等离子体密度对激光拉曼放大机理的影响

等离子体中的背向拉曼散射机理可以用来产生超短超强的激光脉冲. 本文采用粒子模拟方法模拟研究了等离子体密度对激光拉曼放大过程的影响. 研究发现, 过低的等离子体密度会导致等离子体波提前波破而降低能量转换效率; 而过高的等离子体密度又会导致其他不稳定性的快速增长, 限制作用距离和输出能量. 因此, 拉曼放大机理的最佳等离子体密度应处于等离子体波破的密度阈值附近, 可以获得最高的能量转换效率和能量输出. 另外, 空间频谱分析显示放大激光的强度饱和主要来自于自相位调制不稳定性的发展. 利用10¹³ W·cm^-2的抽运激光脉冲, 模拟证实拉曼放大机理可有效地将种子激光的强度从10¹³ W·cm^-2 放大到10¹⁷ W·cm^-2, 脉宽压缩到40 fs, 且能量转换效率达到58%.

Plasma density effect on backward Raman laser amplification

Backward Raman amplification (BRA) in plasma can be used for generating ultra-powerful laser pulses. In this paper, the plasma density effect on backward Raman laser amplification is studied by using particle-in-cell method. It is found that using a low plasma density can lead to the premature Langmuir wave breaking and thus result in a small energy-transfer efficiency. On the other hand, using a high plasma density will enhance the developments of unwanted instabilities, which rapidly disturb the Raman amplification, thus limiting the interaction length and output power. Therefore, an optimal plasma density for BRA is near the threshold of Langmuir wave breaking in order to achieve both high efficiency and large energy flux. The space frequency spectrum analysis shows that the saturated intensity of amplified pulses is limited mainly by the self-phase modulation instability. By using a 10¹³ W·cm^-2 pump pulse, our simulation results show that the initial 10¹³ W·cm^-2 seed pulse can be well be well amplified into a pulse with an energy power of 10¹⁷ W·cm^-2, a duration of 40 fs, and and an energy conversion efficiency of up to 58%.