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Following LIL single beamline commissioning in 2003, where performance in terms of power and energy required for LMJ was demonstrated, we spent year 2004 to qualify the quadruplet (or quad) performance at 1ω/3ω. Over that year, the first quad high power and high energy laser experiments took place on LIL facility. A careful set of test campaigns were conducted to safely ramp up laser performance. The main goal was to measure quad-specific features such as beam synchronization and focal spot (size, smoothing contrast ratio or irradiation nonuniformity σ rms ) versus the LMJ requirements. LIL Quad beam waist was recorded for various pulse durations, smoothing techniques and for a wide range of laser intensities up to LMJ-nominal ones. Now, LIL quad has been commissioned to the center of the target chamber and the first plasma experiments are made.

The Laser MegaJoule (LMJ) under construction at CESTA, a CEA laboratory located near Bordeaux, is designed to produce 240 beams, 1.8 MJ, 500 TW of ultraviolet light for high energy density physics experiments and thermonuclear burn of DT targets. The first step of LMJ program consisted in the construction of a full scale prototype the "Ligne d'Integration Laser" (LIL) which was developed to demonstrate LMJ performances and qualify all its components and systems as well as maintenance procedures. Commissioned in 2002, LIL facility mainly includes a laser bay which houses a bundle of 8 beamlines similar to the LMJ ones and a target bay where are located the 1ω-3ω conversion system, the focusing gratings and the 4.4-meter diameter target chamber equipped with plasma diagnostics. This paper describes the LIL facility as a tool for laser-matter interaction pointing out the laser system.

The requirements for target ignition with megajoule-class lasers (Nakatsuka et al., Second Annual International Conference on Solid-State Lasers for Application to ICF, Paris, October 22–25, 1995 [1] ) create new constraints on the laser technology namely due to the propagation, frequency conversion and focusing of ultrahigh energy pulses. This paper emphasizes some advanced laser topics that arise with megajoule-class lasers such as frequency conversion with narrow and broadband pulses, energy losses due to mechanical sources of beam birefringence and due to a too long length of propagation in air. Wavefront distortion is measured and corrected and a new focusing scheme with high efficiency gratings is presented. The third beamline of the Phebus laser (Fleurot et al., Infrared Technology XVI, 1990, pp. 280–291 [2] ) is dedicated to these laser physics investigations at an energy density level of 3–4 GW/cm2 that is close to the megajoule laser (LMJ) beam density value. This beamline delivers up to 1.2 kJ, 1ω light in ns square shaped pulses, i.e. typically 1 TW pulses. The output beam aperture is reduced from 23 to 18 cm to get the required energy density level. The bandwidth of the laser pulse is varied up to a few angstroms. The beam characteristics are recorded and analysed by means of the output sensor 1ω energy, time profile and spectrum diagnostics and by means of a specific set of diagnostics for the 1ω, 2ω and 3ω beam parameters, located in the switchyard on a workstation called BACF.

The Precision Phebus program, started in 1993, emphasizes a series of laser and target experiment objectives on the two-beam Phebus Nd-phosphate glass laser. Recently, three major objectives that are also very important issues for megajoule-class lasers have been met: First, the balance of the incident beam-to-beam 3ω power is shown to be in the range from 5 to 12% for 3-ns, 3ω-shaped pulses of reproducible high-energy shots; second, the smoothing uniformity of the laser energy deposited on the target, that is, the contrast of the spatial beam modulations, can be kept lower than 5%; and, finally, the tight control of the beam targeting leads to a pointing precision of less than 10 μrd on the target at the target chamber center (TCC) and of 80 μrd on X-ray sources located up to 3 cm from the TCC to improve the space- and time-resolved X-ray shadowgraphy techniques performed for target physics experiments such as implosion and hydrodynamical instability studies.

