In this paper, the behavior of temperature, density, velocity and pressure of the plasma shock front produced by the interaction of a focused Nd: YAG laser beam on a copper target, with an average pulsewidth of 30ns and 140mJ laser energy, have been studied using shadowgraphy technique. For this purpose, the plasma was examined in air at the pressure from 760 to 2x 10-4 Torr by a laser probe beam of 10ns pulsewidth at the wavelength of 532 nm. The results obtained from the measurements and by using the hydrodynamical model indicated that at atmospheric pressure the temperature and plasma pressure near the target surface increase up to 30 eV and 7x104 atm, respectively. The relative plasma density during the interaction time was found to be about 6 times of the air density at the normal atmospheric pressure. By decreasing the pressure from 760 Torr to 50 Torr the shock wave gradually converts to a rarefaction wave. For the pressure less than 0.1 Torr, only a dense region with an electron density higher than »3.94x1021 cm-3 near the target surface was observed.

In this paper, stimulated Raman scattering (SRS) and electron heating in laser plasma propagating along the plasma fusion is investigated by particle-in cell simulation. Applying an external magnetic field to plasma, production of whistler waves and electron heating associated with whistler waves in the direction perpendicular to external magnetic field was observed in this simulation. The plasma waves with low phase velocities, generated in backward-SRS and dominateing initially in time and space, accelerated the backward electrons by trapping them. Then these electrons promoted to higher energies by the forward-SRS plasma waves with high phase velocities. This two-stage electron acceleration is more efficient due to the coexistence of these two instabilities.

Direct laser driven inertial confinement fusion showed numerous anomalies in the laser interacjion process including parametric instabilities, self-focusing (filamentation) and the later confirmed stochastic pulsation in the 10 ps range. Smoothing of the laser beam resulted in a dramatic reduction of parametric processes and filamentation, and a strong increase of direct driven laser-fusion gains without that a convincing explanation was given before. An explanation for the effects of by laser beam smoothing is based here on some calculations for the suppression of the stochastic pulsation intermixed with mechanism of instabilities by frequency modulated laser pulses. The stochastic pulsation of the interaction in the 10 ps range could be analyzed numerically as due to changes between mirror reflection and phase reflection caused by self generated von - laue diffraction by nonlinear force produced density rippling in the plasma and its thermo-kinetic decay. Solving this problem by broad band laser beam smoothing was shown numerically and is applied to the measured suppression of pulsation by random phase plates. One of the key experiments hidden in the broad steam of publications is the question mark experiment of Pant et al. As a tool for testing the degree of elimination of the anomalies. This may permit direct drive low cost laser fusion using three times more energetic laser beams without expensive harmonics generation and its problems in the next facilities of NIF scale.

Proton acceleration process by the generated sheath in the rear-side of an Al target in the interaction of P polarized laser with the pulses duration of ≥ 40 fs and intensity of 3. 6×1019 W cm-2 has been experimentally and numerically studied. The results show that by employing the positively chirped pulse, the proton cut off energy and the number of the accelerated protons are increaesd in comparison with an unchirped and negatively chirped pulse interactions. After that, the electron heating process in interaction of positivly and negativly chirped pulse is investigated and correlation between the chirped parameter and produced electrostatic field in the backside of the target is studied. Particle-in-cell simulation results show that the electron heating process and consequently the electrostatic field for proton acceleration goal are significantly increased by employing the positively chirped pulse campared to negativly unchirped conditions.

The electron-neutral collisions in the plasma become crucial with regard to the generation of THz radiation when thermal motion of the electrons is considerable. If we look at the mechanism of THz emission, this is only the movement/oscillations of the electrons which is responsible for the excitation of nonlinear current that generated the THz radiation. The present work aims to disclose the role of thermal motion of the plasma electrons to the resonance condition and the THz emission when two co-propagating super-Gaussian laser beams beat in the plasma. The dynamics of the plasma electrons and subsequent generation of nonlinear current are discussed in greater detail for the emission of THz radiation.

Nonlinear excitation of Wakefield in plasma by a short laser pulse is considered. A new model on the interaction of laser pulse and plasma particles is presented. Based on this model the electric and magnetic components of emitted radiation generated by the second order nonlinear current in plasma are calculated. Poynting vector of radiation is obtained and radiation pattern is studied for different variables. Results are in good agreement with experimental data.

A new kind of high power tunable radiation source, where the short electromagnetic pulse is generated by a perpendicularly magnetized plasma wake is studied. A gas jet flow is used to generate the sharp boundary plasma. Wakefield is excited by a mode locked Ti:sapphirc laser beam operating at 800 nm wavelength with the pulse width of 100 fs (FWHM) and maximum energy of 100 mJ per pulse with 10 Hz repetition rate. Different nozzles are used in order to generate different densities and gas profile. The neutral density of gas jet flow is measured with a Mach-Zehnder interferometer. Strength of the applied external dc magnetic field varies from 0 to 8 kG in the interaction region normal to the direction of laser pulse propagation. The frequency of the emitted radiation with the pulse width of 200 ps (detection limitation) is in the millimeter wave range. Radiation is polarized perpendicularly to the laser pulse propagation direction and dc magnetic field lines. Radiation propagates mainly in the forward direction. Intensity of the radiation in different plasma densities and different magnetic field strengths has been observed.

Short and intense laser pulses in laser-plasma interaction stimulate various local structures like solitary waves in the plasma. Relative salitons should be given special attention because the amplitude of the electromagnetic field is intense enough to set plasma electrons in relativistic motions. In the interaction of intense laser with plasma, collisions can play an important role in the physical phenomena. In this paper, the effect of the collision on the emission of solitons is investigated by considering the interaction of an intense laser with plasma. Then, the NLS equation is numerically solved and the different results are compared with each other. Also, the stability conditions of individual waves and the effect of the collision on these waves are investigated.

In this paper, the effect of plasma density ramp and laser pulse length on the energetic particle generation and pulse scattering were investigated in an under-dense plasma for short and long pulse lengths, of τ L = 60 fs, and τ L = 300 fs, respectively. In our simulations, we used a kinetic particle in cell simulation 1D-3V code. It is found that the laser pulse length and density ramps play an important role on the energetic particles generation and pulse scattering in plasma. So that, the simulations of the laser pulse length impact indicate that, in the case of short pulse interaction with the under-dense plasma, electrons are accelerated to the higher energy level. Furthermore, among the three different ramps: steplike, ramp 1 with a steep slope, and ramp 2 with a gentle slope, in the case of the step-like density ramp, electrons accelerate to high energies, in comparison with two other ramps. A fourier analysis of the total radiation spectrum indicate that in the case of the step-like density profile, the growth rate of the electromagnetic modes have maximum regular picks and minimum growth rate obtained in the case of the smooth ramp. Meanwhile, the time evaluation analysis of the energy distribution function shows that with the time increment of the pulse propagation, in the shorter pulse case, and for the step density scale length, plasma particles can reach a high energy level.