Population inversion in optically thick,recombining hydrogen plasmas

The plasma condition is investigated theoretically for population inversion between the first two excited states of hydrogen atoms in a recombining plasma. The rate equation, including atom-atom collision terms, is solved consistently with the optical escape factors. The upper bound of the ground level population density (n₁)_max necessary for inversion in the optically thick plasma at specified electron density and temperature is nearly inversely proportional to the mean radius of the plasma r_O. With a decrease in the atom temperature, the upper bounds increase in the optically thin plasma but decrease in the optically thick plasma.     

Population inversions in recombining hydrogen,nitrogen, and oxygen atomic plasmas

The possibility of population inversions in atomic hydrogen, oxygen and nitrogen plasmas has been investigated by solving a system of master equations describing the temporal evolution of the electronic levels exposed to the action of collisional and radiative processes. The results show that population inversions can be obtained for both quasistationary and fast, transient, recombining plasmas. In this case, the lasing durations of the transitions O(I) 3p³P-3s³S, 3p⁵P-3s⁵S and N(I) 3s²P-2p²D, 3s²P-2p²P⁰ are of the same order of magnitude as the collisional relaxation times of the original states.     

Lasing conditions in recombining helium plasmas

Lasing conditions for He have been evaluated numerically. We have used a collisional radiative model to calculate overpopulation densities Δn_ij, which are defined as differences between population densities per unit statistical weight of the upper and lower excited levels i and j, respectively. Laser oscillations for the level pairs 2¹P-3¹S, 2¹P-4¹S, 2¹P-3¹D, 2¹P-4¹D, 3¹D-4¹F, 3¹P-4¹S, 3¹P-4¹D, and 3³P-4³S are possible when the electron densities are within well defined limits at low electron temperature (T_e = 0.1 eV). For level pairs of the singlet state, the inversion mechanism for He is the same as for H. Only collisional processes produce population inversion in the triplet level pair 3³P-4³S.     

A DCN laser interferometer for electron density measurements of plasmas

The main characteristic of this interferometric system is such that the systems can be used for DCN laser (=195m, 190m) and also for HCN laser (=337m) by changing the work medium, regulating the temperature of tube wall and adjusting the optic path, without changing any optic element.     

Population inversion in recombining hydrogen plasma

The collisional-radiative model is applied to a recombining hydrogen plasma in order to investigate plasma conditions in which a population inversion between the energy levels of hydrogen can be generated. Population inversion is expected in plasmas for which three-body recombination makes a large contribution to the recombining processes and the effective recombination rate is larger than a critical value for a given electron density and temperature. Calculated results are presented in figures and tables.     

Enhanced damping of electrostatic waves due to combination scattering from density oscillation in plasmas

Experiments demonstrating the combination scattering of the electrostatic wave from the electron density oscillation in plasmas are described. The spatially periodic energy transfer among the satellite modes and the primary mode is observed, which results in an enhanced damping of the primary mode.     

Quantitative two-dimensional shadowgraphic method for high-sensitivity density measurement of under-critical laser plasmas

We present a quantitative shadowgraphic method that can measure the density of a laser-generated plasma in air. Simultaneous comparison of both shadowgraphy and interferometry has been carried out, allowing the experimental evaluation of the reliability of the shadowgraphic method. Even though limited by absorption in the sample, shadowgraphy performs better in terms of sensitivity.     

Doppler shift of femtosecond laser pulses from solid density plasmas

Doppler-shift of short UV pulses (248 nm, 700 fs) reflected from aluminum, polystyrene and gold targets were measured at focused intensities up to 5×10¹⁵ W/cm². In spite of the fact that the Doppler-shifts are expected to be dependent on the atomic weight of the target material, experimentally similar shifts are found for all targets. A possible explanation is provided by the well known dependence of the Doppler shift on the plasma temperature, if different absorption coefficients are assumed for the different targets. This coefficient shows good agreement with that one deduced from a theory based on collisional absorption in the case of aluminum, but in the case of the other targets, however, the experimentally observed Doppler-shift suggests smaller absorption for lighter and larger absorption for heavier target materials. PACS 52.25.Mq; 52.25.Qt; 52.40.Nk; 52.50.Jm     

Plasma density gratings induced by intersecting laser pulses in underdense plasmas

Electron and ion density gratings induced by two intersecting ultrashort laser pulses at intensities of 10¹⁶ W/cm² or lower are investigated. The ponderomotive force generated by the inhomogeneous intensity distribution in the intersecting region of the interfering pulses produces deep electron and ion density modulations at a wavelength less than a laser wavelength in vacuum. Dependence of the density modulation on the plasma densities, temperatures, and the ion mass, as well as the laser pulse parameters are studied analytically and by particle-in-cell simulations. It is found that the density peaks of such gratings can be a few times that of the initial plasma density and last as long as a few picoseconds. It is also demonstrated that the scattering of signal pulses by such a bulk density grating results in high-harmonic generation. The density gratings may be incorporated into ion-ripple lasers [K.R. Chen and J.M. Dawson, Phys. Rev. Lett. 68, 29 (1992)] to generate ultrashort X-ray pulses of a few angstroms by using electron beams at only a few tens of MeV only. PACS 52.35.Mw; 42.65.Ky; 52.25.Os     

Kr喷气箍缩等离子体的数值研究

 探讨了Z箍缩等离子体辐射磁流体模拟二维三温物理模型及其数值计算方法，针对“强光一号”加速器上Kr喷气实验的具体条件，利用辐射磁流体力学程序Z-pinch 2D-DG模拟了Kr喷气等离子体聚爆过程，分析了X射线辐射的特点，给出了等离子体密度和温度的演化图像以及喷气箍缩中一些带有普遍性的结论。     