Backward stimulated Raman scattering (SRS) experiments have been performed using the optical fiber smoothing implemented on the high‐power Phebus laser facility [Laser Part. Beams 4, 93 (1988)]. The interaction took place in low‐Z plasmas presenting either exponential density profiles (solid targets) or Gaussian‐type profiles. Raman instability was driven with 1.4 ns duration—0.53 μm laser pulses, containing an energy up to 1.8 kJ. Comparative and absolute measurements of SRS reflectivity are reported as a function of the incident laser intensity, both with and without beam smoothing. Near‐backward time‐resolved SRS spectra are also presented. Random phase plates are not used in these experiments. With or without smoothing, SRS reflectivities of the order of 10% have been measured. However, smoothing becomes effective in exponential profiles below 1015 W/cm2. Although the optical fiber efficiently reduces the contrast of the energy modulations in the focal spot, the spectral bandwidth may be insufficient ...

The Laser Integration Line (LIL) was first designed as a prototype to validate the concepts and the laser architecture of the Laser MegaJoule (LMJ). The LIL facility is a 4-beam laser representing a quad structure of the LMJ. A set of test campaigns were conducted to safely ramp up laser performance. The main goal was to measure quad-specific features such as beam synchronization and focal spot (size, smoothing contrast ratio or irradiation nonuniformity) versus the LMJ requirements. Following the laser commissioning, the LIL has become a major instrument dedicated to the achievement of plasma physics experiments for the French Simulation Program and was also opened to the academic scientific community. One of the attributes of the LIL facility is to be very flexible to accommodate the requests of plasma physicists during campaigns. The LIL is constantly evolving to best meet the needs of target physicists. Changes made or planned are either to improve the quality of laser beams, or to increase the LIL Energy-Power operating space. To optimize preparation and design of shot campaigns, the LIL performance status has been elaborated. It gives information about the characteristics of the laser in terms of near field and far field, defines the steps to maintain performance, explains how the facility responds to the request, details settings (smoothing, shaping of the focal spot, energy, temporal pulse shaping, beam pointing) and gives the limits in energy and power. In this paper, an overview of the LIL performance is presented.

Pulses of 20-TW peak power have been generated at 1064 nm using the chirped-pulse-amplification technique with a 90-mm output-aperture Nd:silicate glass amplification line. This system delivers 60 J of energy in a chirped pulse of 600-psec duration, with a capacity to maintain a 3.5-nm output bandwidth. These chirped pulses are compressed to 1.2 psec with an energy of 24 J by using large holographic gold-coated 1740-lines/mm diffraction gratings.

This Letter presents first experimental results of the laser imprint reduction in fusion scale plasmas using a low-density foam layer. The experiments were conducted on the LIL facility at the energy level of 12 kJ with millimeter-size plasmas, reproducing the conditions of the initial interaction phase in the direct-drive scheme. The results include the generation of a supersonic ionization wave in the foam and the reduction of the initial laser fluctuations after propagation through 500 {mu}m of foam with limited levels of stimulated Brillouin and Raman scattering. The smoothing mechanisms are analyzed and explained.

The Phebus laser was used to irradiate gold targets at λ=0.35 μm; the laser pulse was 0.7 or 1.3 ns and the energy on target 1 to 1.8 kJ. The incident intensity was varied from 3×1013 to 3×1015 W/cm2. The time history of x‐ray emission was monitored in the range 0.2–2.5 keV with a streak camera coupled with a transmission grating. Numerical simulations cannot replicate the temporal evolution of soft x‐ray emission (hν

Low-fluence laser interaction with metallic target is investigated in a wide range of irradiance, from 5 × 106 to 1011 W/cm2, with 4- and 30-ns laser-pulse durations and 1.06-μm wavelength. Edge effects are avoided by means of large surfaces of irradiation (5 cm2). Specific diagnostics such as a ballistic pendulum and a piezoelectric quartz gauge are developed to measure the total momentum imparted to the target, as well as the temporal evolution of the stress at the rear side of the target. Specular reflection and sidescattering of the laser light from the target are also measured. The behavior of 2024 aluminum alloy is particularly studied. The results are interpreted in terms of liquid-vapor transition and change in laser absorption.In light of experimental data, the theoretical analysis of surface absorption is reviewed, showing the dependence on metal temperature and laser wavelength. The thermal and mechanical coupling is also calculated.Numerical simulations are performed with a 1–D Lagrangian hydrodynamic code including modifications for low-pressure regimes. The multiphase equation of state is extended to take into account negative pressures. An elastoplastic module is introduced into the code. A quite good agreement is obtained with the measurements of the stress and of the mechanical coupling coefficient.

