Laser-irradiated Targets Research Articles

Investigation of energy transfer in plane laser-irradiated targets with high X-ray conversion efficiency

Results of experimental and computational investigations devoted to energy transfer mechanisms and X-ray conversion efficiency in laser-produced plasma are presented and discussed. The layers of different thicknesses and diameters deposited on the plane mylar substrate were irradiated by the focused beam of Nd:glass laser. Spectrally, temporally, and spatially resolved measurements of soft X-ray emission have been carried out at power densities of 1013-1014 W/cm2. The conditions of “re-emission” zone formation have been established. Radiative heat conductivity is shown to be the important energy transfer mechanism in the experimental conditions under investigation.

The dielectronic satellites to the 2s-3p Ne-like krypton resonance lines

We present an analysis of dielectronic satellite spectra of 2p6-2s2p63p Ne-like krypton resonance lines. The satellite structure was registered with high (better than λ/Δλ > 3500) spectral resolution in the emission of a laser irradiated gas puff target. We perform an unambiguous identification of satellite lines caused by radiative transitions from autoionizing states of sodium-like krypton ions. A total of about 20 spectral features are identified, most of them for the first time. Very good agreement between the satellite structure calculations and experimental emission spectra is obtained.

Imaging of laser-irradiated targets at a wavelength of 33.8 Å using a normal-incidence multilayer mirror

The morphology of the 33.8 Å emission from laser-irradiated targets was studied using a concave mirror with a W/B4C multilayer coating. The mirror had peak normal-incidence reflectance of 1.8% at a wavelength of 33.8 Å. The emissions from a variety of targets were imaged on film with a spatial resolution of 30 μm in the target plane. Radiatively heated, low-density plastic and silica foams, x-ray laser targets, and a gas-filled enclosure were imaged. Several targets were simultaneously imaged at wavelengths of 33.8 and 130 Å using two normal-incidence microscopes.

Investigation of the behavior of particles generated from a laser-ablated silicon target using time-of-flight measurements

We have investigated the phenomena of ArF-laser ablation with two different time-of-flight techniques and optical emission spectroscopy. The results show that the growth of the plume from the laser-irradiated Si target is divided into two components with fast (15–20 km/s) and slow (46 m/s) expansion velocities. These two components are created with different thresholds for the laser fluence: 1.8 J/cm 2 for the fast component and 2.2 J/cm 2 for the slow component. By comparing the results measured with the above three methods and SEM observation, it is thought that the origin of the fast component is a hot plasma and that of the slow component includes neutral materials in liquid phase.

Random-phase plate effects on burn-through measurements of laser-irradiated plane-layered targets

The application of random-phase plates in laser beams to improve the uniformity of target ablation is investigated by combining the simultaneous X-ray and particle emission measurements of ablation characteristics. It is shown that if some hot spots with dimensions of a few µm exist on target they persist at least during the initial stages of the laser interaction process, producing a local maximum in the ablation pressure. Lateral energy transport, inferred from the neutral particle measurements, has little influence on absorbed energy redistribution (only 1% of the energy is transported from the laser focal spot to an outer region of 2 mm size).

X-ray emission from laser-irradiated gas puff targets

This letter reports the first study of x-ray emission from plasma produced by laser irradiation of gas puff targets. The gas puff targets were created by the pulsed injection of gas into a vacuum chamber. To irradiate the gas puff target a Nd-glass laser was used, which generated 1-ns pulses of up to 15 J in energy. Spatial, spectral, and temporal measurements show that intense soft x-ray emission from the laser-irradiated gas puff target is produced.

Intensity ratio of resonance lines as a diagnostic of initial conditions suitable for XUV laser action in recombining plasmas

The time varying ratio of intensities of resonance lines of H- and He-like ions in laser irradiated cylindrical fibre targets is analysed using a hydro/atomic physics code. Correspondence between the ratio and gain on the Balmer α transition of H-like ions in the adiabatically cooling and recombining plasma is found. The line ratios and absorbed energies corresponding to maximum gains for carbon, oxygen and fluorine plasmas are calculated. A useful diagnostic method is proposed for infering the absorbed energies in the plasmas and for monitoring the production of plasmas which should give maximum gain during adiabatic expansion.

