Yingjun Li

Optical diagnostics of femtosecond laser plasmas

Optical diagnostics of evolution of plasmas produced by ultrashort laser pulses is carried out using a femtosecond probing beam. The time sequence of plasma shadowgrams and interferograms are obtained. The filamentation instability in high-density region induces the local density modification. Large-scale toroidal magnetic fields confine plasma expansion in the transverse direction, resulting in the formation of a plasma jet. The plasma expansion along the target normal direction is found to scale as t1/2.

Recent Progress of Research on X‐ray Lasers at the Institute of Physics, CAS

The transient nature of x‐ray laser is studied. For a drive pulse even as short as 500fs, the x‐ray laser gain still shows a quasi‐steady state behavior. A real transient nature can only be seen when the x‐ray laser is driven by a laser pulse with a duration as short as tens of femtoseconds. A femtosecond laser pulse driven collisional Ne‐like Ti x‐ray laser at 32.6 nm is numerically investigated. By using the optimized drive pulse configuration, a gain of 40 cm−1 can be generated from a 5 mm × 50 μm line focus using about 1 J pump energy. Effects of delay time on transient collisional excitation nickle‐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. 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 short pulse duration is more efficient than that o...

Formation and evolution of a pair of collisionless shocks in counter-streaming flows

A pair of collisionless shocks that propagate in the opposite directions are firstly observed in the interactions of laser-produced counter-streaming flows. The flows are generated by irradiating a pair of opposing copper foils with eight laser beams at the Shenguang-II (SG-II) laser facility. The experimental results indicate that the excited shocks are collisionless and electrostatic, in good agreement with the theoretical model of electrostatic shock. The particle-in-cell (PIC) simulations verify that a strong electrostatic field growing from the interaction region contributes to the shocks formation. The evolution is driven by the thermal pressure gradient between the upstream and the downstream. Theoretical analysis indicates that the strength of the shocks is enhanced with the decreasing density ratio during both flows interpenetration. The positive feedback can offset the shock decay process. This is probable the main reason why the electrostatic shocks can keep stable for a longer time in our experiment.

Forward dynamics of water-plasmas produced by femtosecond laser pulses

The evolution of the forward density front of water-plasmas generated by ultrashort laser pulses on an air–water interface is investigated with optical diagnostics. The density front is found to move with supersonic speed at the beginning, as predicted by a hydrodynamics simulation. This plasma-expansion process is followed by a fluid mechanical process. The expansion rate gradually reduces to the acoustic speed in water.

Transient characteristics of a neon-like x-ray laser at 19.6 nm

A transient Ne-like x-ray laser at 19.6 nm (J=0→1, 3p→3s) is numerically investigated using a hydrodynamic code for a 100 μm thick Ge planar target irradiated by a nanosecond prepulse followed by a subpicosecond or a picosecond main optical laser pulse. Simulations are performed for different main pulse lengths of 50 fs, 500 fs, 5 ps, and 50 ps but with fixed pulse energy. For a drive pulse even as short as 500 fs, the x-ray laser gain still shows quasisteady state behaviors. A real transient nature can only be seen when the x-ray laser is driven by a laser pulse with a duration as short as tens of femtoseconds.

Nickel-Like Molybdenum and Niobium Soft X-Ray Lasing Driven by 200 ps Laser Pulses with 50 J of Energy

Intense lasing at 18.9 and 20.3 nm in nickel-like molybdenum and niobium ions has been observed by using two 200 ps laser pulses with total energy of 50 J at 1.053 μm from XingGuang II laser facility. Their gain coefficients were measured to be 1.6 and 1.5 cm-1, respectively. This shows the possibility of extending Ni-like x-ray lasing in low-Z elements and paves the way towards small scale x-ray lasers for applications at university laboratories.