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.

We present some significant results of collisional excitation X-ray laser experiments in plasmas produced by a laser. We studied the amplification in Ne- and Ni-like ions by varying both the nature and the thickness of targets, the irradiation, and the wavelength of the driving laser. Some potentially interesting scalings as a function of the atomic number of the lasing element are demonstrated in the Ne-like system. An order-of-magnitude increase in gain in the Ni-like experiments was determined.

The Nd-glass Phebus laser has the capability of delivering 2 X 10 kJ in a 1.3 ns pulse when using the usual narrow band oscillator. For the purpose of focal spot smoothing the coherence of the pulse is reduced by injecting the beam of a 1 nm bandwidth oscillator into a step index optical fiber. The use of the optical fiber at the laser front end changes the common point laser source into an extended source. The required optical arrangement taking into account the divergence of the extended source beam is described. The smoothing efficiency is measured by the reduction of one order of magnitude of the contrast of the energy modulations in the focal spot. It is experimentally established that non linear effects limit the power density in the amplifying medium to a value depending on the geometrical extension of the source. The identification of the relevant non linear effects is discussed.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An experiment carried out on the LIL facility is presented. The experiment was designed to investigate the propagation of a multi-kJ laser beam through a plasma produced by the irradiation of a low-density plastic foam. Laser beam self-smoothing has been observed and its consequences on laser imprint and Stimulated Brillouin Scattering (SBS) are discussed.

We present an overview of recent experiments fielded on the LIL facility. A key issue for mega-joule class laser facilities is shrapnel fragment generation. A specific collector was therefore developed to capture debris in aerogel and dedicated shots were done to quantitatively evaluate target fragmentation phenomena. The LIL panel of transmitted and backscattered light diagnostics is well suited for laser-plasma interaction (LPI) experiments. This include gas-filled hohlraum configurations relevant to Indirect Drive ignition targets in order to test the specific LMJ longitudinal SSD technique, as well as LPI experiments with foam targets for laser beam smoothing in underdense plasma in the context of Direct Drive. After Visar commissioning a campaign was dedicated to boron Equation of State (EOS).

The PHEBUS facility has operated since 1986 for the French laser matter interaction program at Limeil. The performances of this system, which is the third most powerful one in the world, are presented as well as some new developments which aim at smoothing the laser beam in order to improve the overall laser-target behavior.

The Ligne d'Integration Laser (LIL) facility has been designed as a prototype for the Laser MegaJoule (LMJ) which is a cornerstone of the French 'Simulation Program'. This laser has been intensively used to test and improve the LMJ components. In addition, a large panel of plasma diagnostics has been installed and is currently used to perform laser–plasma experiments. After a brief discussion about the LIL design, we present the last results in various plasma physics domains.

This paper presents experimental results obtained at LULI 2000 and LIL about (i) the compared laser plasma coupling at 526 (2ω) and 351 nm (3ω) and (ii) the early laser imprint suppression using foam targets as plasma smoother of the laser beam. Both experiments are described, part of the experimental results are presented and discussed.

This facility has been operated since 1986 for the French laser matter interaction program at Limeil. It is the third most powerful laser system in the world. We will emphasize in this presentation the 1.06 micron performances of the facility, although it is mainly aimed at working for .53 and .35 micron plasma experiments1.

Amplification of soft x rays in Sr (Z=38) plasmas produced by second-harmonic laser irradiation of exploding foil targets is studied. Gains of 4.4 and 4.0 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively, have been measured for the 164.1- and 166.5-\AA{} J=2 to 1 transitions in Ne-like Sr at laser intensities of 1.3\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$. Numerous x-ray and soft-x-ray transitions were identified in order to demonstrate typical spectra from x-ray laser plasmas and infer information regarding the plasma ionization balance. The J=0 to 1 transition at 159.8 \AA{}, analogous to the well-known Se J=0 to 1 line at 182.4 \AA{}, was not definitively observed due to a wavelength overlap with a Na-like Sr line. Two other J=0 to 1 transitions of interest at 84.9 and 133.0 \AA{} were not observed. Reduced gain on the J=2 to 1 lasing lines was seen for several shots taken at a lower pump laser intensity of 7.0\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$. Possible reasons for this are considered. The results of this work imply the feasibility of a high-output-power cavity geometry Ne-like Sr x-ray laser operating at 164.1 and 166.5 \AA{}.