Gas dynamics of impulsive jets and pressure oscillations of a laser-irradiated target

GAS DYNAMICS OF IMPULSIVE JETS AND PRESSURE OSCILLATIONS OF A LASER-IRRADIATED TARGET N. M. Bulgakova and L. I. Kuznetsov UDC 533.6.011:535.211 I. Introduction. The interaction of an intense millisecond pulse of laser radiation (LR) with a solid target can be accompanied by pressure oscillations, recorded by a piezoelec- tric transducer on the backside of the irradiated target, with frequencies of the order of i0 kHz [i]. This phenomenon occurs on a lead target with LR intensity of about 2 MW/cm 2. In [2] it is shown that pressure oscillations are observed on different dielectric and metal- lic targets in a wide range of LR energies and is accompanied by intense vaporization of the material in the irradiation spot. Possible mechanisms for the appearance of these oscilla- tions are suggested in [3, 4]: bursts of absorption in the plasma in an unstable vaporiza- tion regime or auto-oscillatory regime of self-screening of the LR by the products of erosion. Probe measurements through a narrow channel in the target under high pressures of the sur- rounding medium have shown that oscillations of absorption of the LR in the photoerosion flame are observed in the tail part of the oscillograms in some irradiation regimes [5]. These absorption oscillations can be interpreted in a manner supporting the above-mentioned mechanism of the pressure oscillations on iradiated targets [3]. However, investigations of the attenuation of radiation in the photoerosion flame of ebonite and magnesium targets in air with LR intensities such that pressure oscillations should be observed indicate that there are no pronounced pulsations of the absorption coeffi- cient for laser radiation through the photoerosion flame [6]. Finally, simultaneous measure- ments of the pressure on the target and LR attenuation by the photoerosion flame [7] have shown that the LR transmission curves are quite smooth, and the small nonmonotonic variations on these curves are not related with the pressure oscillations on the target. These investi- gations, as well as measurements performed with different irradiation configurations [8] and change in sign of the pressure vector [2] have made it possible to propose a gas-dynamic mechanism for the appearance of the pressure oscillations on irradiated targets. In the present work, in order to check this mechanism we performed numerical modeling of the gas dynamics of an erosion flame and compared the computational and experimental re- suits in a wide range of irradiation intensities. 2. Experimental Procedure. Detailed experimental investigations of the pressure on irradiated targets were performed on a VIKA vacuum chamber [9]. The 1.06 ~m laser pulses with a width of 0.3 msec at half-height was focused on the target through a long focal length lens. The diameter of the irradiation spot ranged from 5 to 20 mm, and the target had a diam- eter of 20 man. The target was placed on a piezoelectric transducer [i0], which measured the pressure on the irradiated target. A coaxial FK-19 photocell was used to record the initial laser radiation from a beam splitter placed at the output of the laser system. The signals from the piezoelectric trans- ducer and from the FK-19 photocell were fed into an $9-8 two-channel digital oscillograph. The data were transmitted from the oscillograph to an IVK-6 measurement-computing system through a KOP interface. Processing of the signals from the piezoelectric transducer on the IVK-6 system consisted of dividing the area under the oscillogram into 10-~sec long elemen- tary sections and normalizing these elementary areas Pi twice - with respect to both the total area EP i under the oscillogram and the energy E 0, normalized in the same manner, of the initial laser radiation: ~ =

Experimental study of X-ray emission from laser-irradiated planar targets on “Mishen” Facility

X-ray emission from planar targets irradiated by 1.054-μm laser pulses was observed with temporal, spatial, and spectral resolution. The main purpose of these measurements was the investigation of energy transfer in multilayer targets and X-ray conversion efficiency. A mass ablation rate was determined from temporal analysis of multicharged ion line emission and a key role of corona X-ray emission in accelerated material preheating was established.

