The filamentation instability is one of the basic beam-plasma instabilities that play a significant role in the energy deposition mechanism of the relativistic electrons generated by the laser-plasma interaction in the fast ignition scenario. In this paper, the effect of the density gradient into plasma on the filamentation instability was investigated in the Weibel unstable plasma, where the plasma temperature anisotropy can play an important role. Results indicated that the density gradient enhances the instability growth rate so that decreasing the density gradient from the critical surface to the core of fuel leads to instability for longer regions in k space. Also, investigations in the region close to the critical surface showed that for decreasing the beam number density n_b ≤ 0.01n₀, the instability occurs for while this can be different for higher values. Increasing the beam relativistic factor causes a decreasing peak of instability growth rate because of a reduction in beam current, whereas the initial thermal spread of plasma amplifies the filamentation instability.

中文翻译：

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等离子体密度梯度对电流成丝不稳定性的影响

丝状不稳定性是基本的光束-等离子体不稳定性之一，在快速点火场景中由激光-等离子体相互作用产生的相对论电子的能量沉积机制中起着重要作用。在本文中，研究了进入等离子体的密度梯度对 Weibel 不稳定等离子体中成丝不稳定性的影响，其中等离子体温度各向异性可以发挥重要作用。结果表明，密度梯度提高了不稳定性的增长速率，从而降低了从临界表面到燃料核心的密度梯度，导致k空间中较长区域的不稳定性。此外，在靠近临界表面的区域的研究表明，为了降低束数密度n _b ≤ 0.01 n _{0 时}，会出现不稳定，而对于更高的值，这可能会有所不同。由于束流的减少，增加束流相对论因子会导致不稳定性增长速率的峰值降低，而等离子体的初始热扩散会放大丝状结构的不稳定性。