Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm². The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.     

The spectra of helium-like ions in laser-produced plasmas

We calculate intensities of lines arising from the n = 2 and n = 3 levels of highly-charged, helium-like ions in very dense plasmas. We determine the importance of recombination and cascading, and of radiation trapping, on certain line-intensity ratios, and we illustrate the relevance of these calculations to laser-plasma diagnostics. Finally, we outline the pitfalls inherent, at very high densities, in some other techniques commonly used to interpret plasma spectra.     

Surface free energy of laser-produced bond percolators

Pulsed laser irradiation of thin metallic films on non-wetted substrates creates two-dimensional bond percolator structures. We show that these can be understood as frozen minimum free energy configurations. The spacing between nodes is found to depend linearly on the initial film thickness, as verified experimentally for the case of a Ti film on sapphire.     

The zero-point energy for rotation

The Gaussian overlap approach (GOA) becomes inappropriate for describing the rotation of weakly deformed systems. A modification is proposed which allows to maintain the GOA for small deformations. The zero-point energy subtraction, derived from it, provides a simple and reliable approximation for angular momentum projection. It becomes obvious, however, that the projection complicates the equations which determine the motion along the deformation path. These effects are studied in some simple models and the results are condensed into a simple interpolation formula for the total zero-point energy.     

Observation of MeV ions in long-pulse, large-scale laser-produced plasmas

A new approach for investigation of the generation of fast ions and hot electrons inside the same plasma volume in laser-produced plasmas is proposed. It is based on the spectroscopic observation of line radiation from singly and doubly excited levels with simultaneous high spectral and spatial resolution. The experimental results demonstrate the observation of fast ions from highly charged target material inside the plasma volume and suggest that the generally accepted scaling relations are seriously invalid under certain conditions. Even at rather modest intensities ions with energies of several MeV are observed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 262–267 (25 August 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Charge exchange of laser-produced ions in a pulsed gas jet

Results of an experiment on the interaction of laser-produced plasma with a pulsed gas jet are reported. A resonant charge-exchange pumping of the n=3 level of the C³⁺ ion was observed. A spatial structure of the region of intensive interaction was obtained by a short time imaging of filtered plasma radiation. According to independent probe measurements, the interaction was realized at densities of ions and gas particles in excess of 10¹⁶ cm^-3. The obtained data provide a prospect for future experiments on laser gain in the EUV spectral range based on charge-exchange pumping of the C⁵⁺ ion. PACS 52.50.Jm; 52.25.Nr     

Retrieval of currents of multiply charged ions emitted from laser-produced carbon plasma

The emission of ions from laser-produced carbon plasmas is investigated by a deconvolution of ion collector signals. The deconvolution is based on the use of Kelly and Dreyfus function expressing the time-resolved ion current to recover hidden peaks in an ion collector signal. The parameters of recovered C ^q+ (1?≤?q?≤?6) currents make possible the quantification of properties of laser-produced plasmas. The drift and peak velocities of C ^q+ ions, the abundance of ions and the plasma temperature are presented in the dependence on focused laser beam energy. The carbon plasma was generated employing either single 9-ns pulses of second harmonics (532 nm) of Nd:YAG laser or pulses repeated at a stable repetition rate of 30 Hz.     

Recombination scheme in lithium-like ions for X-UV amplification

Recombination in lithium-like ions in a fast cooling plasma is an alternative attractive approach to the X-ray laser because, at a given lasing wavelength, it requires the lowest pumping energy of the main pumping schemes. In this paper we give a status report on recent experiments concerning the Li-like X-ray laser, and we discuss the problem of lengthening of the active medium necessary to obtain a true X-ray laser.     

四程放大系统时间平顶脉冲的能量离散

 针对光学元件参数存在随机误差的多光束高功率固体激光驱动器,利用概率与统计的方法模拟计算了9-6排布四程放大系统输出时间宽度为1ns和13ns的平顶脉冲时主放大器输出的基频光的能量离散情况。计算结果表明,当激光脉冲工作在介质的线性放大区时,输出脉冲的能量离散度较大;而当激光脉冲工作在介质的深度饱和区时,输出脉冲的能量离散度较小。利用放大器的这一性质,可以通过增加注入能量的方法使激光器工作在更深的饱和状态,从而减小输出脉冲的能量离散度。     

Channeling stopping power for high energy ions

Making use of the inelastic scattering function introduced by Ohtsuki, the position dependent stopping power is derived for energetic ions in a channeling condition. We can interprete the Lindhard theory, Esbensen-Golovchenko theory, and Burenkov-Komarov-Kumakhov theory for the channeling stopping power in terms of our method. Our results agree very well with experiment.     

