Polarization Spectral Diagnosing In Laser-produced Plasma

Experimental research of laser-produced high energy X-ray source is one of the most important parts of Inertial Confinement Fusion (ICF).The laser-produced high- temperature plasma contains very abundant information, such as the ion temperature, electron density, ionization, opacity, transport, fluctuation, instability, etc. In order to study laser-produced plasma, it is necessary to develop a key diagnostic technology to measure the parameters of ions and electrons. Polarization spectroscopy of X-ray lines represents a diagnostic tool to ascertain the presence of electron beams in high- temperature plasmas. Polarization spectroscopy focuses on the ratio of two orthogonal components of X-ray. And polarized X-ray is susceptible to the distribution of electrons and magnetic field. The anisotropy of plasma is relevant to the temperature and density of plasma electrons.This paper was supported by National Natural Science Foundation of China under Grant No.(NSAF)10576041. It has an in-depth study in the polarization theory of laser-produced plasma.The spectrograph has been designed based on X-ray crystal diffractive principle and perpendicularly disposed double-crystal method. The first experiment was carried out at the 20J laser facility of Laser Fusion Research Center, China Academy of Engineering Physics (CAEP). The He-like spectrum emitted from the aluminum plasma was recorded by the imaging plate, which could be used to calculate the polarizability and density of plasmas by some arithmetics.The main tasks of author were the designment and experimental studies on polarized X-ray. In this paper, we draw the following conclusions:1. In order to analyze the mechanism of polarized X-ray and the polarization degree of physical parameters in laser-produced plasma, the basic theory of laser plasma polarization and computational model of polarization were researched. The polarization effect on X-ray was discussed which including the interaction effect between laser and charged particles, the collisional excitation between electron and ion in laser-produced plasma. The Zeeman transitions and polarized X-ray occurs whenΔMJ=0 orΔMJ=±1. Three theoretical computing methods of polarized X-ray were analyzed emphatically, which are Stokes parameters, magnetic sublevel atomic kinetics, population-alignment collisional-radiative (PACR). The simulated results of three models are essentially in agreement, which respectively based on Stokes parameters, radiation cross-section and population-alignment.It shows that polarization of X-ray will decrease and gradually approaches zero with the increase of collisional electron energy in laser-produced plasma.2. The modified comprehensive diagnosis of double-crystal was proposed,which was not restricted to equal optical length and Brewster angle and satisfied to practical application of X-ray diagnosis. Compared with the method of measuring angular emission(MAE) and rotatable spectrometer technique(RST), the method of double- crystal is more adapt to diagnose polarized X-ray. The advantages and disadvantages of three common methods for diagnosing polarized X-ray were discussed.The method of equal optical leghth was flexible but needed two equal optical length, the method of Brewster angle normalized spectral lines based on a non-or-weak polarized line. The experiments indicate that the modified method is extraordinary effective diagnostic method of polarization spectroscopy.3. The novel polarized X-ray crystal spectrometer was developed and manufactured with perpendicularly disposed Double-crystal technique. The traditional crystal spectrometer is difficult to diagnose polarization spectrum with only one or symmetric dual channel. In this paper, the new orthogonal double-crystal structure was adopted, that is, the two crystal analyzer was orthogonal layout in the plane perpendicular to the incident electron beam direction. Single channel crystal usually can be divided into three geometric structures, which are flat, elliptical and spherical crystal with different materials.4. The experimental scheme of diagnosing polarization spectrum was explored,which could exam the properties of this spectrometer and verify anisotropic phenomenon in laser-produced plasma. By Stokes parameter algorithm, we have worked out polarization of resonance line Pw = +0 .1204_0.0096^0.0204and intercombination line Py = + 0.0885_0.0916^0.0679 followed by calculating experimental data, which are well identical to the theory. The results show the laser-produced plasma is anisotropy.5. It has experimentally diagnosed electron density of Al plasma by ratio of resonance and intercombination.The electron density of Al plasma is 0.9¹.5×10²⁰cm^-3 which is consistent with the academic value. These experimental results imply that the performance of the spectrometer satisfies the design requirements and it is a good tool for diagnosing laser-produced high-density plasma.