Environmental conditions in near-wall plasmas generated by impact of energetic particle fluxes

High Energy Density Physics 9 (2013) 568e572

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Environmental conditions in near-wall plasmas generated by impact of energetic particle fluxes a, b  O. Renner a, *, M. Smíd , T. Burian a, L. Juha a, J. Krása a, E. Krouský a, I. Matulková a, c, J. Skála a, A. Velyhan a, R. Liska b, J. Velechovský b, T. Pisarczyk d, T. Chodukowski d, O. Larroche e, J. Ullschmied f a

Institute of Physics, Academy of Sciences CR, Na Slovance 2, 182 21 Prague, Czech Republic Czech Technical University in Prague, FNSPE, 115 19 Prague, Czech Republic Faculty of Science, Charles University in Prague, 128 40 Prague, Czech Republic d Institute of Plasma Physics and Laser Microfusion, 00908 Warsaw, Poland e CEA DIF, F-91297 Bruyères le Châtel, Arpajon, France f Institute of Plasma Physics, Academy of Sciences Czech Rep., 182 00 Prague, Czech Republic b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 May 2013 Accepted 21 May 2013 Available online 30 May 2013

Directional flows of energetic ions produced by laser-exploded foils were used to investigate transient phenomena accompanying the plasma interaction with surfaces of solid targets (walls). In experiments carried out on the iodine laser system PALS, the formation of energetic plasma jets from burn-through foils of Al and Ta was optimized using the three-frame interferometry and applied to a design of alternate experimental configurations. The interaction of the directional plasma flows with secondary targets was studied via X-ray imaging, optical and high-resolution X-ray spectroscopy. The environmental conditions in near-wall plasmas created at surfaces of plasma-exposed solids, in particular the velocity distribution of impinging and back-scattered ions, were determined via analysis of the observed spatially-resolved spectra of Al Lya and Hea groups. The validity of the ion velocity gradients derived from the Doppler effect induced shifts and splitting of the spectral lines was supported by theoretical modeling based on a combination of hydrodynamic, atomic and collisional-radiative codes. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Laser-produced plasma K-shell spectroscopy Plasmaewall interaction Particle jet formation Ion back-scattering Plasma simulations

1. Introduction Formation of laser-driven plasma jets and their interaction with gaseous or solid targets is investigated in context with three different scientific disciplines. In high-energy-density laboratory astrophysics based on applications of high power lasers and Zpinches, the jets production and interaction with ambient media are studied to interpret large-scale astrophysical situations [1e3]. In the inertial confinement fusion directed research [4], the energetic particle jets are investigated as possible ignitors in alternate schemes for fast ignition. Further, important fusion-relevant applications are connected with studies of material erosion and migration at inner-walls of fusion devices [5]. The acquisition of precise experimental data on transient phenomena accompanying the interaction of plasma jets with surfaces of solids, generally known as plasmaewall interaction, PWI, contributes to the

* Corresponding author. Tel.: þ420 266 052 136; fax: þ420 286 890 527. E-mail address: [email protected] (O. Renner). 1574-1818/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hedp.2013.05.012

understanding of the material response to heavy loads of charged particles and provides information needed for a development of advanced simulation techniques assisting in a design of fusion reactors [6]. A brief survey of processes accompanying the energy transfer in the near-wall region, e.g., ion deceleration and stopping, shock wave generation, formation of highly excited Rydberg states or hollow ions, charge transfer and ion neutralization, and an overview of previous studies of laser-produced plasmaewall interaction can be found in Ref. [7] and references therein. The aim here is to contribute to the precise X-ray spectroscopic characterization of near-wall plasmas generated by an impact of jets of energetic laserproduced particles onto solid surfaces. Benefiting from optimization of the plasma outflow from the laser-burn-through foils [8], we describe the experiment directed to the investigation of environmental conditions in the colliding and interpenetrating plasmas produced with double-foil targets. As an extension of the previously reported distribution of the plasma density and temperature in the inter-target space [7,9], here we concentrate on the measurements of the velocity profiles of ions impinging onto secondary

