Charge displacement in ultraintense laser-plasma interaction

The distribution of optical held and charge density in the interaction between ultraintense ultrashort pulse laser and plasma is studied by numerical computation. The plasma considered has an exponential density profile. which corresponds to isothermal expanding. Our calculation shows that electrons are pushed forward by the incident laser, but ions, due to their much greater inertia, remain stationary. The resulting charge displacement forms a strong electrostatic field in the plasma. After the interaction of laser pulse and plasma. electrostatic energy still exists even after the laser pulse and will be absorbed by the plasma finally. This serves as an explanation to the mechanism of laser energy deposited into plasma.

Plasma channeling by multiple short-pulse lasers

Channeling by a train of laser pulses into homogeneous and inhomogeneous plasmas is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy. The increased penetration distance can be attributed to the repeated actions of the ponderomotive force, the continuous between-pulse channel lengthening by the inertially evacuating ions, and the suppression of laser-driven plasma instabilities by the intermittent laser-energy cut-offs.

Intense laser beam guiding in self-induced electron cavitation channel in underdense plasmas

In underdense plasmas, the transverse ponderomotive force of an intense laser beam with Gaussian transverse profile expels electrons radially, and it can lead to an electron cavitation. An improved cavitation model with charge conservation constraint is applied to the determination of the width of the electron cavity. The envelope equation for laser spot size derived by using source-dependent expansion method is extended to including the electron cavity. The condition for self-guiding is given and illuminated by an effective potential for the laser spot size. The effects of the laser power, plasma density and energy dissipation on the self-guiding condition are discussed.

Few-cycle relativistic solitons in plasmas

A set of exact one-dimensional solutions to coupled nonlinear equations describing the propagation of a relativistic ultrashort circularly polarized laser pulse in a cold collisionless and bounded plasma where electrons have an initial velocity in the laser propagating direction is presented. The solutions investigated here are in the form of quickly moving envelop solitons at a propagation velocity comparable to the light speed. The features of solitons in both underdense and overdense plasmas with electrons having different given initial velocities in the laser propagating direction are described. It is found that the amplitude of solitons is larger and soliton width shorter in plasmas where electrons have a larger initial velocity. In overdense plasmas, soliton duration is shorter, the amplitude higher than that in underdense plasmas where electrons have the same initial velocity.

Standing ultrashort relativistic solitons in overdense plasmas

By using a one-dimensional self-consistent relativistic fluid model, an investigation is made numerically on relativistic electromagnetic solitons with a high intensity in cold overdense plasmas with an electrons' initial velocity opposite to the laser propagating direction. Two types of standing solitons with zero group velocity are found at the given electrons' initial velocities. One is single-humped with a weakly relativistic intensity; the another is multi-humped with a strong relativistic amplitude. The properties of these two types of solitons are presented in detail.

Improved uniformity of target illumination by combining a lens array and the technique of spectral dispersion

A scheme using a lens array and the technique of spectral dispersion is presented to improve target illumination uniformity in laser produced plasmas. Detailed two-dimensional simulation shows that a quasi-near-field target pattern, of steeper edges and without side lobes, is achieved with a lens array, while interference stripes inside the pattern are smoothed out by the use of the spectral dispersion technique. Moving the target slightly from the exact focal plane of the principal focusing lens can eliminate middle-scale-length intensity fluctuation further. Numerical results indicate that a well-irradiated laser spot with small nonuniformity and great energy efficiency can be obtained in this scheme. (c) 2007 American Institute of Physics.

激光诱导等离子体开关控制脉宽实验研究

利用激光诱导等离子体开关技术，在1064nm的Nd：YAG固体激光器上获得了脉宽4．4～6．4ns的短脉冲激光输出。激光电离空气产生的等离子体开关控制脉冲宽度时，聚焦透镜焦距越短，压缩后的脉宽越窄，但激光能量损耗越大。压缩后的激光脉宽与激光能量近似成双曲线关系。在控制脉宽光路的焦点处放置带孔的Cu薄片可抑制等离子体的扩散，得到了脉宽最短可达4．4ns的激光输出。

355 nm脉冲激光诱导等离子体开关削波

利用激光诱导等离子体开关技术,对355 nm脉冲激光自削波进行了实验和理论研究。分别采用5种不同焦距的透镜,集中讨论了透镜焦距及激光器输出单脉冲能量对脉宽压缩的影响,发现采用焦距为200 mm的透镜能够获得最佳的脉冲压缩效果。在聚焦透镜焦距200 mm,单脉冲能量160 mJ时,获得最短脉宽3.47 ns；在激光电离Cu小孔内壁表面及空气击穿共同作用下,获得了脉宽最短达2.11 ns的脉冲激光输出。此外,根据实验结果得到了355 nm激光空气击穿阈值,并与理论估算值进行比较,两者结果较为一致。

Optical spectral relation between the master and the frequency-locked slave lasers

In this paper, the spectral relation between the master and the frequency-locked slave laser (FLSL) is investigated by the conventional technique of optical intensity modulation and optical heterodyne. Experimentally, we demonstrate that under complete and stable locking condition, the lightwave of the FLSL and the sidebands of the master laser produced by the optical intensity modulation are perfectly coherent (frequency coherence). Referring to our recent studies, the lightwave of the master laser and its corresponding sidebands are also perfectly coherent. Additionally, the spectral structures of two perfectly coherent lightwaves are identical in the level of wave train. Therefore, we indirectly verify that the spectral structures of the FLSL and the master laser are identical in the level of wave train.

