| 题名 | 超强激光与等离子体相互作用中非线性过程的实验研究 |
| 作者 | 汤宇晖 |
| 学位类别 | 博士 |
| 答辩日期 | 2002 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 韩申生 |
| 关键词 | 激光 等离子体 汤姆逊散射 受激拉曼散射 二次谐波辐射 |
| 其他题名 | The Experimental Researches of Nonlinear Processes in the Ultra-short Intense Laser-Plasma Interactions |
| 中文摘要 | 在过去的二十年间,激光技术有了极大的进展,从而使人类能够获得极短脉冲宽度和超高强度的激光.现今,采用了啁啾脉冲放大技术的小型化台式桌面激光系统可以输出飞秒量级和聚焦功率超过10<'18>W/cm<'2>的激光脉冲,其电场强度超过了氢原子的内部场强.在如此强的电场中的电子行为将呈现出强非线性的特征.这一技术使人们进入了激光物质相互作用研究中的新领域,包括激光驱动核聚变,X射线激光,基于等离子体的尾场加速器,高次谐波,自由电子激光及其他许多领域.该文的工作主要涉及了超短超强激光与等离子体相互作用中非线性过程的实验研究. |
| 英文摘要 | Over the past twenty years a revolution has occurred in the laser technique, which allowed people to produce ultrashort and extremely high-intensity pulses. Today a small tabletop laser system with chirped pulse amplification (CPA) can produce fetosencond pulses and achieve focused intensity above 10~(18)W/cm~2. The corresponding electromagnetic intensity is greater than the electric field of H atom. In such strong field the behaviors of the electron will show strongly nonlinear characteristics. This ability opens for study a whole new regime of laser-matter interaction, with applications in laser-driven inertial confinement fusion, X-ray laser, plasma-based wake-field acceleration, high-harmonics generation, the free-electron laser instability (FEL) of the self-amplified spontaneous emission FEL, and many other areas. This dissertation mainly discussed the experimental researches of the nonlinear processes in ultra-short intense laser-plasma interactions. The obtained results are followed: I. We have established the experimental platform based on the 45fs, 2TW laser system. To avoid the laser pulse expansion caused by nonlinear dispersion response while the ultarshort intense pulses propagate in lens, we used reflective system to focus the laser beam. Under vacuum, the focal spot profile was Gaussian with a radius at 1/e~2 in intensity w_0 =10μm. The peak laser intensity reached 5.7 * 10~(17) W/cm~2. A gas jet was used to produce ultrasonic gas blast, which can generate underdense plasma in small region with the laser interaction. With a time-sequence control system, the synchronism among the gas jet, laser pulse and collection system was assured. Using the Thomson scattering diagnostics method, we have studied the electron temperature of the underdense plasma, which produced by the intense ultrashort laser pulses interacting with gas jet. The dependences of plasma temperature on gas density, laser energy and laser pulse length have been researched. Processes that contribute to plasma heating include above threshold ionization (ATI) energy and inverse bremsstrahlung (IB). The experimental results show that for short pulses a low temperature plasma far from collisional equilibrium can be produced, which is of current interest for short-pulse X-ray laser at short wavelengths. And it also proved the contribution from IB can be limited under a small lever for pumping laser pulse lengths less than 100fs. The experimental researches of the nonlinear processes in an ultrashort intense (45fs, 5.7 * 10~(17) W/cm~2) laser pulse interactions with an optically ionized helium gas are presented. 1. The validity of the initial transient theory of SRS has been examined experimentally in 45fs pulse OFI laser-plasma interactions. The stimulated Raman backscattering reflectivity and the phenomena of transition from strongly coupled BSRS to weakly coupled BSRS were studied and agreed with theory. 2. We have observed the phenomena of second harmonic emission (SHE) and its nonlinear structure in an 45fs, 6 * 10~(17) W/cm~2 laser pulse interaction with an optically ionized helium gas. The relationship between the conversion efficiency of SHE and the laser energy were given out, and made good agreement with the theoretical explanation that the origin of SHE is due to the transverse plasma density gradient. The nonlinear structures of SHE, l-3th Stocks satellites, were induced by the stimulated Raman process in laser-plasma interactions. 3. The heating processes of collective effects, including SRS and wakefield wave, have been studied by ion energy spectrum and computer simulation. We measured the relationship between the laser energy and the ion energy, which confirmed the corresponding plasma temperature stayed at a low lever. A one-dimension particle-in-cell was used to simulation the plasma dynamics while a short pulse injected into a gas. The calculation results showed that the electric wave excited by the laser pulses could be neglected while the laser wavelength is comparable with the plasma wavelength. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15382]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 汤宇晖. 超强激光与等离子体相互作用中非线性过程的实验研究[D]. 中国科学院上海光学精密机械研究所. 2002. |
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