KrF laser-plasma interaction experiments with ns and ps pulses

We report on KrF laser-plasma interaction studies at focused intensities up to 4 × 1014 W/cm2 for pulse durations of 1–2 ns and up to 1015 W/cm2 for pulse duration of 100 ps. The longer-pulse experiments are concerned with quantifying two important features of the ablating plasma. Stimulated Brillouin scattering at moderately large L/λ has been measured in detail as a function of intensity, target Z, and angle of incidence θ to compare with modeling calculations of backscatter in inhomogeneous plasma. In addition, electrodynamic charge analyzer measurements have been made for varying intensity and target Z to compare with hydrodynamic calculations of ion expansion and recombination. In the short-pulse experiments, we report on X-ray conversion measurements for 100-ps laser-irradiated targets of varying Z at laser intensities of 1·5 × 1014 and 1015 W/cm2. In particular, it is shown that higher laser intensity leads to a substantial increase in X-ray conversion efficiency.

K- and L-shell absorption spectroscopy of laser-produced plasmas (invited)

Diagnostic by Kα and Lα,β absorption spectroscopy leads to information on the ionization state, the electronic temperature, and matter density of laser-produced plasmas. In particular, a mapping of the ionic populations is possible by recording the spectra on space-resolved x-ray spectrographs. The plasma to be probed and the x-ray backlighter source are produced by different beams of the same frequency-doubled Nd laser. Kα spectroscopy probes F-like to He-like ions and Lα,β spectroscopy is a diagnostic of higher atomic number ions stripped into configurations around the Ca-like structure. Measurements of dense matter radiatively heated at temperatures around 50–100 eV in laser-irradiated multilayered targets or x-ray heated samples evidence the effect of radiative heating on the temperature gradient in the target. The space and time-resolved measurements of the corona of a low temperature aluminum plasma are presented.

Time resolved soft x-ray imaging with submicron spatial resolution (invited)

Recent progress in multilayered mirror technology has allowed the developments of novel imaging systems for probing of high density plasmas in the soft x-ray regime. The experimental system described here basically consists of a spherical multilayered mirror which reflects a narrow band (∓5 eV) of soft x-ray radiation in the 50 to 200 eV energy region. The mirror images either the self-emission of a laser produced plasma and/or the shadow of the expanding plasma generated by a separate laser produced x-ray source with approximately 50× magnification in the image plane and submicron resolution in the target plane. The time resolution of about 150 ps is obtained with either a gated microchannel plate intensifier as detector or a short pulse x-ray backlighter. This imaging technique has been applied to several different studies of laser produced plasmas. Measurements of the overdense plasma profiles were carried out by imaging the self-emission produced by bremsstrahlung. Two-dimensional x-ray radiographs of the density region close to the ablation surface of laser irradiated targets were recorded to observe the uniformity of the overdense plasma blowoff. The hydrodynamics of thin foil targets driven with either laser or soft x-ray radiation was investigated.

Radiative cooling instabilities in the low dense plasma corona of laser-irradiated solid targets

Plasma jets produced from targets with atomic numbers ranging from 5 to 82 have been investigated in detail. Theoretical modeling based on a radiative cooling instability has been used to successfully explain the experimental results. Optical shadowgrams and x-ray pinhole pictures strongly indicate a correlation between the jets and x-ray emission from plasma. Plasma jets are intense in materials such as copper, molybdenum, tungsten, and gold which are characterized by strong x-ray emission. Interferograms indicate a large increase in electron density in the region of plasma jets.

Nonuniformity imprint on the ablation surface of laser-irradiated targets.

The first high-resolution (spatial and temporal) 2D images of the density region close to the ablation surface of laser-irradiated targets are obtained using a novel x-ray imaging technique. These show that laser intensity nonuniformities imprint themselves onto the cold target surface, even if these exist only on short \ensuremath{\simeq}10 ps time scales as is the case with a laser beam smoothed by induced spatial incoherence. This imprint phenomenon is interpreted in terms of the poor thermal smoothing under the conditions prevailing at the beginning of the pulse. It appears that the problem can be combatted by preforming a uniform plasma before interacting with the optical drive.

