Dynamics simulation on interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.     

Study on the interaction of intense femtosecond laser pulses with nanometre-sized hydrogen clusters

The interaction of intense femtosecond laser pulses with hydrogen clusters has been experimentally studied. The hydrogen clusters were produced from expansion of high-pressure hydrogen gas (backed up to 8\tm10⁶Pa) into vacuum through a conical nozzle cryogenically cooled by liquid nitrogen. The average size of hydrogen clusters was estimated by Rayleigh scattering measurement and the maximum proton energy of up to 4.2keV has been obtained from the Coulomb explosion of hydrogen clusters under 2×10¹⁶ W/cm² laser irradiation. Dependence of the maximum proton energy on cluster size and laser intensity was investigated, indicating the correlation between the laser intensity and the cluster size. The maximum proton energy is found to be directly proportional to the laser intensity, which is consistent with the theoretical prediction.     

Three-dimensional simulation on explosions of hydrogen atomic clusters irradiated by an intense femtosecond laser pulse

Using classic particle dynamics simulations, the interaction process between an intense femtosecond laser pulse and icosahedral hydrogen atomic clusters H_{13}, H_{55} and H_{147} has been studied. It is revealed that with increasing number of atoms in the cluster, the kinetic energy of ions generated in the Coulomb explosion of the ionized hydrogen clusters increases. The expansion process of the clusters after laser irradiation has also been examined, showing that the expansion scale decreases with increasing cluster size.     

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.     

Pulsed Gas Jet for Large Size Atomic Cluster Production

A pulsed gas jet of large size noble gas atomic clusters as targets for high intensity femtosecond laser pulses is reported. The jet can work for gas stagnation pressure in excess of 40 atmospheres and with a repetition rate of 10 Hz to fit 10 Hz table-top terrawatt femtosecond Ti∶Sapphire laser. The scaling law indicates that the monomer argon clusters produced in the jet can be as large as 22,000 atoms/cluster at room temperature. Preliminary experiments for argon ionic kinetic energy spectrum indicated that the argon clusters are produced in the jet.     

Pulsed Gas Jet for Large Size Atomic Cluster Production

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Ar原子团簇与飞秒强激光相互作用产生的高能离子计算

利用改进的“等离子体球”模型模拟了Ar原子团簇与飞秒强激光相互作用的物理过程.改进后的模型弱化了原模型在共振吸收附近团簇内部屏蔽电场的异常增强行为，从而使其更为合理，计算得到的Ar离子平均动能与以往的实验结果符合.还定量地研究了Ar离子平均动能及其平均电荷态与团簇尺寸以及激光参数之间的变化关系. 关键词： 团簇 飞秒激光 高能离子 共振吸收     

Thermonuclear fusion in the irradiation of large clusters of deuterium iodide with a field of a superatomic femtosecond laser pulse

A theory of thermonuclear fusion caused by the irradiation of deuterium-iodide clusters with the field of a superatomic femtosecond laser pulse is developed. It is based on considering the process in which the sequential above-barrier multiple internal ionization of atomic ions within a cluster is accompanied by external field ionization. The theory is illustrated by taking the example of a cluster that is formed by 10⁶ molecules of deuterium iodide and which is irradiated with a laser pulse of duration 50 fs and intensity 2×10¹⁸ W/cm² at the peak. This case is dominated by I²⁶⁺ atomic ions. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision upon the passage of a laser pulse is calculated. The result is 10⁵ neutrons per laser pulse. The mean kinetic energy of deuterons is estimated at 50 keV. Owing to induced inverse bremsstrahlung in scattering on multiply charged atomic ions, the electron temperature increases up to 28 keV. The role of the Mie resonance in the heating of the electron component is discussed.     

一维氢原子团簇模型下的兆电子伏离子与库仑爆炸： 简化的数值模拟

本文使用一维双列氢原子团簇模型对强激光场中的团簇动力学过程进行了数值模拟。计算所得的质子最大动能和能谱均与他人的计算或实验结果符合较好,但计算大为简化。表明本文所述模型适用于氢原子团簇模拟。本文亦根据逸出质子数量随入射激光强度变化的规律总结出一个描述团簇库仑爆炸的光强阈值公式。     

飞秒强激光作用下Ar原子团簇离子能谱计算

研究了飞秒强激光与Ar原子团簇相互作用时的电离机制及库仑爆炸过程,建立了一个简化的模型计算相互作用产生的离子能谱,计算结果与实验结果较为吻合,可以较好地解释离子能谱高能端的主要特征。     

Charge composition of a cluster plasma upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse

A theory is developed for calculating the charge composition of a cluster plasma produced upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse. The theory is based on the overbarrier process of a successive multiple internal ionization of atomic ions inside a cluster accompanied by the external field ionization. Collision ionization is also taken into account in the calculations. The theory is illustrated by the example of a cluster consisting of 10⁶ xenon atoms irradiated by a 50-fs laser pulse with a peak intensity of 2×10¹⁸ W/cm². In this case, the Xe²⁶⁺ ions dominate. The amounts of atomic xenon ions with multiplicity up to 31 are calculated.     

