Main Laser Pulse Research Articles

The influence of pre-pulse plasma on ion and x-ray emission from Ta plasma produced by a high-energy laser pulse

The paper describes studies of ion and x-ray emissions from plasma produced by the PALS laser system at different parameters of pre-plasma generated by a laser pre-pulse. The plasma was produced by 0.4-ns laser pulses of about 140 J at 438 nm (3rd harmonic frequency) focused on a slab Ta target. The main laser pulse was preceded by a pre-pulse having either 0.7 main-pulse energy. The main pulse was delayed by 0.6 ns, 1.2 ns, 2.3 ns, or 4.6 ns with reference to the prepulse. The characteristics of the ion streams (energies, charge states and angular distributions of ion emission) were investigated using diagnostics based on the time-of-flight method. The x-rays were measured using various types of semiconductor detectors covered with different filters. It was found that, at small delay times, Δt 0.2 A cm−2 at 1 m) ion stream having angular divergence and ion energy spread significantly smaller than those for the case without the pre-pulse.

Laser-induced adiabat shaping by relaxation in inertial fusion implosions

The theory of laser-induced adiabat shaping is carried out for inertial confinement fusion (ICF) capsules. It is shown that a significant improvement of the stability characteristics of ICF implosions can be achieved by shaping the adiabat inside the imploding shell. The optimized adiabat profile has a maximum on the outer ablation surface to lower Rayleigh–Taylor growth rates, and a minimum on the shell inner surface for high compressibility and high neutron yields. Laser-induced adiabat shaping is produced via relaxation using a weak prepulse followed by laser shut-off and the main laser pulse.

Soft x-ray emission from postpulse expanding laser-produced plasmas

A diagnostic spectrometer has been developed at the Naval Research Laboratory to measure the time resolved absolute intensity of radiation emitted from targets irradiated by the Nike laser. The spectrometer consists of a dispersive transmission grating of 2500 lines/mm or 5000 lines/mm and a detection system consisting of an absolutely calibrated Si photodiode array and a charge coupled device camera. In this article, this spectrometer was used to study the spatial distribution of soft x-ray radiation from low Z elements (primarily carbon) that lasted tens of nanoseconds after the main laser illumination was over. We recorded soft x-ray emission as a function of the target material and target orientation with respect to the incoming laser beam and the spectrometer line of sight. While a number of spectral features have been identified in the data, the instrument’s combined temporal and spatial resolution allowed observation of the plasma expansion from CH targets for up to ∼25 ns after the cessation of the main laser pulse. The inferred plasma expansion velocities are slightly higher than those previously reported.

Laser-induced nonlinear excitation of collective electron motion in a cluster

We consider the nonlinear collective electron dynamics inside a largecluster irradiated by a strong linearly polarized short (≲0.1 ps) laser pulse. The equation of the centre-of-mass motion of the electron clouddriven by the strong laser field is derived using the approximation of anincompressible medium. The analysis of this equation demonstrates the presenceof odd harmonics of the fundamental frequency in the cluster dipole moment, andin both the internal and the scattered electric field. Both neutral and ionizedclusters are considered. For clusters with radii R ≃ 100 Åirradiated by a femtosecond titanium–sapphire laser with a peakintensity of I ≃ 1016 W cm−2,the internal electric field strength near the tripled fundamental frequencyis shown to be of the same order as the field of the fundamental. Thereason is that both for metallic and for laser-ionized van der Waalsclusters the Mie surface-plasmon energy ℏωMis around 5 eV, which is close to three times the energy of a titanium–sapphirelaser-field quantum. On the other hand, the condition for first-order resonancewith the Mie frequency is not met during the presence of the main laser pulse, butonly temporarily, either at the first onset of inner ionization on the leading edge ofthe pulse (for van der Waals clusters) or during the subsequent Coulombexplosion. In both cases, the ion density is reduced. The presence of a strong thirdharmonic leads, in particular, to the enhanced production of multiply charged ionsin clusters irradiated by a strong laser field, as compared with isolatedatoms. This point is discussed in the light of recent experimental results onthe production of multiply charged ions in laser–cluster experiments.Third-harmonic generation by a cluster in a strong laser field, as a function ofboth the cluster and the laser-field parameters, is also considered.

On the role of prepulses during solid target heating by picosecond laser pulses

X-ray emission spectra of the plasma created at the surface of magnesium, aluminum, copper, and zinc targets heated by 1-ps laser pulses with a peak power density of up to 1016 W/cm2 were measured. The effect of a picosecond prepulse on the spectra was studied for various power densities and intensity contrasts of the main laser pulse. It is established that the emission spectra of laser plasmas are weakly affected by a change from 105 to 107 in the main pulse contrast relative to the first prepulse. Variations in the parameters of emission from aluminum and magnesium plasmas were calculated using relative intensities and widths of the resonance lines of H-and He-like ions and their two-electron satellite peaks.

