ИСТИНА

We present our recent experimental and numerical data on the interaction of high contrast relativistically intense laser pulse with a preplasma formed by an additional nanosecond laser pulse. The preplasma extent was controlled by changing time delay between the pulses and energy density of the nanosecond pulse. Experimental techniques included hard x-ray and gamma-ray measurements, direct detection of accelerated electrons, as well as angularly resolved second and three halves harmonic spectra measurements. We also used plasma shadowgraphy and interferometry to access plasma parameters before and at the instant of the femtosecond pulse action. We studied two specific set of parameters then electron heating is very efficient and gamma quanta as high as 7-10 MeV appeared at intensity of 2000 PW/cm2. Prominent role of the plasma field ionization by femtosecond pulse was revealed both numerically and experimentally. At certain conditions the femtosecond beam experienced severe ionization defocusing, that causes drop in intensity by at least order of magnitude. We also studied interaction of the chirped laser pulse (duration of a few picoseconds at the same pulse energy) with the same preplasma. We observed extremely bright hard x-ray and gamma-emission at the proper experimental conditions.