O. Renner et al. / High Energy Density Physics 9 (2013) 568e572

targets and back-reflected from their surface. The ion velocities are derived from Doppler shifts of the emitted spectral lines, the observed splitting of the Al Hea emission is interpreted in terms of the radial expansion of the ions back-scattered from the near-wall plasma. This interpretation of the experimental data is supported by plasma simulations based on a combination of hydrodynamic, atomic and collisional-radiative codes. 2. Overview of the experiment The experiment was carried out on the Prague iodine laser system PALS [10]. The laser beam delivering 25e70 J of frequencytripled radiation (438 nm) in a pulse length of 0.25e0.3 ns FWHM and intensities 2  1014e5  1015 W/cm2 was incident onto the primary, jet-producing target, with the composition of 0.8-mmthick Al or 0.5-mm-thick Ta foil, inclined by an angle of 30 from the target normal. The secondary target, composed of 2-mm-thick Mg foil or 250-mm-thick pyrolytic graphite foil, was positioned in a distance of 0.5e1 mm from the primary foil. Hydrodynamic simulations indicate that the irradiated foil burns through well before the laser pulse maximum [9], and the expanding plasma plume created at the rear (non-irradiated) foil surface propagates primarily along the direction of the target normal. Consequently, the target-beam configuration, schematically shown in Fig. 1, determines the regime of the plasma interaction with the secondary target. At normal incidence, the plasma jet strikes the secondary target pre-ionized by the action of the transmitted laser light, i.e., the PWI effects are complemented by the near-wall interaction of two counter-propagating plasmas. In the oblique incidence case, the laser beam does not hit the secondary target and the expanding plasma interacts with the unperturbed surface, though potentially pre-heated by fast particles emitted from the primary plasma and by its radiation, thus creating a better-characterized environment for PWI studies. Further we demonstrate that by optimizing the laser interaction with the primary target, well collimated plasma jets can be produced. The standard diagnostic complex used in the PALS PWI experiments (time-resolved X-ray imaging of plasma expansion, optical spectroscopy, survey and high-dispersion X-ray spectrometers [7]) was complemented by a three-frame laser interferometer. The temporally- and spatially-resolved electron density ne of the expanding plasma was determined by applying the Abel transformation on the measured 2D phase distribution of the plasma probing radiation. The diagnostic beams were split-off from the main iodine laser beam and frequency-doubled, thus limiting the maximum measurable electron density to 2.6  1021 cm3. The delay between contiguous frames was set to 3 ns.

VJS Al

streak camera

C

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0

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optical spectroscopy

z Al C plasma plume

optical interferometry

Fig. 1. Schematic drawing of the plasma formation at the laser-irradiated double-foil targets and the configuration of the diagnostic complex.