Experiments with laser-irradiated cylindrical targets

Results of novel experiments with laser-heated capillary targets are presented. In these experiments the interior of gold capillaries having a 200- or 700-μm inner diameter and a 2–12-mm length was axially irradiated by injection of the laser energy through one of the end openings. A frequency-doubled Nd:glass laser (λ = 0.53 μm) was employed, delivering 8-J energy in 3 ns. The experiments showed no significant backreflection of laser light. Depending on the capillary diameter and length, most of the laser energy is either transmitted or absorbed inside the capillary. The transmission of laser light was measured as a function of capillary length and found to be in good agreement with the predictions of a simple theoretical model. Two extreme cases could be identified. Capillaries with a 700-μm diameter show uninhibited laser light propagation due to multireflections off the inner wall. In contrast, at the entrance of capillaries with a 200-μm inner diameter a plasma plug forms that absorbs most of the laser energy. In both cases significant energy transport was observed to occur in the axial direction. A stable and strongly radiating plasma column is formed along the capillary axis by the collision of the radially imploding plasma. During the collision, part of the hydrodynamic energy of the plasma is converted into radiative energy. In a special case-a lower limit of ≊7% could be inferred for the conversion efficiency from laser light into X-ray radiation emitted from the rear opening of the capillary.

Time-resolved study of hot dense germanium by L-shell absorption spectroscopy.

We have used time-resolved 2p-3d absorption spectroscopy to probe the ionization degree of a germanium plasma heated by the soft x rays emitted from the rear side of laser-irradiated gold targets. We have determined the average ionization degree and the distribution of ion stages as a function of time during the expanding, cooling phase of the plasma. They have been compared with collisional-radiative distributions based on hydrodynamic simulations of the plasma parameters. Results show a slow decrease of the ionization degree from 〈Z〉=11.5 to 10.0 during a 3-ns period after gold heating. Comparison with theoretical spectra show matter densities around 0.01 g/${\mathrm{cm}}^{3}$ and temperatures of about 50 eV.

Reduction of 3ω/2 emission from laser-produced plasmas with broad bandwidth, induced spatial incoherence at 0.53 μm

Measurements of the 3ω0/2 emission from laser-irradiated targets at 0.53 μm were made at three angles over a wide range of laser bandwidths with and without induced spatial incoherence (ISI) echelons. The 3ω0/2 emission was found to be correlated with hard x rays but not Raman spectra, suggesting that the 3ω0/2 radiation was due to two-plasmon decay. Reduction of both 3ω0/2 emission and the accompanying hard x rays by ISI required five to ten times larger bandwidths than needed to suppress stimulated Raman scattering and stimulated Brillouin scattering.

Investigations of pressure nonuniformity in laser-irradiated targets using incoherent optical smoothing

From analytic models of the ablation process in laser-plasma experiments, as well as simulations with Fokker–Planck and Spitzer codes, we have identified a new effect, thermal inertia smoothing. This effect results in a smoother Fokker–Planck temporal and spatial variation of the ablation pressure than that for Spitzer, for the same applied timevarying laser intensity. It is therefore advantageous for incoherent optical smoothing schemes. This is especially true at early times in the laser pulse (≤100 ps).

X-ray emission by the rear side of laser-irradiated gold targets

We present results of X-ray emission by the rear side of gold thin targets irradiated by the Phebus laser at λ = 0.35 μm, τ = 0.7 and 1.3 ns, and EL = 1.5 kJ. A streak camera coupled with a transmission grating gave the time-resolved X-ray emission of the rear side. Also, a streak camera coupled with a slit allowed us to obtain information about the space and time evolution of the plasma. Some other diagnostics gave information about the energy balance and the X-ray conversion efficiency. The results are in good agreement with previous ones obtained with the Octal laser, particularly on optimum thicknesses for X-ray conversion efficiencies. Values of the thermal flux limiter are deduced. Simulations with FCI 1 code with multigroup radiative transfer and non-LTE ionization reproduce the experimental results only about some points. A number of reasons, such as 2-D effects and problems of opacity, are invoked.

Soft X-ray continuum from laser-irradiated high-Z targets

X-ray data were acquired from intermediate- and high-Z element targets with the Pharos III laser system. Targets, mounted at the tip of thin glass stalks, were microscopic fragments or foils of single or multiple high-Z element composition. The laser irradiance was 4 × 10 14W/cm 2 with about 300J of a focused, 1.05 μm wavelenght laser beam. A convex KAP crystal spectrograph was used to collect high-resolution spectra in the 4–20 Å soft X-ray region. The recorded spectral film densities were microdensitometered and converted by computer-processing to absolute continuum intensities. The spectral features were identified with the aid of ab initio atomic structure calculations. The continuum background was evaluated as a source of pseudo-continua for absorption studies of soft X-rays.