Dynamics of Coulomb explosion of hydrogen clusters in a high-intensity femtosecond laser pulse

Using Bethe model, the dynamics of the ionization and Coulomb explosion of hydrogen clusters (0.5-5nm) in high-intensity (1015-1017 W/cm2) femtosecond laser pulses have been studied theoretically, and the dependence of energy of protons emitted from exploding clusters on cluster size and laser intensity has been investigated. It is found that the maximum proton energy increases exponentially with the cluster size, and the exponent is mainly determined by the laser intensity. For a given cluster size, the maximum proton energy increases with increasing laser intensity and gets saturation gradually. The calculation results are in agreement with the recent experimental observation.     

A plasma channel scheme for the interaction of an intense femtosecond laser pulse with a deuterium cluster jet

We propose a plasma channel scheme to obtain an improved table-top laser driven fusion neutron yield as a result of explosions of large deuterium clusters irradiated by an intense laser pulse. A cylindrical plasma channel is created by two moderate intensity laser prepulses at the edge of a deuterium cluster jet along which an intense main laser pulse propagates several nanoseconds later. With the aid of this plasma channel, the main laser pulse will be allowed to deposit its energy into the central region of the deuterium gas jet where the cluster sizes are larger and the atomic density is higher. The plasma channel formation and its impact on the deuterium ion energy spectrum and the consequent fusion neutron yield have been investigated. The calculated results show that a remarkable increase of the table-top laser driven fusion neutron yield would be expected.     

Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet

Using interferometry, we investigate the dynamics of interaction of a relativistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 microm in diameter, with a radial gradient of electron density approximately 5 x 10(22) cm(-4) and with an on-axis electron density approximately ten times less than its maximum value of 8 x 10(19) cm(-3). A high radial velocity of the surrounding gas ionization of approximately 3.8 x 10(8) cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas excitation and ionization. Comparing the model predictions with the interferometric data, we reconstructed the axial profile of laser channel and on-axis laser intensity. The estimated maximum energy of accelerated ions is about 500 keV, and the total energy of the fast ions is 5% of the laser pulse energy.     

Electron gas compression and Coulomb explosion in the surface layer of a conductor heated by femtosecond laser pulse

The hypothesis is put forward on the basis of experimental data that strong inhomogeneous heating of the skin layer of conducting materials by a femtosecond pulse gives rise to a double electrical layer that is formed of a “surface” layer of positive ions and a thin (about 1 nm) “subsurface” layer of a superdense (10²³–10²⁵ cm^?3) degenerate electron gas. The double layer breaks within one picosecond through the Coulomb explosion.     

Interaction of a high-intensity ultrashort laser pulse with extended nanofilaments of dense plasma

Generation and propagation of fast electrons in laser targets consisting of thin nanofilaments are studied numerically and analytically. Such targets completely absorb laser radiation and exhibit a large coefficient of laser-energy conversion to kinetic energy of a flow of fast electrons. Analytical estimates show that the optimal thickness of the filament is on the order of the skin depth of the laser plasma, while an optimal distance between filaments is on the order of the Debye radius of hot electrons. A bunch of relativistic electrons can propagate as far as several hundred micrometers in such targets, while the fastest electrons can propagate several millimeters. Upon bending of filaments, the flow of electrons propagates along the filaments and can be focused by bringing the filaments together. Laser targets of the discussed composition are used as sources of dense bunches of relativistic electrons and subsequent generation of high-intensity X-ray radiation with their help.     

On the possibility of the amplification of subterahertz electromagnetic radiation in a plasma channel created by a high-intensity ultrashort laser pulse

The evolution of the electron energy distribution function in a plasma channel in a xenon plasma at atmospheric pressure created by radiation of a KrF femtosecond laser has been considered. It has been shown that, owing to the existence of the Ramsauer minimum in the transport scattering cross section, such a channel can be used to amplify electromagnetic waves up to the terahertz frequency range at relaxation times of the energy spectrum of ～10^?7 s. The gain factor has been calculated as a function of the time and radiation frequency.     

Formation of plasma channels in the interaction of a nanosecond laser pulse at moderate intensities with helium gas jets

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].     