中等光强纳秒激光电离苯团簇产生多价碳离子的数值模拟和实验研究

飞秒强激光与团簇相互作用产生多价离子的现象已被广泛报道,然而近期多个研究小组发现当功率密度低至1010W/cm2的纳秒激光照射团簇时,同样也观察到了多价离子的存在.虽然可以用"多光子电离引发-逆韧致吸收加热-电子碰撞电离"电离机理对这种现象进行解释,但是缺乏相应的数值模拟.建立了一个简化的数值模型,根据有质动力势Up计算团簇内电子能量,再由Lotz公式计算出相应的电离截面,最后由动力学反应速率方程计算出团簇内多价碳离子随时间的演变.详细分析了团簇尺寸、电子密度等关键参数对多价离子产生的影响.数值模拟结果表明:团簇电离在小于0.7 ns时间尺度内完成,C2+,C3+和C4+多价离子强度达到平衡后,离子相对强度由大到小依次为C2+,C3+,C4+,这与实验结果相一致;多价离子的价态随着团簇尺寸的增加而升高,半径为5.6 nm的苯团簇比半径为3 nm的苯团簇更容易产生高价态的离子,这也与实验结果相一致.     

Control parameters for ion heating and X-ray emission from laser induced cluster explosion

The highest energies of the ions obtained from the explosion of an atomic cluster in an intense femtosecond laser field can vary greatly depending on the cluster size, atomic species and the peak intensity, duration and shape of the laser pulse. By careful choice of these parameters the ion energies, electron energies or X-ray emission can be optimised. A relationship is described that allows for rapid determination of the optimum experimental parameters. We present experimental data of keV X-ray emission from Argon clusters, which investigate intensity and pulse duration effects. In addition we present the first results from closed-loop optimal control, pulse-shaping experiments that optimise X-ray emission and show a significant enhancement in the X-ray yield. PACS 36.40.Gk; 52.50.Jm     

Influence of laser frequency chirp on deuteron energy from laser-driven deuterated methane cluster expansion

The simulations of three-dimensional particle dynamics are carried out to investigate the Coulomb explosion dynamics of deuterated methane clusters under the irradiation of an ultrashort intense laser pulse. The final kinetic energy of deuterons produced from the cluster explosion is calculated as a function of the pulse width, the laser intensity and the pulse chirp. It is found that the deuteron energy obtained in an intense laser pulse with negative chirp is higher than that with positive chirp, which agrees qualitatively with the experimental results reported by Fukuda et al. [Y. Fukuda et al., Phys. Rev. A 67, 061201 (2003)].     

Emission of thermally activated electrons from rare gas clusters irradiated with intense VUV light pulses from a free electron laser

The ionization dynamics of Ar and Xe clusters irradiated with intense vacuum ultraviolet light from a free-electron laser is investigated using photoelectron spectroscopy. Clusters comprising between 70 and 900 atoms were irradiated with femtosecond pulses at 95 nm wavelength (approximately 13 eV photon energy) and a peak intensity of approximately 4 x 10(12) W/cm2. A broad thermal distribution of emitted electrons from clusters with a maximum kinetic energy up to 30-40 eV is observed. The observation of relatively low-energy photoelectrons is in good agreement with calculations using a time-dependent Thomas-Fermi model and gives experimental evidence of an outer ionization process of the clusters, due to delayed thermoelectronic emission.     

Effect of laser spot size on fusion neutron yield in laser--deuterium cluster interactions

The effect of the laser spot size on the neutron yield of table-top nuclear fusion from explosions of a femtosecond intense laser pulse heated deuterium clusters is investigated by using a simplified model, in which the cluster size distribution and the energy attenuation of the laser as it propagates through the cluster jet are taken into account. It has been found that there exists a proper laser spot size for the maximum fusion neutron yield for a given laser pulse and a specific deuterium gas cluster jet. The proper spot size, which is dependent on the laser parameters and the cluster jet parameters, has been calculated and compared with the available experimental data. A reasonable agreement between the calculated results and the published experimental results is found.     

Generation of even harmonics in a relativistic laser plasma of atomic clusters

It is shown that the irradiation of atomic clusters by a superintense femtosecond laser pulse gives rise to various harmonics of the laser field. They arise as a result of elastic collisions of free electrons with atomic ions inside the clusters in the presence of the laser filed. The yield of even harmonics whose electromagnetic field is transverse is attributed to the relativism of the motion of electrons and the consideration of their drift velocity associated with the internal ionization of atoms and atomic ions of a cluster. These harmonics are emitted in the same direction as odd harmonics. The conductivities and electromagnetic fields of the harmonics are calculated. The generation efficiency of the harmonics slowly decreases as the harmonic number increases. The generation of even harmonics ceases when the drift velocity of electrons becomes equal to zero and only the oscillation velocity of electrons is nonzero. The results can also be applied to the irradiation of solid-state targets inside a skin layer.     

飞秒激光氘团簇聚变

简要介绍了原子团簇的产生方法和特点以及激光惯性约束核聚变的基本原理,氚团簇在飞秒激光的作用下,大量吸收激光能量,产生了高离化态高能离子,从而实现了氚团簇桌面台式聚变。     

飞秒强激光脉冲中氩团簇库仑爆炸特性研究

利用Bathe模型，理论模拟了氩团簇在飞秒强激光中(100 fs, 1016 W/cm2)的电离和爆炸过程.研究结果显示，在团簇尺寸较小时，离子平均能量与团簇初始半径平方成正比，爆炸机制为典型的库仑爆炸.随着团簇尺寸的增加，能量增加的速度趋缓并在一定团簇尺寸后趋于饱和.模拟结果与实验数据有较好的吻合.     

Vacuum heating of large atomic clusters by a superintense femtosecond laser pulse

The analytic approach of vacuum (Brunel) heating mechanism is generalized to the case of large atomic clusters irradiated by a superintense femtosecond laser pulse. The hydrodynamic cluster expansion is taken into account in this approach. Simple universal expressions are obtained for the absorbed laser energy by a cluster and for the radius of an expanding cluster. The absorption of laser energy and the cluster expansion are determined by only one dimensionless field parameter.