569

By varying the laser beam energy and the focal spot radius R, the optimum conditions for the plasma jet generation in the transmission target geometry were found. To demonstrate this, in Fig. 2 we present the ne distribution recorded when irradiating the double-foil Ta/Mg target by the laser at an oblique incidence (42 J, R ¼ 150 mm, foil spacing 1 mm). The frame timing relates to the laser pulse maximum, the distance r is measured along the foil surface and z along its normal. The electron density distribution is indicated by equidensity contours, the outer plasma contour refers to the ne ¼ 1018 cm3. The frequency-tripled beam of the PALS is characterized by a central depression in its transverse intensity distribution. When increasing the focal spot radius, the central intensity drop is deeper and pinching effects of the axial plasma expansion cause a larger part of the generated plasma to collide on the axis [11]. The dense core of the Ta plasma propagates between both foils in a laminar flow without large-scale plasma turbulence. The interferograms clearly demonstrate the influence of the secondary target on the axial plasma outflow. Contrary to the free plasma expansion, where the density monotonically decreases with the distance from the primary target, the presence of the secondary target results in stopping the axial plasma flow. At t ¼ 4 ns, the Ta jet forms a counterpropagating plasma at the secondary Mg target. By t ¼ 7 ns there is a continuing weaker plasma outflow that further increases the electron density near the secondary target where the Ta plasma dissipates laterally. This effect, however, introduces only small perturbations into the observation of the energetic plasma jet interaction with the secondary target. Similar characteristics of the jet formation were found when irradiating the Al foils. Consequently, jets launched from the burnt-through foil targets represent a well-defined model environment for investigating transient phenomena at surfaces of plasma-exposed solids. 3. Spectroscopic results and discussion The optical spectra characterizing the ionization and ablation of atoms at the surface of secondary targets due to the impact of energetic particles were recorded using the Oriel Instruments imaging spectrometer MS257. The line of sight of the micro-lens connected to the optical fiber cable was approximately perpendicular to the plasma jet propagation. The lens covered the plasma emission area with the diameter of approximately 1.5 mm, the spectral resolution of the grating used was 1.5  103 eV. The images were taken from single laser shots using the CCD detection system (iCCD; iStar 720, Andor). The spectrum shown in Fig. 3 was recorded at the double-foil Ta/Mg target, for a 69 J, R ¼ 150 mm laser shot on to a target with an inter-foil spacing of 1 mm. The spectrum was taken 30 ns after the laser pulse maximum with the acquisition time of 200 ms, i.e., it characterizes the plasma recombination phase in the full inter-target volume. The interaction of the Ta jet with the counter-propagating Mg plasma is visualized via the absorption structure in the Ta quasi-continuum, the main pumped transitions in the singly- and doubly-ionized Mg are identified. The simulations performed using the PrismSPECT code [12] indicate the presence of the two-component recombining plasma. Its parameters were estimated to be ion density 1  1017 cm3, temperature in the range of 7 and 1 eV, and, average charge in the range of 2.65e 0.98. With respect to the delayed and relatively broad temporal duration of the spectrum exposure, these parameters characterize the plasma recombining instead of the impact phase. On the other hand, the observed absorption features prove the excitation and ionization of the secondary Mg target and indicate prospective application of the optical methods in investigation of the plasma recombination processes accompanying the PWI. The principal X-ray diagnostics applied in the present PWI study was a vertical-geometry Johann spectrometer (VJS) [13] fit with the

O. Renner et al. / High Energy Density Physics 9 (2013) 568e572

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Fig. 2. Electron density distribution in the optimized plasma jet observed when the laser impinges on the 0.5-mm-thick burn-through Ta foil at an oblique angle (42 J, R ¼ 150 mm) and its interaction with the secondary Mg target positioned in the distance of z ¼ 1 mm.

cylindrically bent crystal of quartz (100). By combining the high spectral (l/Dl z 6000) and 1D spatial resolution (5 mm), the VJS simultaneously provides two sets of spectra symmetrically displaced about the central wavelength l0 (see Fig. 1). The first application concerns the precise measurements of the ion deceleration close to the walls. The double-foil Al/Mg target was irradiated using the laser intensity 4.3  1015 W/cm2. The ions produced by oblique incidence of the laser onto the Al foil were incident on the secondary Mg target positioned at a distance of 550 mm, the plasma interaction with the wall was investigated via the X-ray self-emission of the H-like Al ions. The spatially resolved spectra presented in Fig. 4 were observed at an angle of 0.8 to the Al foil surface, the spatial resolution was obtained along the normal to this (z-direction). The time-integrated spectra were recorded on X-ray film Kodak Industrex HS800, digitized using the scanner with the resolution of 4800 dpi and recalculated to the linear wavelength and intensity scale by using the algorithm described in Ref. [13]. A distinct satellite structure (including the strongest J-satellite) observed in the Al Lya group emission is restricted to the relatively cold plasma regions within 80 mm from the Al foil, the intensity of the Al Lya resonance line gradually drops with the increasing distance z from the primary target. Starting at z z 300e350 mm, the Al Lya intensity grows again and attains its maximum at the distance of approximately 100 mm from the Mg foil, i.e., in the strongly collisional plasma zone where the forward streaming Al ions interpenetrate the counter-propagating plasma cloud. Simultaneously the resonance line shifts to higher frequencies (dot-dotdash line). The detailed analysis taking into account the z-