Hot and cold electron dynamics following high-intensity laser matter interaction

The characteristics of fast electrons laser accelerated from solids and expanding into a vacuum from the rear target surface have been measured via optical probe reflectometry. This allows access to the time- and space-resolved dynamics of the fast electron density and temperature and of the energy partition into bulk (cold) electrons. In particular, it is found that the density of the hot electrons on the target rear surface is bell shaped, and that their mean energy at the same location is radially homogeneous and decreases with the target thickness.     

Observation of laser satellites in a plasma produced by a femtosecond laser pulse

Laser satellites are detected in the emission spectra of magnesium and aluminum plasmas produced by femtosecond laser pulses. This is made possible by the realization of picosecond time resolution in a high-luminosity x-ray spectrograph with a spherically curved mica crystal. The temporal characteristics of these newly recorded spectral lines show unequivocally that they are formed as a result of nonlinear processes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 454–459 (10 October 1997)     

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 10¹⁶W/cm². The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 10¹⁶–10¹⁷ W/cm² and also when extrapolated with the results of three-dimensional particle simulations at 10²⁰–10²¹ W/cm². Energetic proton generation in low-density plastic (C₅H₁₀) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm³. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 10¹⁸ W/cm². A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.     

微通道脉冲管中He气交替振荡的分子动力学模拟

使用非平衡分子动力学模拟方法分析了微通道脉冲管(MPT)中由正弦速度活塞提供驱动力时He气交替振荡的微观动力学过程,并对MPT的冷却机制进行了分析.结果表明,MPT的压缩和膨胀过程之间存在一个交替的振荡过程,两个过程具有不对称的属性分布,膨胀过程具有比压缩过程更大的轴向压力梯度.当充气压力较低时,循环时间对冷端温度的影响很小,但是当充气压力高于20 bar时,冷端温度对时间较为敏感,随着时间的减少,冷端温度进一步降低,而冷端瞬时平均温度随着充气压力的增加而增加.另外,压比随着时间的减少而增加,并且明显不受充气压力的影响,但它会在MPT的轴向上产生较大的温度梯度.综上所述,在热端使用不同形式的换热器和调相元件会释放或回收额外的声功率.固定工作模式和尺寸参数的MPT具有最佳频率,可以在冷端获得最低的空载温度.仿真结果增进了对脉冲管制冷机的认识,并为微通道脉冲管制冷机的优化设计提供理论支持.     

Nonlinear polarization response of an atomic gas medium in the field of a high-intensity femtosecond laser pulse

Polarization response that appears in silver vapors in the field of a high-intensity femtosecond Ti:sapphire laser has been studied by the direct numerical integration of the time-dependent Schrödinger equation. The regions of applicability have been determined for perturbation theory and the power series expansion of the polarization in the field. The contribution of free electrons to the response at the frequency of the interacting field has been calculated, which is due to the photoionization process and limits the Kerr effect. An important contribution of laser-excited atomic states to nonlinear atomic responses of neutral atoms has been demonstrated.     

Nonlinear propagation of an intense Laguerre–Gaussian laser pulse in a plasma channel

The nonlinear propagation of an intense Laguerre–Gaussian(LG) laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method. The evolution equation of the spot size is derived including the effects of relativistic self-focusing, preformed channel focusing, and ponderomotive self-channeling. The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size, periodically focusing and defocusing oscillation,catastrophic focusing, and solitary waves are obtained. Compared with the laser pulse with fundamental Gaussian(FG)mode, it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse, while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse. Thus, the matched condition for the intense LG laser pulse with constant spot size is released obviously, while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.     

Nanosecond laser pulse interactions with breakdown plasma in gas medium confined in a microhole

The previous investigations on nanosecond laser pulse interactions with breakdown plasma in a gas medium confined in a microhole have been limited. This kind of plasma has been studied in this paper. Due to the significant measurement difficulty resulted from the very small spatial and temporal scales involved, a physics-based computational model has been employed as the investigation tool. The model is developed by solving gas dynamic equations numerically using the finite difference method based on an essentially non-oscillatory scheme. The gas dynamic equations are coupled with suitable equation of state, where the electron number density for plasma region is calculated through the Saha equation. Using the model, the spatial confinement effects of the microhole sidewall on the plasma evolution under laser radiation have been investigated. It has been found that under the studied conditions the hole sidewall confinement can greatly enhance the plasma temperature, pressure, and thrust (over the same surface area). The enhancement should be due to the sidewall’s restriction on the plasma lateral expansion and the sidewall’s reflection of the pressure wave induced by plasma. This study implies potential advantages of the breakdown plasma confined in a microhole in many relevant applications, such as laser propulsion and laser-induced breakdown spectroscopy. The developed model also provides a useful guiding tool for future fundamental research and practical applications in many areas related to laser interactions with gas breakdown plasma.     

