飞秒激光驱动X射线源中超热电子与靶相互作用研究

飞秒激光与靶相互作用产生超热电子,随后超热电子与靶原子碰撞,通过kα、kβ等散射过程,可辐射高亮度、飞秒级X射线,在原子与分子物理、生物及医学等领域均有广泛的应用前景.论文首先对飞秒激光驱动X射线源的发展进行简要叙述,然后对X射线源中的超热电子与靶相互作用进行研究.超热电子的产生由靶材对光脉冲的非碰撞吸收机制决定,X射线的产生由超热电子决定.研究超热电子、靶参数对X射线产额的影响,确定最佳参数值,可指导驱动激光脉冲参数的选择,以获得更大的X射线光子产额.使用蒙特卡洛模拟方法可研究超热电子动能及入射角、靶材(Cu靶)厚度对靶材上、下表面X射线辐射光子产额的影响,分析确定最佳超热电子动能及最佳靶厚.驱动激光强度与超热电子动能的定标关系表明：需要合理选择驱动激光参数,使真空加热机制主导超热电子产生过程,以在合适的激光脉冲强度下获得最大X射线光子产额.     

超热电子的产生与输运背向辐射的实验研究

报道了在100TW fs激光器上采用电子磁谱仪和光学CCD积分成像相机分别对激光-固体靶相互作用在靶背方向产生的超热电子能谱及其光学渡越辐射进行的测量.能谱测量结果显示:超热电子能谱呈单温类-麦克斯韦分布,拟合的温度为107 keV;光学渡越辐射(OTR)测量结果显示:OTR是由于超热电子输运穿越固体靶所致,而辐射区域呈圆盘状、有发散角、有光强分布;如果考虑超热电子的产生和加热机制,则占主导地位的加热机制是共振吸收对电子的加热.     

超快脉冲激光辐照金属薄膜热-力效应的模拟研究

基于双曲双温两步热传导和热电子崩力模型,考虑到超快脉冲激光辐照金属薄膜材料过程中的热-力耦合效应,得到了完全耦合的、非线性的超快热弹性模型.运用具有人工粘性和自适应步长的有限差分算法,以脉宽为100 fs的脉冲激光辐照200 nm厚金膜为例,对薄膜体内的电子-晶格温度及温度梯度、热应力和电子热流进行了数值模拟研究.结果表明：脉冲辐照早期为明显的非平衡加热阶段,同时形成较大的热电子崩力;电子热流出现双峰现象;超快加热引起的热应力是导致薄膜力学损伤的主要原因.     

超热电子的产生与输运背向辐射的实验研究

报道了在100TW fs激光器上采用电子磁谱仪和光学CCD积分成像相机分别对激光-固体靶相互作用在靶背方向产生的超热电子能谱及其光学渡越辐射进行的测量。能谱测量结果显示：超热电子能谱呈单温类-麦克斯韦分布,拟合的温度为107keV;光学渡越辐射(OTR)测量结果显示：OTR是由于超热电子输运穿越固体靶所致,而辐射区域呈圆盘状、有发散角、有光强分布;如果考虑超热电子的产生和加热机制,则占主导地位的加热机制是共振吸收对电子的加热。     

超短脉冲激光与乙醇微滴相互作用中超热电子的双叶状角分布

测量了聚焦光强为1016W/cm2的超短脉冲激光与乙醇微滴相互作用中产生的能量大于50?keV的超热电子的角分布和电子能谱.观察到的超热电子角分布明显依赖于激光的偏振特性，在与激光偏振平面平行的平面上超热电子相对于激光入射方向呈对称的双叶状分布.超热电子的能谱显示超热电子的最大能量大于750?keV.以上超热电子的角分布可用一个基于共振吸收机制的模型加以解释. 关键词： 超短脉冲激光 超热电子 微滴 共振吸收     

