In transient collisional excitation scheme, a long (nanosecond) prepulse is used to perform and ionize plasmas. After a delay time, a short (sub-or picosecond) intense laser pulse is used to rapidly heat the plasma. This results in transient x-ray lasers with high gain. Effects of delay time on transient collisional excitation nickel-like x-ray lasers are investigated analytically using a simple model. The calculations show that the longer delay time can greatly relax the density gradient. This is very critical for the propagation of x-ray lasers. However, a too long delay will reduce the electron temperature of the plasma before the arrival of the short pulse. Increasing the intensity of the long pulse or extending the pulse duration can keep the temperature required to maintain a high percentage of Ni-like ions while the delay time is longer. Similarly, increasing the intensity of the short pulse or extending the duration can also raise the electron temperature, resulting in higher gain coefficient. Our results indicate that extending the pulse duration is more efficient than that of increasing the intensity.

A femtosecond laser driven collisional Ne-like Ti x-ray laser at 32.6nm is numerically investigated using a hydrodynamic code coupled with an atomic data package for a 100-mm-thick Ti planar target irradiated by a single or double prepulse followed by an intense femtosecond laser pulse. By using an optimized drive pulse configuration, a gain of 40cm-1 can be generated from a 5mm×50μm line focus using only about 1 J pump energy.

With ultrashort laser pulses (25 fs) and low energy (E=15mJ), we observe laser plasma channels with a length over 5m in air. The diameter of the plasma channel is measured to be 120mm. The average electron density in the channel is inferred to be about 10 18W/cm3 with an interferometric method. At the same time, the temporal evolution of the electron density in the plasma channel is investigated. The resistivity in the plasma channel is measured to be less than 1V cm. It is suggested that the lifetime of the plasma channel can be greatly prolonged by launching another long laser pulse along the plasma channel.

A simple similarity model is presented to study the hydrodynamics of transient collisional excitation nickellike x-ray lasers. Scaling laws for the temperature, scale length, and electron density are obtained by analytic derivation. The hydrodynamic characteristics of transient Ni-like Pd x-ray lasers are investigated using this model. The calculations agree well with Dunn's experimental results.

用光路追踪程序对具有一定尺度的光源通过变栅距凹面光栅后的成像情况进行了研究。证明了对于非点光源，在特定的实验安排下，其焦面仍可以在一个平面内，同时给出了对于不同入射距离的非点光源形成平焦面的波长范围。结果还表明，当光源尺度与入射距离之比小于1:1000时，光源尺度的变化对焦面位置没有实质性的影响。