Ultrafast nuclear dynamics and non-uniform Coulomb explosion of heteroclusters

This paper reports on some unique features of the ion spatial distribution, energetics and time-resolved dynamics in Coulomb explosion of multicharged light-heavy heteroclusters, consisting of light, low-charge and heavy, high-charge, ions, e.g. hydroiodic acid [image omitted] and its isotopic substituents [image omitted] and [image omitted]. In these clusters, extreme multielectron ionization in ultraintense laser fields (peak intensity I = 1015 - 1020 W cm-2) results in highly charged heavy ions, e.g. qI ? 7 at I = 6×1015 W cm-2 and qI = 25 at I = 1019 W cm-2. Molecular dynamics simulations based on the cluster vertical ionization (CVI) initial conditions, together with complete simulations involving both electron and nuclear dynamics of heteroclusters subjected to a Gaussian laser pulse, which were conducted for Coulomb explosion of [image omitted] and [image omitted] ionic clusters, reveal expanding, thin, two-dimensional spherical shells of the light D+ or H+ ions, with the monolayer expansion occurring on the femtosecond time scale. The expanding spherical nanoshells of light ions are analogous to a 'soap bubble', characterized by negative surface tension and driven by Coulomb pressure. The energetic data for the light ions reveal high energies with a narrow energy distribution, characterized by a lower energy cut-off, e.g. average energy Eav = 23 keV at width ΔE/Eav = 0.16, and a cut-off energy of EMIN = 19.2 keV for Coulomb explosion of [image omitted] clusters. These dynamic, structural and energetic data for exploding multicharged light-heavy heteroclusters arise from kinematic overrun effects of the light ions.     

The effect of plasma channel on the self-distortion of laser pulse propagating through the collisional plasma channel

In the present paper, laser pulse distortion/breakup and the effect of the plasma channel on the laser propagation through the collisional plasma have been studied by using moment theory approach. Second order nonlinear differential equations of the beam width parameter have been derived for the propagation of the laser through uniform homogenous plasma and preformed plasma channel having parabolic density profile. Differential equations of beam width parameter have been solved numerically using Runge Kutta method. It has been observed from analysis that when the laser pulse propagates through the homogenous plasma, the low intensity front and rear parts of the laser get defocused/diffracted and the high intensity central/main portion of the laser pulse gets self-guided. As a result of this, the laser pulse gets distorted. This distortion of the laser has not been observed when the laser pulse is propagated through the plasma channel having density minimum at the axis and maximum at the edges. The laser pulse is guided as a whole, even the low intensity front and rear parts of the laser are also guided. Therefore, the plasma channel is useful to prevent the distortion/breakup of the laser.     

Laser plasma propulsion generation in nanosecond pulse laser interaction with polyimide film

The plasma propulsion generated in nanosecond pulse laser interaction with polyimide film is investigated. A comparison of coupling coefficient and specific impulse with glass layer and water layer confinement is given. It shows that polyimide has a higher efficiency in water confinement ablation. Through doped carbon black in polyimide film, a higher coupling coefficient is obtained. In ablated surface images, less re-deposited products on polyimide surface have been observed compared with other polymers at the same laser intensity.     

Spectral characteristics of interaction between a short intense laser pulse and ionizable gas

An expression for the mean-square frequency of a short intense laser pulse passing through an ionizable gas is derived for arbitrary 3D irradiation and observation geometries and pulse intensity. It is found that the resulting blue shift depends on the lasing intensity at the instant of ionization and on the ionization energy loss. Taking into account stimulated Raman backscattering increases the predicted value of the blue shift.     

Plasma ion evolution in the wake of a high-intensity ultrashort laser pulse

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.     

Energetic-ion generation by the combination of laser pressure and Coulomb explosion

A scheme of generating energetic ions by the interaction of an ultrahigh-intensity laser pulse and a thin solid foil is studied. The combination of the effects of radiation pressure and Coulomb explosion makes the ion acceleration more effective. The maximum ion velocity variation with time is predicted theoretically while the temporal evolution of the electrostatic field due to the Coulomb explosion is taken into consideration. Two-dimensional particle-in-cell simulations are done to verify the theory.