“神光Ⅱ”首轮基频光驱动内爆实验超热电子诊断

在“神光Ⅱ”装置上进行了激光直接驱动爆推型和烧蚀型DT气体的玻璃微球靶内爆实验.采用多道滤波荧光谱仪(FFS)测量15—250keV硬x射线谱,由高能x射线谱通量和斜率推算出这两种内爆靶产生的超热电子份额ηhe和超热电子温度Th分别为ηhe=25%—30%,Th=30—40keV和ηhe=05%—4%,Th=10—20keV.并给出了不同内爆靶型在不同激光能量EL和不同调焦方式下超热电子产生的特征,由爆推靶产生超热电子份额与实验测量靶的能量吸收效率ηa=29%—34%比对,证明爆推靶吸收的激光能量是以超热电子能量沉积为主,同时实验观测中子产额Yn随超热电子能量Ehe的增大而增大,从而证明了爆推靶是依靠超热电子加热玻璃球壳实现内爆的 关键词： 1.053μm激光 直接驱动 超热电子 爆推靶和烧蚀靶     

超强飞秒激光与固体靶产生的超热电子加热机制

 在SILEX-1激光器上测量了超强飞秒激光与Ta靶相互作用产生的出射超热电子能谱及角分布，研究了出射超热电子加热机制。激光脉宽为 30 fs，激光功率密度为8.5×10¹⁸ W/cm²。靶前法线方向超热电子温度为550 keV。从实验结果可知：共振吸收是靶前法线方向超热电子主要加热机制，这与靶前存在大密度标长预等离子体的实验条件吻合。靶厚为6～50 μm时，靶后超热电子沿法线方向出射；靶厚为2 mm时，该发射峰消失。     

飞秒激光脉冲和液体等离子体相互作用产生的高能电子特性研究

文章研究了单脉冲和多脉冲飞秒激光和水等离子体相互作用产生的高能电子特性 ,发现多脉冲激光构型可以大幅度地增强超热电子的产生和提高其温度 .实验观测到在激光偏振面内 ,沿与激光轴反向夹角 4 6°的方向 ,对称地喷射出两束能量大于 2 5keV的高能电子 .二维粒子模拟结果和实验符合很好 .实验和理论都表明 ,这些超热电子是通过后续脉冲与前面脉冲形成的球形液滴相互作用产生的 ,具体机制为共振吸收 .     

激光入射角对靶面方向超热电子发射的影响

实验研究了超短超强激光脉冲与薄膜靶相互作用中产生的超热电子角分布随激光入射角的变化.在靶面方向观测到一束方向性很好的高能超热电子.该高能超热电子束的电子数目随着激光入射角的增大而增大.对结果的分析表明，表面准静态磁场是导致表面电子产生的主要原因. 关键词： 超热电子 表面准静态磁场 超强激光脉冲与等离子体相互作用     

飞秒激光-固体靶相互作用中超热电子输运的实验研究

报道了在100TW超短脉冲掺钛宝石激光装置上,完成的飞秒激光-固体靶相互作用中超热电子在靶内输运的实验研究结果.获得了超热电子的产额、注量和总能量.结果表明,超热电子的注量和总能量随靶厚的增加而减少,超热电子约80%的能量主要沉积在靶内的前10(m,对以上形成的原因进行了分析指出,是由于静电场对超热电子输运影响所致.     

Study of x-ray emission enhancement via a high-contrast femtosecond laser interacting with a solid foil

We observed the increase of the conversion efficiency from laser energy to Kalpha x-ray energy (eta(K)) produced by a 60 fs frequency doubled high-contrast laser pulse focused on a Cu foil, compared to the case of the fundamental laser pulse. eta(K) shows a strong dependence on the nonlinearly modified rising edge of the laser pulse. It reaches a maximum for a 100 fs negatively modified pulse. The hot electron efficient heating leads to the enhancement of eta(K). This demonstrates that high-contrast lasers are an effective tool for optimizing eta(K), via increasing the hot electrons by vacuum heating.     

