ИСТИНА

Experimental & numerical studies of interaction of femtosecond laser radiation with intensity up to 5х1018 W/cm2 with dense plasma, conducted recently using terawatt femtosecond laser facility at MSU, are presented. Main stress was on the control of plasma parameters (luminosity in X-ray and gamma ranges, generation of bunches of relativistic electrons and fast multicharged ions) and their optimization by choosing interaction regime and preplasma parameters. The latter is unavoidable due to action of prepulses with different intensity and duration always pertaining in the temporal structure of a powerful femtosecond pulse. That is why we are paying a lot attention to the contrast of the laser pulse at nano- and picosecond scales. 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. A substantial increase both in gamma yield and “temperature” were obtained by the proper adjustment of the time delay between the two pulses (0-5 ns), while gamma yield dropped to almost zero values if the nanosecond pulse came 10-20 ns in advance of the femtosecond one. Comprehensive optical diagnostics (shadowgraphy, interferometry, angular resolved self-emission measurements) data allowed us to estimate the electron density profile. The latter profile was used for making numerical Particle-in-cell simulations which describe the gamma yield enhancement well. We also illustrate how the observed drop in gamma yield within a certain range of delays was due to ionization defocusing of the femtosecond beam in an expanding longscale (L/>1) preplasma. For clarification of the electron acceleration mechanisms numerical simulations were done using fully relativistic 3D3V PIC code for laser-plasma interaction and electron acceleration. Parameters of nanosecond and femtosecond laser pulses coincided with the experimental ones. We also considered photo induced near threshold nuclear reactions induced by corpuscular emission of laser produced plasma as a diagnostic tool for high energy particles and photons. While for a single particle detection plenty of methods were developed in nuclear physics, laser produced plasma demands new approaches as it emits huge amount of particle in a time much less than the temporal response of standard nuclear techniques. “Low” threshold gamma-D and gamma-Be photonuclear reactions were studied experimentally. We also consider gamma-gamma reactions (long-lived isomer production) for plasma diagnostics. We present numerical simulations using GEANT 4.0 package supporting our estimates and experimental data. It is worth mention that our research is relevant to laser-plasma interaction at higher intensities, since the pulse contrast is naturally limited by ASE, prepulses, or parametric luminescence, etc. Our findings may pave the way to further optimization of high energy particle sources based on extremely intense laser plasma interaction.