Research on EUV spectra of highly ionized titanium

The spectrum of highly ionized titanium was studied by means of the beam-foil technique. Titanium ions were provided by the HI-13 tandem accelerator at China Institute of Atomic Energy. The experimental results are compared with those of laser-produced plasmas. Numerous lines attributed to Ti XVI to XVIII ransitions have been identified, and three of them were newly measured, which were attributed to 2s2p(24)P(3/2)-2p(32)D(3/2), 2s2p(21)S(0)-2sp(31)P(1) and 4p(1)P(0)-5d(1)P(1) transitions.

激光诱导等离子体光谱法检测合金钢组分的实验研究

激光诱导击穿光谱技术(LIBS)具有无需样品制备,原位快速分析,可进行实时控制的特点使其在钢铁冶炼控制中具有巨大的实际应用价值。本文以波长为1 064 nm的Nd∶YAG调Q固体激光器为激发光源,CCD为探测器,高合金钢GBW01605—01609系列为样品,在建立的LIBS实验装置上研究激光与合金钢之间的相互作用。系统地研究了观测距离、激光能量对高合金钢样品中激光诱导击穿谱特性的影响,并分析了LIBS信号的时间分辨特性,确定了将LIBS用于合金钢微量元素定量分析时的最佳实验条件。

Time-resolved investigation of low-density plasma channels produced by a kilohertz femtosecond laser in air

By employing pump-probe back longitudinal diffractometry, the electron density and decay dynamics of a weak plasma channel created by a 1-KHz fs laser in air has been investigated. With ultrashort laser pulses of 50 fs and low energy of 0.6 mJ, we observe weak plasma channels with a length similar to 2 cm in air. An analytical reconstruction method of electron density has been analyzed, which is sensitive to the phase shift and channel size. The electron density in the weak plasma channel is extracted to be about 4x10(16) cm(-3). The diameters of the plasma channel and the filament are about 50 and 150 mu m, respectively, and the measurable electron density can be extended to less than 10(15) cm(-3). Moreover, a different time-frequency technique called linearly chirped longitudinal diffractometry is proposed to time-resolved investigate ultrafast ionization dynamics of laser-irradiated gas, laser interaction with cluster beam, etc.

Angular distribution and conversion of multi-keV L-shell X-ray sources produced from nanosecond laser irradiated thick-foil targets

An experimental study on the angular distribution and conversion of multi-keV X-ray sources produced from 2 ns-duration 527nm laser irradiated thick-foil targets on Shenguang II laser facility (SG-II) is reported. The angular distributions measured in front of the targets can be fitted with the function of f(theta) = alpha+ (1- alpha)cos(beta) theta (theta is the viewing angle relative to the target normal), where alpha = 0.41 +/- 0.014, beta = 0.77 +/- 0.04 for Ti K-shell X-ray Sources (similar to 4.75 keV for Ti K-shell), and alpha = 0.085 +/- 0.06, beta = 0.59 +/- 0.07 for Ag/Pd/Mo L-shell X-ray Sources (2-2.8 keV for Mo L-shell, 2.8-3.5 keV for Pd L-shell, and 3-3.8 keV for Ag L-shell). The isotropy of the angular-distribution of L-shell emission is worse than that of the K-shell emission at larger viewing angle (>70 degrees), due to its larger optical depth (stronger self-absorption) in the cold plasma side lobe Surrounding the central emission region, and in the central hot plasma region (emission region). There is no observable difference in the angular distributions of the L-shell X-ray emission among Ag, Pd, and Mo. The conversion efficiency of Ag/Pd/Mo L-shell X-ray sources is higher than that of the Ti K-shell X-ray sources, but the gain relative to the K-shell emission is not as high as that by using short pulse lasers. The conversion efficiency of the L-shell X-ray sources decrease, with increasing atomic numbers (or X-ray photon energy), similar to the behavior of the K-shell X-ray Source.

Long lifetime plasma channel in air generated by multiple femtosecond laser pulses and an external electrical field

The lifetime of a plasma channel produced by self-guiding intense femtosecond laser pulses in air is largely prolonged by adding a high voltage electrical field in the plasma and by introducing a series of femtosecond laser pulses. An optimal lifetime value is realized through adjusting the delay among these laser pulses. The lifetime of a plasma channel is greatly enhanced to 350 ns by using four sequential intense 100fs( FWHM) laser pulses with an external electrical field of about 350kV/m, which proves the feasibility of prolonging the lifetime of plasma by adding an external electrical field and employing multiple laser pulses. (c) 2006 Optical Society of America.