فهرست مطالب سید ابوالفضل قاسمی

This paper applied two intense relativistic beams to low-density plasma under different conditions. The effect of the distance and the angle between these two beams on the acceleration of electrons were investigated. The simulations showed that when the distance between two beams is zero, and there is no relative angle between them, after some adequate amount of time, it seems that a powerful beam travels through the entire plasma, increasing the acceleration length of the electrons and their kinetic energy. The best overlap occurs between the pulses at and , naturally, the hot electrons will have the highest temperature and the highest cut-off energy. On the other hand, as the pulse distance increases, the useful overlap decreases (the total amplitude of the laser fields decreases). Consequently, the temperature of the hot electrons will reduce. The results showed that the conditions  and  the rate of pulse erosion, and the increase in the kinetic energy of the electrons are slightly greater than the non-zero angles, which can be due to the excitation of the stronger plasma waves.

In ultra-high power laser interaction with plasma, quantum electrodynamics phenomena such as high energy photon emission by electrons, radiation reaction trapping, and anti-particle creation can affect the interaction mechanism. In the present work, the interaction mechanism of the circular and linear laser with an intensity higher than 1023 W/cm2 with rarified plasma in the presence of the radiation reaction force has been investigated using particle in cell simulation. The results indicate that radiation reaction trapping for circular polarization is more effective than the linear one. Also, photons emitted by electrons have a higher density for circular polarization. For both polarizations, at later times of the interaction, considering photon emission and radiation reaction effects lead to the significant decrement of the cut-off energy of electrons. The cut-off energy of the emitted photons for circular polarization is higher than that of linear polarization.

In the present work, the effect of the external magnetic field of = ( T 0 -1000 Bext ) on the energy deposition of the relativistic electrons into high density fuel of 3 ρc ( ) 200 1000 g.cm   in fast-shock ignition concept has been investigated by using the Geant4 simulation toolkit. In our simulation model, two types of the energy distribution function including exponential energy distribution and quasi-two temperature energy distribution function, for relativistic electrons with an initial kinetic energy of 0 E   ( ) 1.5 6.5 MeV were considered. The results of simulations in the presence of an external magnetic field show that the dynamics of the relativistic electrons are strongly affected by the external magnetic field which results in the optimum energy transfer into the fuel. The comparison of the results indicates that the optimal energy deposition is obtained for the density of 3 ρc 300 g.cm in the presence of a magnetic field of about Bext 600 T with an ignitor laser wavelength of λif   0.35 m and laser intensity of 21 2 I 10 W.cm  . It can therefore be concluded that by increasing the external magnetic field of 600 T ext B  , the energy deposition rate increases slowly. Keywords: Geant4 simulation toolkit, Fast-shock ignition, External magnetic field, Exponential energy distribution, Quasi-two temperature energy distribution.

In this paper, the effect of the initial electron energy with E0=1-10 MeV, its distribution function and pre-plasma background temperature T=0.5-10 keV on the optimum transport into the dense fuel with the density ρc=292-828 g.cm-3 have been investigated analytically for fast-shock ignition concept. The analytical results showed that for Te ≥ 5 keV, the Coulomb logarithm of the charged particle is weakly dependent on the pre-plasma temperature, and it seems that the plasma stopping power is approximately independent of background temperature. Therefore, it could be concluded that pre-plasma temperature is not a key parameter for the electron penetration improvement, and the electron penetration can be optimized by a decrement of fuel density and increment of electron incident energy; in a way that the optimal condition obtained about E0≈4.5 MeV for electron incident energy and ρc=300 g.cm-3 for pre-plasma density. Furthermore, investigating the impact of the fast ignitor wavelength and electron energy distribution function showed that the electron distribution function is almost independent of the background temperature and by considering quasi two-temperature distribution function for electron and fast ignitor wavelength λif ≈ 0.35 µm, the optimized penetration may be obtained. The analytical results showed an acceptable agreement with those of Monte Carlo simulations.

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 τL</sub> = 60 fs, and τL</sub> = 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: step-like, 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.

In this paper, plasma main characteristics such as electron mean temperature, electron number density, and oscillation frequency have been measured experimentally using the double Langmuir probe diagnostic system. In our experiment, the plasma was generated by applying the low pressure dc glow discharge in several common gases. The experimental results indicated the highest plasma density and oscillation frequency for the plasma originating from Argon and the highest mean value of temperature for Hydrogen plasma. The experimental results were then confirmed by COMSOL simulator, showing reasonable consistency with the simulations. The data were then used to compare the degree of ionization with the measured plasma parameters.

