Application of multi-pulse optical imaging to measure evolution of laser-produced counter-streaming flows

A counter-streaming flow system is a test-bed to investigate the astrophysical collisionless shock (CS) formation in the laboratory. Electrostatic/electromagnetic instabilities, competitively growing in the system and exciting the CS formation, are sensitive to the flows parameters. One of the most important parameters is the velocity, determining what kind of instability contributes to the shock formation. Here we successfully measure the evolution of the counter-streaming flows within one shot using a multi-pulses imaging diagnostic technique. With the technique, the average velocity of the high-density-part (n_e≥ 8-9×10¹⁹ cm^-3) of the flow is directly measured to be of ～ 10⁶ cm/s between 7 ns and 17 ns. Meanwhile, the average velocity of the low-density-part (n_e ≤ 2×10¹⁹ cm^-3) can be estimated as ～ 10⁷ cm/s. The experimental results show that a collisionless shock is formed during the low-density-part of the flow interacting with each other.