dependent angle of the spectral observation explains this blue shift of the Lya line by a macroscopic Doppler effect connected with the variable velocity of the Al ions. The observed line shift corresponds to the gradual deceleration of Al ions from the maximum axial velocity w5  107 cm/s to their full stopping at the Mg surface. The velocity decrease in the 100-mm-thick plasma layer close to the Mg target amounts to approximately 7.6  106 cm/s, thus resulting in the velocity gradient 7.6  108 s1. In contrast to previously reported experiments with double-foil targets irradiated at the normal laser incidence [14], the ion velocity profile is not affected by the relatively dense counter-streaming Mg plasma generated by a direct impact of the laser beam transmitted through the burnthrough Al foil. Instead, the energetic Al ions are decelerated due to collisions with the diluted plasma consisting of the Al ions trapped in the near-wall region and Mg ions ejected by the impinging Al jet from the secondary target surface, thus providing more relevant data for PWI studies. The second VJS spectral record presented in Fig. 5 indicates a more complicated interaction scenario. The plasma jet was produced on the Al foil irradiated with a 27 J, R ¼ 40 mm, and 2  1015 W/cm2 pulse at oblique incidence, the secondary C target was positioned at a distance of 600 mm. The emission of the Al Hea group e including the resonance, w, intercombination, y, and Li-like satellite transitions with the strongest being the j-satellite e was observed tangentially to the Al surface and recorded on X-ray film Kodak Industrex AA400. Again, the emission of satellites is restricted to the plasma regions close to the Al foil, whereas the w and y lines extend throughout the inter-target space. The broadening and splitting of these lines was attributed to the Al ions deceleration, trapping and back-scattering from the C surface.

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wavelength [A] Fig. 3. Absorption structure in the Ta quasi-continuum characterizes the ionization and ablation of surface atoms of the secondary Mg target due to the impact of energetic Ta particles.

Fig. 4. Spatially resolved spectra of the Al Lya emission indicates the Al ions deceleration close to the Mg foil surface. The laser beam strikes the Al foil from above.

O. Renner et al. / High Energy Density Physics 9 (2013) 568e572

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wavelength [A] Fig. 5. Spatial distribution of the Al Hea emission from the double-foil Al/C target irradiated at the oblique laser incidence.

To validate this scenario, the plasma evolution and its X-ray emission were modeled using a combination of four codes. The initial phase of the plasma expansion was calculated using the Prague Arbitrary Lagrangian-Eulerian hydrodynamic code PALE [15]. This 2D fluid code does not include the plasma interpenetration, thus the simulation proceeds until the Al plasma reaches the C target (t z 0.3 ns). At the time the Al plasma reaches the C target, the calculated distribution of hydrodynamic parameters is transferred to the MULTIF hydrocode [16] to define initial conditions for further modeling including plasmas interpenetration. Since the MULTIF is a 1D code, a pseudo-2D regime has been modeled by interpolating the PALE output onto a rectangular Eulerian grid and