类铍离子激发态的能量计算

采用多组态相互作用方法及Rayleigh- Ritz变分法,计算了类铍O4+离子1s22s2p3Po态的非相对论能量,并详细分析了如何选择质量最好的变分波函数.运用截断变分方法进一步饱和函数空间得到能量的改进量,并考虑相对论修正、质量极化效应等获得了相对论能量计算值.计算结果表明,此理论计算方法可得到高精度的能量计算值.     

Fokker-Planck analysis of energy deposition of laser-produced relativistic electrons in dense plasmas

Evaluation of the energy deposition region of a laser-produced relativistic electron beam in dense core plasma is essential in the fast ignition approach to ICF. Recently, we have developed a two-dimensional relativistic Fokker-Planck code “RFP-2D” for fast electron transport in dense plasmas to investigate the energy deposition profiles. In this paper, we compare the sizes of the energy deposition region derived from RFP-2D and another RFP-type calculation model with stochastic treatments. It is found to be considerably important to take into account the influence of the self-generated electromagnetic field when evaluating the energy deposition region.     

Universal function for the brilliance of undulator radiation considering the energy spread effect

Angular and spatial profiles of undulator radiation have been investigated to derive a universal function that evaluates the brilliance of undulator radiation and takes into account the effects of electron beam emittance and energy spread. It has been found that the effects of energy spread on the angular divergence and source size can be expressed by simple analytic expressions, and a universal brilliance function has been derived by convolution with the electron beam distribution functions. Comparisons with numerical results have been carried out to show the validity and applicability of the universal function.     

多条形电极束流能散度探测器的张角优化

采用Surperfish对北京正负电子对撞机的多条形电极束流能散度探测器进行了电磁场模拟计算。在带模拟束流的情况下,计算分析了探测器对束流位置和能散度的分辨能力与电极张角之间的关系,首次计算了条形电极间的电容耦合综合强度。电极张角优化结果表明：在不考虑噪声水平的情况下,电极张角为20°时,探测器的灵敏度和分辨率达到最优。而当噪声影响很大时,可以适当增加电极张角,同时考虑减小噪声的措施。     

多条形电极束流能散度探测器的张角优化

 采用Surperfish对北京正负电子对撞机的多条形电极束流能散度探测器进行了电磁场模拟计算。在带模拟束流的情况下,计算分析了探测器对束流位置和能散度的分辨能力与电极张角之间的关系,首次计算了条形电极间的电容耦合综合强度。电极张角优化结果表明：在不考虑噪声水平的情况下,电极张角为20°时,探测器的灵敏度和分辨率达到最优。而当噪声影响很大时,可以适当增加电极张角,同时考虑减小噪声的措施。     

Observation of Stark-shifts of Lyman-α lines of low-Z ions in laser-produced plasmas

Red-shifts have been observed for Lyman-α lines of Z = 4–7 ions at an electron density of 10²¹ cm^-3. The plasma was produced by a 0.1 TW 100 ps glass laser. The shift was measured relative to the line center of the spatially resolved unperturbed line. The results are compared with theory.     

A scheme for the implementation of unconventional geometric phase gates with trapped ions

We propose a scheme for the realization of unconventional geometric two-qubit phase gates with two identical two-level ions, In the present scheme, the two ions are simultaneously illuminated by a standing-wave laser pulse with its pulse frequency being tuned to the ionic transition. The gate operation time can be much shorter, making the system robust against decoherence. In addition, we choose the appropriate experimental parameters to construct the geometric phase gate in one step, and thus avoid implementing the pure geometric single qubit operation.[第一段]     

Poloidal rotation of main ions in the CT-6B tokamak

The poloidal rotation velocity of neutral hydrogen atoms is measured using the Doppler shift of the Hα spectral line emitted in the CT-6B tokamak. The poloidal rotation of hydrogen atoms is generated through the collisions and charge-exchanges with main ions (protons). Therefore, the rotation direction of main ions can be deduced from that of neutral hydrogen atoms. The experimental results show that the main ions rotate in the electron diamagnetic drift direction, the same as the impurity ions, in the plasma core. The neutral hydrogen atoms rotate also in the electron diamagnetic drift direction in the edge region of the plasma. However, the rotation direction of main ions in the edge region cannot be judged from the experimental result due to the long mean free path of hydrogen atoms in the edge region. An inward diffusion flux of hydrogen atoms toward the torus inside with a velocity of the same order of magnitude as their poloidal rotation is also observed.