Laser imprint reduction with a shaping pulse, oscillatory Richtmyer–Meshkov to Rayleigh–Taylor transition and other coherent effects in plastic-foam targets

A substantial reduction of the laser imprint with a short, low-energy “shaping” laser pulse incident upon a foam–plastic sandwich target prior to the main laser pulse has been demonstrated to be possible [Metzler et al., Phys. Plasmas 9, 5050 (2002)]. Nonuniformity of this shaping pulse, however, produces standing sonic waves in the target. Laser-imprinted seeds for the Rayleigh–Taylor (RT) instability growth then emerge from the interaction of these waves with the strong shock wave launched by the drive laser pulse. Such coherent interaction between different waves and modes perturbed at the same wavelength is shown to be important in a variety of situations relevant to the inertial confinement fusion studies. As an example, an oscillatory transition from the classical Richtmyer–Meshkov shock-interface instability development to the RT growth exhibiting a characteristic phase reversal in a target of finite thickness is described. Another example refers to the feedout mechanism of seeding the perturbations that come from the nonuniformities of the rear (inner) surface of the laser target. The coherent interaction between the strong shock wave from the main laser pulse and the rippled rarefaction wave produced by a low-intensity foot of the pulse produces observable effects, such as an extra phase reversal compared to the case of no foot. Some of these predictions are shown to be consistent with our new experimental results obtained in the feedout geometry on the Nike laser facility [S. P. Obenschain et al. Phys. Plasmas 3, 2098 (1996)].

Transient characteristics of a neon-like x-ray laser at 19.6 nm

A transient Ne-like x-ray laser at 19.6 nm (J=0→1, 3p→3s) is numerically investigated using a hydrodynamic code for a 100 μm thick Ge planar target irradiated by a nanosecond prepulse followed by a subpicosecond or a picosecond main optical laser pulse. Simulations are performed for different main pulse lengths of 50 fs, 500 fs, 5 ps, and 50 ps but with fixed pulse energy. For a drive pulse even as short as 500 fs, the x-ray laser gain still shows quasisteady state behaviors. A real transient nature can only be seen when the x-ray laser is driven by a laser pulse with a duration as short as tens of femtoseconds.

Propagation issues and energetic particle production in laser–plasma interactions at intensities exceeding 1019 W/cm2

A series of experiments recently carried out at the Rutherford Appleton Laboratory investigated various aspects of the laser–plasma interaction in the relativistic intensity regime. The propagation of laser pulses through preformed plasmas was studied at intensities exceeding 1019 W/cm2. The transmission of laser energy through long-scale underdense plasmas showed to be inefficient unless a plasma channel is preformed ahead of the main laser pulse. The study of the interaction with overdense plasmas yielded indication of collimated energy transport through the plasma. The production of fast particles during the interaction with solid density targets was also investigated. The measurements revealed the presence of a small-sized directional source of multi-megaelectron volt protons, which was not observed when a plasma was preformed at the back of the solid target. The properties of the source are promising in view of its use in radiographic imaging of dense matter, and preliminary tests were carried out.

Optimization of Drive Pulse Configuration for a High-GainTransient X-Ray Laser at 19.6 nm

An Ne-like transient collisional excitation x-ray laser at 19.6 nm (J = 0→ 1, 3p → 3s) was investigated numerically using a sophisticatedhydrodynamic code for a 100 µm thick Ge planar target irradiated by ananosecond pre-pulse followed by a picosecond main optical laser pulse. Thesimulations indicate that for a given peak intensity, the main pulse has anoptimal duration to generate the maximum effective gain. An effective gainas high as 200 cm-1 was obtained for the optimized drive pulseconfiguration.

Spectroscopic measurements of target preheating on OMEGA

The preheating of laser-heated microballoon targets has been measured by time-resolved x-ray and extreme ultraviolet (euv) spectroscopy on the 30 kJ, 60-beam (maxima), 351 nm, laser-fusion system at the University of Rochester Laboratory for Laser Energetics. Thin coatings of aluminum overcoated with magnesium served as indicators. Both the sequence of the x-ray line emission and the intensity of euv radiation were used to determine a preheating peaking at ∼10 nsec prior to onset of the main laser pulse, with a power density ≲1% of the main pulse. The measurements are supported by numerical modeling. Further information is provided by absorption spectra from the aluminum coating, backlighted by continuum from the heated surface. The exact source of the preheating energy remains unknown at present, but most likely arrives from early laser leakage through the system. The present target diagnostic is particularly useful when all beams cannot be monitored directly at all laser wavelengths.

Two-dimensional model of thermal smoothing of laser imprint in a double-pulse plasma

The laser prepulse effect on the thermal smoothing of nonuniformities of target illumination is studied by means of a two-dimensional Lagrangian hydrodynamics simulation, based on the parameters of a real experiment. A substantial smoothing effect is demonstrated for the case of an optimum delay between the prepulse and the main heating laser pulse. The enhancement of the thermal smoothing effect by the laser prepulse is caused by the formation of a long hot layer between the region of laser absorption and the ablation surface. A comparison with experimental results is presented.

