Source-size measurements and charge distributions of ions accelerated from thin foils irradiated by high-intensity laser pulses

We report on measurements of source sizes and charge state distributions of ions accelerated from thin foils irradiated by ultrashort (100–300 fs) high-intensity (1-6×10¹⁹ W/cm²) laser pulses. The source sizes of proton and carbon ion beams originating from hydrocarbon contaminants on the surfaces of 5 m thick aluminum foils were investigated using the knife-edge method. For low-energy protons and low-carbon charge states, the source area was found to exceed the focal spot area by a factor of 10⁴. For the determination of charge state distributions, sandwich targets consisting of a 25 m thick tungsten layer, a 2-nm thin beryllium layer, and again a tungsten layer whose thickness was varied were used. These targets were resistively heated to remove the light surface contaminants. Peaked energy spectra of oxygen and argon ions corresponding to the equilibrium distribution after propagation through matter were observed. PACS 41.75.Jv; 52.38.Kd; 52.25.Jm; 52.50.Jm; 52.70.Nc; 41.75.Ak     

X-Rays emission by high-intensity pulsed lasers irradiating thin foils at PALS laboratory

X-rays and forward ion emission from laser-generated plasma in the Target Normal Sheath Acceleration regime of different targets with 10-μm thickness, irradiated at Prague Asterix Laser System (PALS) laboratory at about 10¹⁶ W/cm² intensity, employing a 1,315 nm-wavelength laser with a 300-ps pulse duration, are investigated. The photon and ion emissions were mainly measured using Silicon Carbide (SiC) detectors in time-of-flight configuration and X-ray streak camera imaging. The results show that the maximum proton acceleration value and the X-ray emission yield growth are proportional to the atomic number of the irradiated targets. The X-ray emission is not isotropic, with energies increasing from 1 keV for light atomic targets to about 2.5 keV for heavy atomic targets. The laser focal position significantly influences the X-ray emission from light and heavy irradiated targets, indicating the possible induction of self-focusing effects when the laser beam is focalized in front of the light target surface and of electron density enhancement for focalization inside the target.     

Ion acceleration at the front and rear surfaces of thin foils with high-intensity 40-fs laser pulses

Ion acceleration from the front and rear sides of a foil target is observed by measurements of the ions’ spectral and spatial emission characteristics when irradiating the targets with ultrashort (40-fs) high-intensity laser pulses. The experimental results show that the origin of accelerated ions, from both the front and rear surfaces of the target, strongly depend on the laser energy absorption mechanism. In particular, laser pulse parameters such as pulse duration and contrast are crucial and determine the entire acceleration scenario. Thus, the experimental outcome can be controlled by selection of the irradiation conditions. The text was submitted by the authors in English.     

Radiative heating of thin Al foils by intense extreme ultraviolet radiation

The effect of induced transparency of thin Al foils radiatively heated by intense extreme ultraviolet (EVU) radiation has been observed. The radiation of the plasma of Z-pinches appearing under the compression of tungsten liners at the Angara-5-1 facility has been used as the radiation that heats the Al foil (peak illumination on the foil ~0.55 TW/cm²) and is transmitted through it. The photoabsorption has been studied in the formed aluminum plasma at temperatures of ~10–30 eV in the density range of ~1–20 mg/cm³ in the wavelength range of ~5–24 nm. Absorption lines of Al^4+...7+ ions have been identified in the experimental spectrum. In addition, radiative gas-dynamic simulations of the foil heating and expansion have been performed taking into account radiation transfer processes.     

Heating of thin foils with a relativistic-intensity short-pulse laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Ilambda (2)=2 x 10(18) W microm(2)/cm(2), laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At /=500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (approximately 100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.     

Forward ion acceleration in thin films driven by a high-intensity laser

A collimated beam of fast protons, with energies as high as 1.5 MeV and total number of greater, similar10(9), confined in a cone angle of 40 degrees +/-10 degrees is observed when a high-intensity high-contrast subpicosecond laser pulse is focused onto a thin foil target. The protons, which appear to originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the back side in a direction normal to the target surface. Acceleration field gradients approximately 10 GeV/cm are inferred. The maximum proton energy can be explained by the charge-separation electrostatic-field acceleration due to "vacuum heating."     