超强短脉冲激光在不同密度分布等离子体中的自聚焦

讨论高斯型强激光束在具有初始柱对称密度分布的低密度冷等离子体中传播时，等离子体密度分布的不同对激光自聚焦的影响.推导出可以判断更有利于自聚焦发生的评价函数，这样通过比较不同密度分布的评价函数值就可以判断哪种密度分布更有利于自聚焦的发生.为了说明这种方法的有效性，对评价函数进行分析得出：在相同的激光场中等离子体柱的轴心密度给定时(以激光的光轴为轴)，离轴越远的地方密度越大及密度变化越陡，自聚焦越容易发生；相对论效应与有质动力共同作用比相对论的单独作用，自聚焦更容易发生.数值模拟证实了评价函数能准确的预测在不 关键词： 自聚焦 相对论效应 有质动力 评价函数     

Possibility of plasma density diagnostics using Langmuir-wave-caused dips observed in dense laser plasmas

A systematic study of the Langmuir-wave-caused dips (L dips) observed in profiles of the Al Ly γ line emitted from laser irradiated sandwich targets indicates that these fine spectral features can be used as a tool for density diagnostics in intermediately coupled plasmas. The spectroscopic data required for a reliable identification of L dips were collected by a vertical-geometry Johann spectrometer providing high spectral and spatial resolution. The electron densities deduced from the evolution of the L dips along the laser target axis compare well with those derived from hydrosimulations and from measurements of the line widths and shifts.     

Kr喷气箍缩等离子体的数值研究

探讨了Z箍缩等离子体辐射磁流体模拟二维三温物理模型及其数值计算方法,针对“强光一号”加速器上Kr喷气实验的具体条件,利用辐射磁流体力学程序Z-pinch 2D-DG模拟了Kr喷气等离子体聚爆过程,分析了X射线辐射的特点,给出了等离子体密度和温度的演化图像以及喷气箍缩中一些带有普遍性的结论。     

Optimization of laser propagation in optical-field-ionization plasmas for X-ray laser generation

Optical-field-ionization X-ray lasers are investigated numerically with a three-dimensional wave propagation code considering the effects of photoionization, energy depletion due to ionization, and refraction on pump laser pulses in spatially and temporally varied plasmas. By focusing the pump laser with small f-number optics at the optimal position, simulations show that diffraction and ionization-induced refraction in plasmas are compensated to keep the pump beam propagating at the optimal size for a longer distance. An amplification length as long as 5 mm can be achieved in Pd-like xenon and Ni-like krypton X-ray lasers at a pump energy of 160 mJ in 50-fs and 30-fs pulses, respectively. The significant reduction of the pump energy is a desirable step toward low-threshold and practical high-repetition-rate operations. PACS 42.55.Vc; 52.50.Jm     

High density plasmas for recombination X-ray lasers

The scaling of recombination XUV lasers to shorter wavelengths requires laser plasmas produced at initial electron densities close to solid. With pump laser pulses longer than a few tens of picoseconds the hydrodynamic motion of the plasma during the interaction makes this difficult to achieve. In contrast, when picosecond laser pulses are used the laser energy is absorbed close to solid density since the plasma expansion is insignificant during the laser pulse. This results in hot near solid density plasmas which are needed for hydrogenic recombination X-ray lasers operating in the water window. Experimental observations have shown that a fully ionized aluminium plasma with a temperature of about 400 eV and a density well above 10²³ cm^–3 is produced when an aluminium target is irradiated with a single 3.5 ps high power KrF laser pulse.     

Relativistic laser plasmas for novel radiation sources

Relativistic laser-plasma interaction results in new sources of short-pulsed x-ray radiation. Here we consider two options. The first one is betatron radiation of electrons accelerated in underdense plasmas and oscillating in transverse fields of the laser wake. This radiation is incoherent and broadband, the pulse duration is comparable with that of the driving laser. The second option is the high harmonic generation (HHG) from overdense plasma surfaces. This radiation is coherent. The relativistic high harmonics are phase locked and emerge in the form of (sub-)attosecond pulses. One- and three-dimensional regimes of relativistic HHG from overdense plasmas are considered.     

Numerical investigation of relativistic self-focusing of intense laser beam with LG01 mode in plasmas

In this paper, we numerically investigate the relativistic self-focusing of laser beam with LG₀₁ mode in plasmas. An effective potential is introduced to identify the critical power. Numerical methods are employed to solve the governing equations. To verify the numerical procedures, we compare the known analytical solution of the relativistic critical power for TEM₀₀ mode with our numerical method. It is shown that there is a good agreement between our numerical results and the analytical solution. The critical power for relativistic self-focusing of a LG₀₁ laser beam is about 6 times of that for a TEM₀₀ laser beam.     

Two-color pump of laser plasmas for harmonic generation

Various laser-produced metal plasmas were used for high-order sum and difference harmonic generation using the mixing of tunable mid-infrared pulses from optical parametric amplifier and 810 nm ultrashort pulses. We show that, regardless of non-optimal spatio-temporal overlap of two sources of radiation in the plasmas, the application of proposed technique allows a significant growth of harmonic yield and broadening of the number of different combinations of interacting waves in the extreme ultraviolet region, which could be useful for observation of the resonance induced enhancement of some frequency components.