An improved version of the initial x-ray ultraviolet spectrograph SPARTUVIX1 has been developed to record the time evolution of spatially resolved spectra in the soft x-ray range (from 30 to 300 {Angstrom}). This diagnostic can be applied to the new class of laser produced plasma experiments now performed in order to better understand the physical evolution of x-ray indirect drive plasmas. Instead of a one-dimensional streak camera used for the first version, a new widely used 2-D soft x-ray time gated imager allows us to follow the time evolution of the spatially resolved soft x-ray spectra. This instrument was designed for plastic foam soft x-ray opacity measurements. The soft x-rays, emitted by an auxiliary radiography laser plasma source, are transmitted through the foam and dispersed into two parts (+1 and {minus}1 order of the transmission grating). Each dispersed spectrum is placed on two different gated striplines of the soft x-ray imager and thus recorded at two different times (exposure time 300 ps, adjustable interframe 600 ps). The spatial resolution is obtained with a slit added in front of the spectrograph, that images the plasma onto the detector with a magnification ratio of 15 and a spatial resolution of 50 {mu}m.more » A detailed description of this instrument and the main results obtained for two different plastic foam opacity measurements (undoped and doped with chlorine) will be presented. This work is supported by European Community Contract No. CEE/CHGF-CT-92-0016. {copyright} {ital 1997 American Institute of Physics.}« less

The hydrodynamic behavior of double-foil targets irradiated with 0.35 μm laser wavelength at irradiance of 2×10 14 W cm -2 has been investigated by means of time and space resolved X-ray backlighting. Comparison with a 1-D lagrangian code shows discrepancies attributed to energy losses in the transfer between two foils. First results obtained with a 2-D code are presented for a single thin target. We infer that the hydrodynamic lateral expansion of the accelerated layer is the dominant process of energy loss.

The effect of laser-beam spatial modulations on plane targets is analyzed as a function of modulation scale length and target material by means of x-ray shadowgraphy. At short laser wavelength it is shown that lateral energy smoothing is clearly improved because of the strong x-ray emission from the irradiated high-Z side of layered targets. The results, interpreted in terms of radiation-driven thermal transport, are in agreement with numerical simulations.

Les etudes relatives aux phenomenes de transport de l'energie sont poursuivies au CEL, dans le cadre des experiences d'implosion par rayonnement laser. Nous presentons une methode fondee sur l'analyse spectrometrique du rayonnement X, le plasma etant cree par l'irradiation de cibles constituees d'un depot d'Al, d'epaisseur variable, sur un substrat de verre. Nous decrivons d'abord le dispositif experimental et les moyens de diagnostic utilises pour l'etude du rayonnement X dans la gamme 4-8 A. Apres avoir precise l'equilibre d'ionisation caracteristique d'un tel plasma, nous exposons la methode de calcul de son emission utilisant les distributions de Te et de Ne obtenus par ailleurs a l'aide d'un code hydrodynamique lagrangien. La comparaison avec l'experience conduit a la determination de parametres caracteristiques des phenomenes de transport de l'energie.

First experimental evidence of a radiative wave propagating ahead of a supercritical shock is given. An aluminum piston driven by laser light at wavelengths of 1.06 and 0.35 \ensuremath{\mu}m induces the shock wave in xenon gas. The plane piston velocity ranges from 2.5 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ to 1.2 \ifmmode\times\else\texttimes\fi{} ${10}^{7}$ cm\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{s}}^{\ensuremath{-}1}$ and the gas pressure is up to 6 bars. Experimental results are in good agreement with a simple analytical model of the propagation of a radiative wave in front of a supercritical shock wave when the approximation of radiation heat conduction is assumed.