Symmetrising effect of an x-ray prepulse in laser target compression

Two-dimensional numerical calculations were made in a study of the symmetrising effect of an x-ray prepulse on the inhomogeneity of laser target compression. It is suggested that a two-stage target, with its outer stage formed by a thin metal layer, can be used to generate such a prepulse. Correct selection of the parameters of this layer and of a laser prepulse can ensure symmetrisation of the inner working target with minimal undesirable preheating of the fuel. Such a target is accelerated and compressed at the expense of the energy of the main laser pulse. The influence of a large-scale asymmetry of irradiation, associated with a specific geometry of the radiation reaching the target, may be reduced by a suitable variation of the shell thickness.

Experimental observation of the self-stabilization of a synchronously pumped dye laser

A passive stabilization technique for a synchronously pumped dye laser is demonstrated. Using a linear external cavity a minute fraction of the pulse is split off and superimposed back to the main laser pulse. Even an extremely weak feedback pulse — whose energy corresponds to that one of a few single photons — leads to the generation of nearly Fourier-transform limited pulses and to a drastic reduction of the high frequency components of the pulse energy fluctuations.

X-ray absorption spectroscopy of laser-produced plasmas: A study of the experiment and data analysis

The novel technique of point-projection backlighting is used to probe a laser-produced plasma. A spatially restricted source of x rays is created by irradiating a fiber which is used to backlight a plasma. Temporal resolution of the absorption spectrum is obtained by creating the backlight with a pulse that is delayed in time from the main, plasma-producing, laser. The backlight can also be of shorter duration than the main laser pulse providing for enhanced time resolution and therefore less temporal averaging. The results for low-Z aluminum and silicon targets are presented. The low-Z absorption spectra using both low-Z (e.g., aluminum) and high-Z (e.g., molybdenum) elements for backlighting are studied. A description of this technique and the analysis of the data obtained is presented.

Characteristics of plasma flow from laser irradiated planar thin foil targets

Filamentary structures or jetting in the plasma flow generated from laser irradiated thin foil targets have been observed using an optical probe technique. Laser wavelength, pulse duration, and intensity range used in the experiments were 1.06 μm, 5 ns, and 5×1012 W/cm2, respectively. A 0.53-μm-wavelength optical pulse of 50-μJ energy and 1-ns duration was used as an optical probe. Optical shadowgrams showed that the filamentary structures or jets in the plasma flow were strongest in the region about 300 μm away from the target surface, and in time, much later than the peak of the main laser pulse. It was also observed that jetting does not affect the stability of the ablatively accelerated thin foils. Jetting was found to be stronger for targets with a higher atomic number. A time-resolved interferometric study of the plasma produced from targets with high atomic numbers also showed the existence of an abrupt change in fringe shift or plasma density in the region of these jets.

Spectroscopic and interferometric measurements of laser–plasma produced blast waves

A laser-produced plasma generates blast waves as it expands supersonically into a stationary photoionized background gas (N2) at a pressure of 1–5 Torr.1 Using a combination of spectroscopic and interferometric measurements, time- and space-resolved values of the temperature and density are obtained. This study was performed on targets in the NRL Pharos III laser facility with laser energies of 20–120 J and pulse durations of ∼5 ns. A 1-m spectrograph equipped with three photomultiplier channels, which are calibrated on an absolute scale, is used for the spectroscopic measurements. The interferometry is done with a folded-wavefront interferometer. It uses an optical probe pulse at 5270 Å that is split off of the main laser pulse, reduced in pulse duration (∼300 ps), and time delayed. Interferometric measurements can be made simultaneously with the spectroscopic measurements. Experimental data and the technique of analysis will be shown. This work was supported by the Defense Nuclear Agency.

The importance of the pulse shape for the laser-beam target interaction

Theoretical analysis of a pulsed CO 2 laser beam aluminum target interaction demonstrates increased efficiency if the main laser pulse is preceded by a pulse of enough power to induce breakdown in air and create plasma in front of the metal target surface.

The technology of solid-fuel-layer targets for laser-fusion experiments

An apparatus which produces uniform solid-fuel layers in glass-shell targets for laser irradiation is described. A low-power cw laser pulse is used to vaporize the fuel within a previously frozen target which is maintained in a cold-helium environment by a cryogenic shroud. The rapid refreezing that follows the pulse forms a uniform fuel layer on the inner surface of the glass shell. This apparatus and technique meet the restrictions imposed by the experimental target chamber. The method does not perturb the target position; nor does it preclude the usual diagnostic experimets since the shroud is retracted before the main laser pulse arrives. Successful laser irradiation and implosion of solid-fuel-layer targets at KMSF have confirmed the effectiveness and reliability of this system and extended the range of laser-target-interaction studies in the cryogenic regime.

Tolerance of laser-driven microshell targets to fluorescence and prepulse energy

Glass-shell targets currently being used for laser-fusion experiments are susceptible to damage by preenergy from the laser. This energy can result from amplified spontaneous emission (ASE) throughout the laser chain during the period of high-population inversion or it can take the form of a short prepulse nanoseconds before the main laser pulse. We point out the energy levels which a typical target can tolerate in the form of ASE and prepulses. These energies are low enough that special precautions must be taken to prevent significant perturbations of the target or its environment before the main laser pulse.