‘‘tail’’ of the velocity distribution function, and nonlinear transformation of the entire velocity spectrum. The latter regime occurs when plasma is throttling through the fine mesh screen. Our observations show that the reported phenomenon...

Instabilities of pulsed low‐temperature laser‐induced plasmas expanding in the low‐density gas

Rayleigh‐Tailor instability of a plasma induced by a microsecond laser pulse and expanding into a gas with mass density of a few μg/cm3 has been experimentally using a streak camera. Both homogeneous targets and those with pre‐fabricated periodic inhomogeneities were used. The instability growth time was found to be close to 200 ns, and the observations for times up to 3 μs have been made. Changes in the observed instability spatial period in the course of plasma expansion and deceleration are reported and compared for different targets. Ignition of a laser supported detonation in ambient gas results in stabilization of the expanding target plasma front.

Interaction of flows of laser ablation plasmas

Our studies of laser pulsed deposition of thin films, using 20-ns second-harmonic Nd:YAG laser operated at a few GW/cm2, show that introduction of an obstacle like a grid screen into ablation plume with typical density 1018 cm−3 and temperature 3 eV substantially affects dynamics of plasma expansion and results in dramatic changes of plasma parameters. In this work, we report the effects typical for the later stage of plasma-grid interaction—effects related to the interaction of multiple plasma flows. Model experiments, performed employing method of emission spectroscopy, time-of-flight technique, and plasma imaging with CCD camera, show significant variation of plasma composition and transformation of velocity distribution due to plasmas interaction. Two modes of such transformation have been observed: linear growth of density in the resulting flow, and formation of the additional slow peak in the velocity distribution. The effect is shown to be velocity-resonant.

Effect of laser ablation plasma on dynamics of a laser absorption wave in a gas

Experimental results indicate deceleration of the detonation wave under conditions of partial absorption of laser radiation when both plasmas--detonation wave and ablation plasma--are ignited simultaneously. The observed effect is attributed to a reduction of laser radiation intensity, reflected by the target surface and transmitted back to the detonation wave, due to the absorption in the dense ionized vapor of the target material. Also, we report results of studies on dynamics of axial and radial shock waves formed in a gas during propagation of the detonation wave. The observed differences between experimental results and self-similar computations are attributed to the effect of the hot gas transfer along the laser beam axis out of the zone of absorption of laser radiation. All the experiments were performed using carbon-dioxide laser with pulse width of a few microseconds and energy of 1 J. We employed streak camera, fast diodes, CCD camera imaging, and plasma transparency measurement techniques for data acquisition.

Methods to control plasma parameters in experiments on pulsed laser deposition of carbon films

Studies of laser pulsed deposition of thin films show that introduction of an obstacle like a grid screen into ablation plume substantially affects dynamics of plasma expansion and results in dramatic changes of plasma parameters. Two regimes of plasma-grid interaction have been studied in detail -- regime of the free-molecular flow and the throttling. The grid screen may break plasma plume into several interacting with each other flows. Some features of plasma-grid interaction (like transformation of velocity distribution function and variation of plasma temperature and density) can be attributed to the interaction of plasma flow with the generated shock wave. Different dimensional effects controlling plasma-grid interaction have been considered. We report the effect of velocity distribution transformation on the stage of interaction between multiple plasma flows behind the screen. Two modes of such transformation have been observed: linear growth of density in the resulting flow, and formation of the additional slow peak in the velocity distribution. We associate these two modes of transformation with two different regimes of plasmas interaction -- with regime of lateral loss reduction, and regime of expansion in the background gas.

In-situ monitoring of carbon dimer formation within ablation plume

Carbon dimer formation during ablation plume expansion and subsequent interaction with a substrate is recognized to affect substantially the process of pulsed laser deposition (PLD) of thin films. In this work we report formation of C2 molecules during laser ablation from graphite targets under conditions typical for PLD. Two types of lasers were used for ablation -- 5 microsecond carbon-dioxide laser operated at 1 J and 10 ns Nd-YAG laser operated at 100 mJ in the second harmonic. We present results of time-resolved (50 ns) emission spectroscopy, plasma imaging and time-of-flight techniques. C2 molecules formation has been studied in a variety of experimental conditions, including ablation plume interaction with target surface, plume interaction with the surface of a substrate and two-plasmas interaction. Particularly no molecules formation was observed at maximum energy and there was an optimum laser pulse energy for dimers formation for short pulses of Nd:YAG laser. The plumes in both cases had shell like structure with high ions inside and neutrals located in the outer layers. Molecules formation occurred to be more efficient in the outer layers of the plume in the case of carbon-dioxide laser.

Interaction of laser-induced quasi-stationary plasma flow with substrates

The effects of interaction of quasi-stationary carbon plasma flow with a substrate have been studied using time-integrated and time-resolved emission spectroscopy and ion time-of-flight technique. Plasma flow with typical electron density of order of 1018 cm-3 and temperature of 5 eV was created with 5 ms pulses of TEA carbon-dioxide laser. Plasma- substrate interaction has been shown to result in significant variation of plasma parameters and plasma composition near the surface of the substrate.

Multidimensional dynamics of laser-ablated materials

We experimentally investigated the expanding CO2 laser produced carbon plasma interaction with a substrate and the interactions occurring when two laser produced plasmas collide. The electron density and temperature of the single plasma source were measured and ion composition of plasma was determined by means of optical spectroscopy. The carbon dimers were observed near the target and substrate surfaces and during the collision of two plasmas.