An investigation of an arc generated from a liquid‐potassium cathode in an inert gas (argon) at a pressure between 0.16 and 6 Torr is given. A theoretical description of the phenomena arising in the vicinity of the cathode leads us to a model which allows the construction of an existence diagram of the arc. As predicted by the theory, the cathode fall potential is about 7 V. By means of Langmuir probe measurements, it is shown that the voltage reduction mainly occurs in the cathode fall region. A comparison has been made of excitation and electron temperatures, as derived from spectroscopic measurements using spectral lines from the alkali‐metal vapor, with the theoretical results and verified that LTE prevails under the conditions of our experiments. The measured electron densities are higher than 3×1015 cm−3 and link up with the electron temperature if Saha equilibrium is assumed in the plasma just above the cathode spots.

Energy streaming around and through an Al disk target irradiated with a Nd‐glass laser delivering a 0.8‐ns pulse is discussed as a function of target thickness, disk diameter, and laser irradiance from 5×1012 to 5×1014 W cm−2. Front and rear surface behaviors of the disk are studied by means of frame and streak shadowgraphy, short‐time exposure interferometry, ion calorimetry, and charge collectors. Experimental data clearly show that preheat of the rear side of a 500‐μm‐diam disk is due to plasma encompassing the target and occurs even at laser irradiance as low as 3×1013 W cm−2. Moreover, it is seen on time‐resolved photography of double‐disk targets, that the second foil expands before being struck by the first disk (irradiated disk) at laser irradiance as low as 1013 W cm−2. This effect is ascribed to preheating by particle bombardment. Numerical simulations do not match the experimentally observed temporal behaviors for the 500‐μm‐diam disks even with some preheat included in the hydrodynamic code, but they agree quite closely with the experimental results when the irradiated disk diameter is much larger than the characteristic density gradient length.

The hydrodynamic behavior of thin A1 disk targets irradiated with a 1 ns frequency-tripled Nd-laser pulse is experimentally studied at irradiances ranging from 10 13 W cm -2 to 5 x 10 14 W cm -2 and compared to numerical simulations from a one-dimensional lagrangian hydrodynamic code. Velocity measurements as a function of target thickness — ranging from 0.75 μm to 25 μm — are performed by means of time and space resolved X-ray backlighting. Comparison is done with free surface velocity measurements deduced from visible interferometry and streak shadowgraphy, and with dense target velocities inferred from the double-foil technique. Results are discussed in terms of preheat effects, beam inhomogeneities and energy smoothing.

Filamentary and jet-like structures are observed in the interaction of a laser beam with aluminum and gold targets. The range of irradiance is 1013 to 5 × 1014 W cm−2 with nanosecond pulses. According to theoretical predictions these structures can be attributed to thermal instabilities. We show that the electrothermal instability grows more probably at the 0.35 μm laser wavelength whilst the magnetothermal instability develops at 1·06 μm. The jet-like structures occurring on the rear side of thin targets are discussed.

The experiments reported in this paper demonstrate that the PHEBUS laser facility is now currently being operated with high performances (4 TW with 250 ps pulses at 0·527 μm wavelength).The output energy of the 2-beam PHEBUS laser system can be focused either in a small focal spot (80% of the incident energy is in a 220 μm diameter focal spot) for high intensity experiments (≥5 × 1015 W cm−2) or in very large spots (a few mm in diameter) at moderate intensities (1013 − 2·5 × 1014 W cm−2), for large scale experiments. It is shown that the spatial intensity distribution in the target plane is primarily due to intensity independent aberrations and to diffraction. Laser light absorption in plane aluminum and gold targets are interpreted in terms of inverse bremsstrahlung absorption that may account for 70 to 90% of absorbed energy. Finally, the plasma expansion is shown to be very planar and comparison with one-dimensional Lagrangian simulations gives flux limiter values of 0·03 and 0·02 respectively for Al and Au targets.

Spatially resolved X-ray line emission allows a detailed study of axial-energy-transport processes in layered Al-SiO2 targets irradiated with 1.06-μm, 80-ps FWHM laser pulses and flux levels in the 1014–1016Wcm−2 range. By means of a double-laser-pulse technique, a comparison between simulations and experiments enables a thermal-flux inhibition factor and the fast-electron pre-heating characteristics to be determined.