The stopping power, penetration and scattering of high energy electrons with different energy distribution functions into dense fuel and hot-spot (fuel core) have been considered for a fast-shock ignition scenario. The analytical calculations indicate that fast electrons with two-temperature energy distribution function penetrate more into the dense fuel, in comparison with the monoenergetic and exponential function, where it is consistent with the MCNPX simulation results. Furthermore, the scattering of energetic electron beams toward the outer surface of the fuel for five various fuel density and two fast ignitor wavelengths of 0.53 and 0.35 micron have been investigated. The results show that for the fuel mass smaller than  2 mg, the scattering of electrons reduce for the electrons with smaller energies and fast ignitor of smaller  wavelengths. Meanwhile, for the electrons with energy of the order ~3.5 MeV, two-temperature and monoenergetic energy distribution function deliver the highest and lowest energy to the main fuel and the central hot-spot, respectively.

In this paper, semi-analytical relations of total energy, fuel gain and hot-spot radius in a non-isobaric model have been derived and compared with Schmitt (2010) numerical calculations for shock ignition scenario. in nuclear fusion. Results indicate that the approximations used by Rosen (1983) and Schmitt (2010) for the calculation of burn up fraction have not enough accuracy compared with numerical simulation. Meanwhile, it is shown that the obtained formulas of non-isobaric model cannot determine the model parameters of total energy, fuel gain and hot-spot radius uniquely. Therefore, employing more appropriate approximations, an improved semianalytical relations for non-isobaric model has been presented, which are in a better agreement with numerical calculations of shock ignition by Schmitt (2010).

This paper deals with the role of fast ignitor in fast-shock ignition (FSI) concept. The semi-analytical model indicates that the FSI target gain is a function of fast ignitor laser wavelength. If the energy of fast ignitor driver is and the laser wavelength is less than 0. 53 micron، then with a fuel mass about 2 mg the FSI has a considerable advantage over pure shock ignition and the figure of merit is better than 1. 2. When the wavelength of fast ignitor becomes shorter، the approaches، and for wavelengths shorter than 0. 25 micron no additional is advantage is obtained.

A new concept for inertial confinement fusion called fast-shock ignition (FSI) is introduced as a credible scheme in order to obtain high target gain. In the proposed model، the separation of fuel ignition into two successive steps، under the suitable conditions، reduces required ignitor energy for the fuel ignition. The main procedure in FSI concept is compressing the fuel up to stagnation. Then، two high intensity short pulse laser spikes with energy and power lower than those required for shock ignition (SI) and fast ignition (FI) with a proper delay time are launched at the fuel which increases the central hot-spot temperature and completes the ignition of the precompressed fuel. The introduced semi-analytical model indicates that with fast-shock ignition، the total required energy for compressing and igniting the fuel can be slightly reduced in comparison to pure shock ignition. Furthermore، for fuel mass greater than، the target energy gain increases up to 15 percent and the contribution of fast ignitor under the proper conditions could be decreased about 20 percent compared with pure fast ignition. The FSI scheme is beneficial from technological considerations for the construction of short pulse high power laser drivers. The general advantages of fast-shock ignition over pure shock ignition in terms of figure of merit can be more than 1. 3.

In this project, the activity concentration of some radionuclides in rice samples, collected from Guilan province have been analyzed and the results have been compared with rice samples imported from other countries. For this purpose, sixteen types of rice samples from different region of Guilan and four imported rice samples have been collected. The collected samples have been analyzed in order to determine the activity concentration of number of radionuclides such as 40K, 226Ra, 232Th and 137Cs. A gamma spectroscopy system with a high purity germanium detector (HPGe) has been used for the radioactivity measurement. The highest activity concentration in the rice samples belongs to potassium-40. The mean activity of 40K, 220Ra, 232Th and 137Cs were (30.23±5.51), (3.30±1.75)10-2, (3.78±1.65)10-2 and (2.65±0.99)10-2 Bq.Kg-1, respectively. It can be concluded that all the activity concentrations are comparable with the values that have been, imported for other countries.