Energy spectra of protons driven by ultra-short laser interaction with thin gold foils

在能量11 mJ、波长744 nm、脉宽120 fs、功率密度6×1016 W/cm2的超短脉冲装置上,开展了超短脉冲激光与2.1 μm和5.0μm金薄膜靶相互作用产生质子束的实验研究.利用Thomson谱仪测量了产生的质子能谱,发现利用2.1 μm金薄膜靶时,质子能谱由于质子源数量不足而在74 keV附近出现单能峰,5.0 μm的金薄膜靶产生的质子计数和能谱均比2.1 μm的金薄膜靶产生的低,主要原因是超热电子穿过薄膜靶时出现的能量损失和几何倾斜降低了电子回流所致.     

超短激光与金薄膜靶相互作用产生的质子能谱

在能量11 mJ、波长744 nm、脉宽120 fs、功率密度61016 W/cm2的超短脉冲装置上,开展了超短脉冲激光与2.1 m和5.0 m金薄膜靶相互作用产生质子束的实验研究。利用Thomson谱仪测量了产生的质子能谱,发现利用2.1 m金薄膜靶时,质子能谱由于质子源数量不足而在74 keV附近出现单能峰,5.0 m的金薄膜靶产生的质子计数和能谱均比2.1 m的金薄膜靶产生的低,主要原因是超热电子穿过薄膜靶时出现的能量损失和几何倾斜降低了电子回流所致。     

Spectroscopic studies of plasmas produced from thin foils by fast risetime nanosecond laser pulses

Studies of emission spectra in the region 20–250 ? from plasmas produced from thin foils of various materials 6?Z?26 by fast risetime nanosecond laser pulses are reported. Ionization and recombination occuring in these plasmas as deduced from the identification of the spectral lines and their intensities is discussed. Estimates of the plasma temperature are made. The results are compared with the predictions of a computer code based on a thermal wave model for the initial burn through the foil and subsequent hydrodynamic expansion of the plasma.     

Enhancement of proton acceleration by hot-electron recirculation in thin foils irradiated by ultraintense laser pulses

MeV-proton production from solid targets irradiated by 100-fs laser pulses at intensities above 1x10(20) W cm(-2) has been studied as a function of initial target thickness. For foils 100 microm thick the proton beam was characterized by an energy spectrum of temperature 1.4 MeV with a cutoff at 6.5 MeV. When the target thickness was reduced to 3 microm the temperature was 3.2+/-0.3 MeV with a cutoff at 24 MeV. These observations are consistent with modeling showing an enhanced density of MeV electrons at the rear surface for the thinnest targets, which predicts an increased acceleration and higher proton energies.     

Control over the space-time structure of electron beams by high-intensity femtosecond laser radiation

The space-velocity distribution of electrons propagating in vacuum can be deformed by the ponderomotive potential produced by high-intensity femtosecond laser pulses, which makes it possible to subsequently separate such electrons from the initial beam. It is shown that optical modification of electron beams with kinetic energies on the order of 100 eV by femtosecond laser radiation with an intensity from 10¹⁴ to 10¹⁸ W/cm² makes it possible to form electron beams with a duration on the order of 50–100 fs. Examples of optical control over the shape of electron beams, based on deflection, reflection, focusing, and splitting of electron beams, are considered.     

Proton acceleration from high-intensity laser interactions with thin foil targets

Measurements of energetic proton production resulting from the interaction of high-intensity laser pulses with foil targets are described. Through the use of layered foil targets and heating of the target material we are able to distinguish three distinct populations of protons. One high energy population is associated with a proton source near the front surface of the target and is observed to be emitted with a characteristic ring structure. A source of typically lower energy, lower divergence protons originates from the rear surface of the target. Finally, a qualitatively separate source of even lower energy protons and ions is observed with a large divergence. Acceleration mechanisms for these separate sources are discussed.     

