We present particle-in-cell (PIC) simulations of laser plasma instabilities (LPIs) with a laser pulse duration of a few picoseconds. The simulation parameters are appropriate to the planar-target LPI experimental conditions on SG-II. In this regime, the plasmas are characterized by a long electron density scale length and a large electron density range. It is found that when the incident laser intensity is well above its backward stimulated Raman scattering (backward SRS, BSRS) threshold, the backscattered light via the primary BSRS is intense enough to excite secondary SRS (Re-SRS) in the region below one-ninth of the critical density of the incident laser. The daughter light wave via the secondary BSRS (Re-BSRS) is amplified as it propagates toward the higher-density region in the bath of broadband light generated through the primary BSRS process. A higher intensity of the incident laser not only increases the amplitude of the BSRS light but also increases the convective amplification lengths of the Re-BSRS modes by broadening the spectrum of the BSRS light. Convective amplification of Re-BSRS causes pump depletion of the primary BSRS light and may lead to an underestimate of the primary BSRS level in SP-LPI experiments. A significant fraction of the generation of energetic electrons is strongly correlated with the Re-BSRS modes and should be considered as a significant energy loss.

中文翻译：

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皮秒激光等离子体相互作用中受激拉曼散射的对流放大

我们介绍了激光脉冲持续时间为几皮秒的激光等离子体不稳定性（LPI）的单元格（PIC）模拟。仿真参数适合于SG-II上的平面目标LPI实验条件。在这种情况下，等离子体的特征在于长的电子密度标度长度和大的电子密度范围。已发现，当入射激光强度远高于其后向激发拉曼散射（后向SRS，BSRS）阈值时，通过主BSRS的后向散射光强度足以激发低于1的区域中的次要SRS（Re-SRS）。入射激光的临界密度的九分之一。经由次BSRS（Re-BSRS）的子光波在通过主BSRS过程生成的宽带光浴中向较高密度区域传播时被放大。入射激光的强度更高，不仅增加了BSRS光的幅度，而且还通过加宽BSRS光的光谱来增加Re-BSRS模式的对流放大长度。Re-BSRS的对流放大导致主要BSRS光的泵浦损耗，并可能导致SP-LPI实验中的主要BSRS水平低估。高能电子产生的很大一部分与Re-BSRS模式密切相关，应被视为重要的能量损失。更高强度的入射激光不仅增加了BSRS光的幅度，而且通过加宽BSRS光的光谱而增加了Re-BSRS模式的对流放大长度。Re-BSRS的对流放大导致主要BSRS光的泵浦损耗，并可能导致SP-LPI实验中的主要BSRS水平低估。高能电子产生的很大一部分与Re-BSRS模式密切相关，应被视为重要的能量损失。入射激光的强度更高，不仅增加了BSRS光的幅度，而且还通过加宽BSRS光的光谱来增加Re-BSRS模式的对流放大长度。Re-BSRS的对流放大导致主要BSRS光的泵浦损耗，并可能导致SP-LPI实验中的主要BSRS水平低估。高能电子产生的很大一部分与Re-BSRS模式密切相关，应被视为重要的能量损失。