محمدجعفر جعفری

Low-intensity laser prepulse (<10^13 W cm^−2, nanosecond duration and <10^17 W cm^−2, picosecond duration) significantly impact experiments on laser-induced proton generation, often constraining the performance of proton sources from high-intensity lasers. Depending on the intensity regime and target thickness, various effects can result from the prepulse. This study focuses on hydrodynamic simulations using a fluid code to replicate real-world conditions, specifically targeting the prepulse parameters of the ATLAS laser facility (including a 20-picosecond duration) across different target thicknesses (1-0.12 μm). The simulations extract target parameters at the end of the prepulse phase. For these specific prepulse conditions, the results indicate that a target thickness of 0.25 μm optimizes the interaction with the main laser pulse. Thinner targets exhibit altered density profiles on the rear surface, thereby reducing acceleration efficiency.

The common mechanism of proton laser acceleration is Target Normal Sheath acceleration. This study investigates the geometry of the aluminum target used in high-intensity laser pulse interaction with plasma. The target with an exponential profile on the front and back sides and with different scale lengths from 0 to 0.85 μm is used as an input to the two-dimensional particle simulation code. The results of their proton energy distribution have been compared. It was found that proton energy decreased with smooth changes behind the target. This is due to the reduction in the electrostatic field amplitude behind the target. In addition, there is a threshold scale length (here 67/0 μm), after which the proton energy does not change with the increase of this quantity and the energy distribution of the proton beam almost overlaps. The transverse laser field emission and the same electron energy distribution were also observed for two targets with zero and 0.67 μm scale lengths.

Target Normal Sheath Acceleration (TNSA) mechanism is one of the most common proton acceleration mechanisms in the experimental setup. In this work, the effect of field ionization on the proton acceleration performance in result of the interaction of high intensity laser pulses with different pulse lengths was studied using two-dimensional particle in cell (PIC) simulations. For this purpose, two solid (neutral) targets as well as fully ionized plasma target, made of aluminum layer with thickness of 0.5 μm, and paired with a thin layer of hydrogen with thickness of 50 nm are considered.  Simulation results showed that considering the constant pulse energy and a medium laser intensity (here a0 =10 for pulse width of 25 femtoseconds) the use of solid structure leads to an increase in the maximum energy of the protons by about 36%. In addition, the difference in proton cut-off energy between the solid target and the ideal plasma one decreases with increasing the laser pulse width.

In the present work, the formation of the wakefield during the interaction of intense laser pulse with a gas medium has been investigated by using PIC simulation code, including the ionization process. The results have been compared with those corresponding to the case of the pre-formed plasma medium. Although in previously published works, the strong launch of forwarding Raman's instability was shown to be as a result of plasma density fluctuations during ionization and the subsequent strong laser pulse modulation, our results indicate that the wakefield amplitude in gas in comparison with plasma considerably depends on the laser pulse shape. For laser pulse with a high slope, the amplitude of the wake electric field is quite the same in gas and plasma mediums. However, as the slope of the laser pulse decreases (soft slope), the wakefield is generated with a larger amplitude in the plasma. A further decrease in the laser pulse slope leads to a larger wake electric field in gas than in a plasma environment.

Laser produced plasma can be used as the sources of soft X-ray laser. The ability to control the laser quality and its gain coefficient by controlling laser and plasma’s parameters is one of the advantages of this method. In this study, a pump pulse assistant along with a pre-pulse is irradiated on a geranium target as the plasma active medium, thenthe gain of soft X-ray laser at wavelength 19.6 nm is calculated. In order to analyze the effect of laser parameters such as intensity, pulse length, and time delay between two pulses, MED103 hydrodynamic code has been used. The simulation results show that there is optimal pulse duration for the pre-pulse as well as the main pump pulse to achieve the maximum gain of soft X-ray laser. In addition, according to the results, by increasing the pre-pulse intensity the amount of soft X-ray laser gain initially increases and then decreases, while by enhancing the main pulse intensity, it keeps increasing. Also, the optimal spatiotemporal regions of the soft X-ray laser gain for different time delays of two pulses are given.

One of the most common laser proton acceleration mechanism is Target Normal Sheath Acceleration (TNSA) method. The use of a foam layer in front of the main target plays an important role in the amount of laser energy absorption by the electrons and consequently the acceleration of the proton. The front layer can be either uniform and homogeneous or nano-structured. In this study, by assuming a nanostructured foam layer, and using two-dimensional particle simulations code, the effect of nanoparticle’s radius on the proton cut-off energy is investigated. Particles with radii of 10, 60 and 120 nm and random sizes in the range of 10 to 120 nm have been studied and simulated in a front layer with thickness of 10 and 20 μm with near-critical average density at laser intensity (I≈1020W/cm2). According to the results, in the case of thin foam layer, the differences of electron and consequently proton spectra are negligible. However, by increasing the foam thickness, the influence of nanoparticle radius causes a further dissociation in the final proton energy spectra. So that, the proton energy increases almost 45% by reducing the nanoparticle size from 120 nm to 10 nm.

Generally, laser ignitior parameters in shock ignition concept are above the threshold for laser-plasma instability and therefore are prompting the generation of fast electrons. Depending on the electron energy spectrum and compressed fuel density, the electrons can cause fuel preheating or increase the shock wave amplitude. In this paper, first, the target hydrodynamic parameters are calculated before driving ignitor pulse. Then, electron energy spectrum in the interaction of ignitor with plasma for laser intensity of 1016 Wcm-2 is considered. For density and temperature profiles of the compressed fuel, fluid approach and to evaluate ignitor pulse interaction with plasma, the kinetic approach are adopted. Considering the actual situation of the plasma at the moment of maximum compression, PIC simulations of high-scale, high-temperature and non-homogeneous plasma show bi-Maxwellian electron distribution.

In this research work, target performance under different spike pulse(s) time behavior is studied. It is shown that by applying two equal spike pulses with appropirate time delay, leads to enhancement of the target gain and reduction of the total ignitor energy. The lower spike energy is required for the lower total driver energy to be used for commercial energy production. Two trapezoidal pulses with 50 picoseconds rise time, different power levels, pulse duration, launching and delay time have been applied on a HiPER baseline target. A one- dimentional simulation by MULTI code has shown that the figure of merit for a double shock ignition scheme varies in a range of 1.1<FM<1.7. The FM>1 demonstrates that for the same compression energy the splitting of the ignitor pulse into two equal lower energy pulses has more benefit compared with the conventional method of shock ignition.