Femtosecond and continuous-wave laser performance of a diode-pumped Yb3+:CaYAlO4 laser

The cw and femtosecond laser operations of Yb(3+):CaYAlO(4) (Yb:CYA) are demonstrated. The laser emitted a maximum cw power of 1.94 W with a slope efficiency (η(slope)) of 71% and an optical-to-optical efficiency (η(opt)) of 51%. Under mode-locking operation, the laser emitted near transform-limited pulses with 156 fs pulse width, 8.1 nJ pulse energy and 0.74 W average power. The η(slope) and η(opt) of the mode-locked laser were 37% and 20%, respectively.     

Effects of atomic number Z on the energy distribution of hot electrons generated by femtosecond laser interaction with metallic targets

The effects of atomic number Z on the energy distribution of hot electrons generated by the interaction of 60fs, 130mJ, 800nm, and 7×10^17W/cm^2 laser pulses with metallic targets have been studied experimentally. The results show that the number and the effective temperature of hot electrons increase with the atomic number Z of metallic targets, and the temperature of hot electrons are in the range of 190-230keV, which is consistent with a scaling law of hot electrons temperature.     

Short-pulse laser ablation of solids: from phase explosion to fragmentation

The mechanisms of laser ablation in silicon are investigated close to the threshold energy for pulse durations of 500 fs and 50 ps. This is achieved using a unique model coupling carrier and atom dynamics within a unified Monte Carlo and molecular-dynamics scheme. Under femtosecond laser irradiation, isochoric heating and rapid adiabatic expansion of the material provide a natural pathway to phase explosion. This is not observed under slower, nonadiabatic cooling with picosecond pulses where fragmentation of the hot metallic fluid is the only relevant ablation mechanism.     

Coherent quantum control of excitons at ultracold and high density in Cu2O with phase manipulated pulses

By phase manipulation of a short laser pulse, it is possible to selectively generate ultracold excitons in a two-photon process while quenching the multiphoton excitation of hot electrons and holes. We show how this technique allows us to suppress the heating of n=1 orthoexcitons in Cu(2)O at high density. Using a laser pulse having an energy of a few microJ and duration of 100 fs, we are thus able to produce a cold exciton gas up to a density of 10(15) cm(-3).     

Electron energy spectrum in the field-ionized plasma

In this article, the effect of ionization on the energy spectrum of electrons within the interaction of a laser pulse with hydrogen atoms is investigated using particle-in-cell simulation codes. The results show that the behaviour of electrons' energy distribution function in the field-ionized plasma, which occurred due to the field ionization, compared with that in the pre-plasma strongly depends on the pulse shape. For short rise-time pulses (here 30 fs), due to the rapid enhancement of laser electric field, ionization occurs quickly, and as a result, there is not much difference in the electron energy in both the media. However, for pulses with rise time of 40 fs, in the pre-plasma state, the electron population reaches higher energies compared with the field-ionized plasma state. The main reason for this difference is the nonlinear wave breaking that happens earlier due to density inhomogeneity in the field-ionized plasma. On the other hand, at longer rise-time pulses (here 60 and 70 fs), electrons achieve higher energies in the field-ionized plasma than those in the case of pre-plasma. In this case, because of density fluctuations in the field-ionized plasma, the Raman backscattered radiations are seeded by a strong initial noise at the earlier times and the Mendonca condition for chaos threshold is met sooner. Therefore, the electrons gain more energy through the stochastic mechanism that is in agreement with chaotic nature of the motion.     

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.     

Enhanced betatron radiation by a modulating laser pulse in laser wakefield acceleration

We propose a new idea to enhance and control the betatron radiation by using a modulating laser pulse in laser wakefield acceleration. In this scheme, a high-power laser pulse is used for self-trapping and acceleration of the plasma electrons and the accelerated electron beam is modulated by a separately-propagating laser pulse for large amplitude betatron oscillations and microbunching. In this way, the relatively low power modulating laser pulse can enhance the X-ray photon flux and energy significantly. We performed two-dimensional particle-in-cell simulations to demonstrate the idea and the results show that a sub-TW laser pulse is enough for electron beam modulation and it can generate easily-controllable fs X-ray pulses with a wide range of photon energies from soft X-rays to hard X-rays.