Monte-Carlo Simulation On Diagnosis Of Steep Density Gradients By Charged-particle Radiography

During an implosion in inertial confinement fusion (ICF), the Rayleigh-Taylor instability (RTI) tends to break up the shell at the early stage, and will prevent the formation of a central hot spot at the later stage. High spatial resolution on the order of 1 ?m is required for precise observation of density profile or hydrodynamic instability.Ultrashort intense laser-generated charged-particle (electron or proton) beam has been widely applied for probing plasmas, which has the properties of small source size, quasi-monoenergetic, short duration, and synchronization to laser pulse. The electromagnetic field in plasma can be determined by measuring the deflection of the charged-particle trajectory. The spatial structure of a target is projected on a radiograph and in this way the areal density of the target can even be quantified from the energy loss of a backlighted charged-particle beam. In addition to this, scattering of the charged particles in matter has received an extensive attention. Since the scattering cross sections of electrons or protons by other charged particles or atoms are much higher than that of x rays, electron or proton beam scattering may be applied in diagnosis of density profile or interface in principle.In this dissertation, we prepose a novel method based on scattering of the charged particles to probe the steep density gradient in a target through Monte-Carlo simulations. It shows that the charged-particle radiography is of high spatial resolution, sensitivity in diagnosis of the steep density gradient.1. We firstly simulate MeV charged-particle radiography of the layers with different linear density gradients. Simulation results show that different density gradients modulate the charged-particle beams at different levels, and the modulation becomes stronger when the density gradient is shorter. The distinguishable density gradient is?AL< 1?m for electron radiography or ?L< 0.5?m for proton radiography. Compared to the proton beam, an electron beam can produce stronger modulation under the same target condition due to larger scattering angle.2. Secondly we simulate MeV charged-particle radiography of a laser-ablated polystyrene target. It is shown that the regions of the target with steep density gradients (AL<0.5 or 1 ?m) can be characterized accurately. However, the regions of the target with long density gradients (?L> 1 ?m) can't be discriminated for both electron and proton radiography, which is consistent with the result obtained in the preceding section.