seven axial cuts of this grid were used for separate MULTIF calculations. These simulations that were performed at various radii within the range r ¼ 0e240 mm are separated by white solid lines in Fig. 6. The output from the PALE and from the full set of MULTIF runs has been interpolated on a smooth 2D Eulerian grid and inserted into the CRETIN collisional-radiative solver [17]. By using the detailed atomic data calculated with the HULLAC [18] atomic model builder, the CRETIN simulated the population dynamics and synthesized the space- and time-resolved X-ray spectra. Results of simulations for the Al (a) and C (b) ions distribution together with the Al He-y emission (c) inside the inter-target volume are shown for three characteristic times in Fig. 6. In the period of the laser maximum (t ¼ 0 ns), the plasma cloud formation and its emission are restricted to the region near the Al foil. At t z 0.6 ns, the forward streaming Al ions reach the C foil and the secondary plasma consisting of a mixture of both elements expands backwards to the Al foil during the rest of the simulation. The details of the simulations will be published elsewhere [19], here we only point out that starting from t z 1.5 ns, a prevailing part of the line radiation is emitted from the near-edge dense plasma regions with the largest ion velocities, while the inner plasma portions produce less emission. The pseudo-2D MULTIF simulations do not include radial ion velocities which, combined with the annular structure of the strongly emitting plasma, provide a tentative explanation of the observed line splitting. The needed radial velocity profiles vrad were included into the modeling via parametrization vrad(r,t) ¼ k r/rmax (t  t0), where rmax ¼ 240 mm is the maximum radial position included in the simulation, and t0 and k are fitting parameters used to obtain the best agreement with the measurement. The comparison of the experimentally observed (a) and the simulated (b) spatially resolved Al He-

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wavelength [A] Fig. 7. Observed splitting of the Al He-y emission (a) and its comparison with simulations (b).

y emission is presented in Fig. 7. The best fit between the simulations and the experimental data has been achieved for a combination of parameters t0 ¼ 0.7 ns and k ¼ 1.2  1016 cm/s2. The fitted z-dependent radial ion velocities are comparable with the maximum axial velocities predicted by the MULTIF simulations, thus implying semispherical scattering of impinging ions from the secondary target. The agreement between the experiment and simulations is qualitative rather than quantitative. The maximum experimentally observed radial ion velocities (up to 3  107 cm/s) exceed theoretical predictions by a factor of 2, the underestimated emissivity of the forward streaming Al ions indicates the insufficiency of the plasma modeling. Despite this, the similarity between the experimental and synthesized line profiles clearly identifies the Doppler effect induced frequency shifts due to the radial velocity components of the backscattered Al ions and an enhanced emission from the expanding plasma surface as the principal mechanisms of the observed spectral line splitting. To our knowledge, these precise measurements represent the first X-ray spectroscopic observation of the ions backscattering from the surfaces of secondary targets. 4. Conclusions Spectroscopic investigation of phenomena accompanying the impact of directional flows of energetic plasma particles onto solid targets requires an application of precise instruments and methods in combination with well-defined experimental conditions. In experiments performed at the PALS laser facility, the production of plasma jets at laser-burnt-through foils was optimized using the three-frame interferometry. The modified experimental configuration benefiting from the use of an oblique laser incidence onto the primary, plasma-jet-producing target, provided a better characterized environment for plasmaewall interaction studies. The absorption structure observed in optical spectra emitted from the

laser-irradiated double-foil Ta/Mg target confirmed the ionization of the secondary target surface due to the impact of the collimated beam of Ta particles and provided information on plasma parameters in the recombination phase. The high-resolution X-ray spectroscopy was used to characterize the ion velocity distribution in the near-wall plasmas produced at Al/Mg and Al/C targets. The measurements of the velocity gradients of the Al ions impinging onto the secondary Mg target were based on the analysis of the spatially resolved Al Lya spectra. The splitting of the Al Hea resonance and intercombination lines observed at the Al/C target was attributed to the radial expansion of the Al ions back-scattered from the secondary C target. The validity of the ion velocity gradients derived from the Doppler effect induced shifts and splitting of the spectral lines was supported by theoretical modeling using a combination of hydrodynamic, atomic and collisional-radiative codes. To conclude, the reported experiments prove the diagnostic potential of X-ray spectroscopy for detailed characterization of phenomena connected with the impact of directional flows of energetic plasma particles onto solid targets. The investigation of environmental conditions in the near-surface plasmas provide complex information on behavior of materials at extreme situations and open new possibilities for their applications. Acknowledgments This research was supported by the Czech Science Foundation grant No. P205/10/0814, by the Academy of Sciences of the Czech Republic Programme of internal support of international collaboration, project M100101208, and by RVO 68407700. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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