Numerical modeling of a high-gain x-ray laser in an avalanche recombination high density plasma

复合型x射线激光器由于具有发展紧凑型x射线激光器的潜力而得到广泛的研究。然而，大多数研究都是在准稳态近似下进行的。在这些等离子体中，粒子数反转不能在高于一定值的电子密度下产生。因此，激光振荡的增益不能增加超过一个有限的值。我们的项目的目的是通过数值计算证明，在相当瞬态（雪崩）复合的高密度等离子体中，准稳态近似是无效的，在增益性能的显着改善成为可能的复合x射线激光。主要研究成果如下：编制了适用于低z离子雪崩复合等离子体的原子物理程序。在完全电离的碳高密度等离子体的复合阶段，求解了含时的碳离子速率方程。初始电子温度设定为10 eV，相当于氢等离子体的0.28 eV。当电子密度大于10 μ cm-3时，C-V_1 18.2nm的增益超过5000cm-3，并随电子密度的增加而增加，而粒子数反转持续时间从1ps减小到10 fs.<19><-3><-1>这表明由于雪崩复合，瞬时高增益X射线激光是可行的。速率方程中还考虑了完全电离的碳离子与氦原子之间的电荷交换过程。计算表明，CV1 18.2 nm线的增益高达3500 cm-2，而没有电荷交换过程的相应增益下降到约200 cm-2。<-1><-1>这清楚地表明，电荷交换过程在产生雪崩复合x射线激光中也起着非常有用的作用。

The recombination x-ray laser has been extensively studied because this scheme has potentiality to develop compact x-ray laser. However, most investigations have been made in plasmas under the quasi-steady state approximation. In these plasmas, the population inversion cannot be generated for higher electron density than a certain value. Consequently, the gain for laser oscillation cannot be increased over a limited value. The objective of our project by numerical calculation is to demonstrate that significant improvement in gain performance becomes feasible of the recombination x-ray laser in quite transient (avalanche) recombination of high density plasma in which the quasi-steady state approximation is invalid. An outline of our research results is as follows :An atomic physics code is developed for the avalanche recombination plasma with low-z ions. Time-dependent rate equations for carbon ions were solved in the recombination phase of the fully ionized carbon high density plasma. The initial electron temperature was set to be 10 eV, equivalent to 0.28 eV for the hydrogen plasma. For the electron density higher than 10^<19>cm^<-3> the gain for the C Vl 18.2 nm becomes over 5000cm^<-1>, and increases with the electron density while the duration of population inversion decreases from 1 ps to 10 fs. This shows transient high-gain x-ray laser is feasible due to the avalanche recombination. Charge exchange process between fully ionized carbon ions and helium atoms was also included in the rate equations. The calculation shows that the gain for the CVl 18.2 nm line is as much as 3500cm^<-1> while the corresponding gain without the charge exchange process decreases down to about 200cm^<-1>. This clearly indicates that the charge exchange process plays also a very useful role in producing the avalanche recombination x-ray laser.