Lateral electron transport in high-intensity laser-irradiated foils diagnosed by ion emission

An experimental investigation of lateral electron transport in thin metallic foil targets irradiated by ultraintense (>or=10(19) W/cm2) laser pulses is reported. Two-dimensional spatially resolved ion emission measurements are used to quantify electric-field generation resulting from electron transport. The measurement of large electric fields ( approximately 0.1 TV/m) millimeters from the laser focus reveals that lateral energy transport continues long after the laser pulse has decayed. Numerical simulations confirm a very strong enhancement of electron density and electric field at the edges of the target.     

Experimental study of the acceleration of protons emitted from thin foils irradiated by ultrahigh-contrast laser pulses

Experimental results are presented for proton acceleration from the back of a target irradiated by laser pulses with intensities up to 2 × 10¹⁹ W/cm² generated by the SOKOL-P facility. The proton acceleration efficiency increases with decreasing of the target thickness. However, thin targets are destroyed by the amplified spontaneous emission (ASE) prepulse before the main pulse arrival. An additional optical switch based on a Pockels cell has been used in the amplification section to carry out the experiments with ultrathin foils. As a result, the energy contrast with respect to the ASE prepulse has been increased up to 4 × 10⁶. Owing to high contrast, the experiments on studying proton acceleration from foils with thicknesses less than 100 nm have been carried out.     

Effect of a prepulse on ablation-pressure smoothing in laser heating of thin foils

The results of theoretical and experimental studies of plasma heating by nonuniform laser radiation and the symmetrizing effect of a laser prepulse on the uniformity of heating of thin foils, which simulated thin-shell laser inertial-confinement fusion (ICF) targets, are presented. The high efficiency of symmetrization by a prepulse was demonstrated both experimentally and theoretically. __________ Translated from Preprint No. 16 of the P. N. Lebedev Physical Institute, Moscow (2006).     

Six MeV proton acceleration from plasma generated by high-intensity laser using advanced thin polyethylene targets

Proton acceleration can be induced by non-equilibrium plasma developed by high-intensity laser pulses, at 10¹⁶ W/cm², irradiating different types of thin polyethylene targets. The process of proton acceleration and directive yield emission was investigated, optimizing the laser parameters, the irradiation conditions, and the target properties. The use of 600 J pulse energy, a laser focalization inducing self-focusing effects and advanced targets with embedded nanoparticles and optimal thicknesses, has permitted to accelerate forward protons up to the energies of about 6 MeV and amount of the order of 10¹⁵ H⁺/pulse. High proton energy is obtained using thin foils enriched with gold nanoparticles, whereas high proton yield is obtained using targets with a thickness of about 10 μm. The plasma diagnostics using SiC semiconductor detectors in time-of-flight configuration was fundamental to monitor the optimal conditions to improve the plasma processes concerning the ion acceleration and the X-ray and relativistic electron emission.     

切向气流对激光加热金属板非熔化穿孔效应的影响

针对切向气流加载导致激光加热金属板在熔化前的穿孔效应,利用金属薄板的弹性弯曲理论,推导出了两种典型光束（方形和圆形）照射下的弯曲挠度表达式,利用Mises理论给出了非熔化穿孔的破坏判据。研究结果表明：激光加热下材料强度降低是出现非熔化穿孔破坏的主要机理;薄板在光斑区的最大变形与气流速度、光斑直径、板厚与弹性模量（U2a4/Eh3）相关,穿孔破坏温度与气流速度、光斑直径及板厚（Ua/h）2相关;与方形光斑辐照相比,圆形光斑辐照的破坏阈值稍高一些。数值计算结果表明：0.8 Ma切向气流作用下,铝合金壳体的激光破坏能量阈值大大降低（可达40%～50%）,典型不锈钢壳体的破坏阈值降低相对较小（20%左右）,气流作用导致金属板破坏阈值的下降是需特别关注的问题。     

Comparison of laser ion acceleration from the front and rear surfaces of thin foils

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (approximately 1-6 x 10(19) W/cm2) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.