Using a long‐duration transverse discharge (300 ns) preionized by a short‐pulse electron‐beam gun (10 ns), laser emission has been obtained in the second positive system of nitrogen at λ=3371 A in nitrogen alone and at λ=3577 A in a N2‐CF4 mixture. In the latter mixture, the laser emission lasts for 70 ns which is much longer than the radiative lifetime of the upper laser level C 3Πu(τF=40 ns). We think that the blottleneck of the terminal laser level is avoided thanks to the vibrational deactivation of the nitrogen B 3Πg state due to the irreversible transfer of vibrational energy of N2 to CF4 during collisions.

A transmission grating streaked spectrograph (SPARTUVIX) has been developed at the Centre d’Etudes de Limeil‐Valenton for measuring the temporal evolution of soft‐x‐ray spectra in laser‐produced plasmas. In this article the ideas behind the instrument are described as well as a description of the actual working diagnostic and its relevant components. This diagnostic has been used to look at the time behavior of x‐ray laser emission, and a number of very interesting results have been obtained. A representative sample of these results will also be shown.

Experiments and analyses on high-Z targets X-ray emission are reported. Two kinds of targets have been used : spherical targets with Octal laser (1.06 µm ; 1013 to 1015 w/cm2) and planar targets with P102 laser (0.35 µm ; 3 × 1013 to 3 × 1014 W/cm2). The influence of the different parameters (laser flux, laser wavelength, atomic number) on emission rates and spectra is discussed. The results about the emission rates are compared with data from other laboratories and an experimental scaling law is proposed. Temperatures can be evaluated from experimental spectra and related to the different plasma regions. Space-time resolved measurements with a streak camera, from spherical targets, in the (1 – 1.3 keV) energy range allow to determine a velocity : about 6 × 106 cm/s for the emission region.

Nous développons un modèle de gaine cathodique dans un arc généré à partir d'une cathode liquide en potassium, dans un gaz inerte (argon) à une pression de quelques torrs ; à partir d'un bilan général des échanges existants au voisinage de la cathode, nous avons construit les diagrammes d'existence des spots constituant la tache cathodique ; nous avons envisagé des intensités de décharge allant jusqu'à 100 A ; nous avons également abordé le problème de la part de l'émission thermoélectronique par rapport à celle de l'émission de champ dans la formation d'électrons à la cathode.

Since 1984 when soft x-ray amplification was convincingly demonstrated at XUV wavelengths near 200 Å, much effort has gone into obtaining soft x-ray gain at shorter wavelengths. One motivation is the possibility of using a high brightness source of amplified spontaneous emission to perform holography of live biological specimens over short time scales.

Recent Ne-like ion x-ray laser experiments carried out at the Centre d'Etudes de Limeil-Valenton (CEL-V) with both 0.53-μm and 0.35-μm laser light will be described. This paper will concentrate primarily on the observation of amplification in Ne-like Sr (Z=38) in plasmas produced by 0.53-μm laser light. Small signal gains of 4.4 cm-1 and 4.0 cm-1 have been measured for the Ne-like Sr J=2-1 transitions at 164.1 and 166.5 Å, respectively. In addition, the effects of pumping the Ne-like Se laser with 0.35-μm light have been investigated. The resulting Se spectra do not differ significantly from those observed with 0.53-μm light. A brief summary of other experiments carried out at CEL-V will also be given.

Electron temperatures and densities, measured in a dense arc plasma generated between a solid amode and a liquid potassium cathode, are compared with results given by a theoretical model of the sheath.

Ablation parameters such as velocity, mass, momentum, pressure, and hydrodynamic efficiency have been investigated with plane targets irradiated in the range 3×1011‐1015 W cm−2 with 1 nsec pulses and laser wavelengths of 1.06 μm and 0.35 μm. We show that ablation velocity, ablated mass, and momentum are in good agreement with ablation scaling laws deduced from analytical models taking into account inverse bremsstrahlung absorption below the critical density. Nevertheless, processes such as lateral conduction, hot spot, and preheat effects make inaccurate the comparison between ablation pressures, mass ablation rates, or hydrodynamic efficiencies measured for different laser wavelengths. Laser illumination nonuniformities are transmitted to the target in terms of pressure variations. The harmful consequence of a reduced lateral energy flow in 0.35 μm experiments can eclipse the increasing of ablation pressure and hydrodynamic efficiency.