Efficient Multi-keV X-ray Sources Generated By Nanosecond Laser Pulses Heated Plasma

X-rays are widely used in diagnosing high density plasmas in the fields of inertial confinement fusion (ICF) and high-energy-density physics (HEDP). X-ray sources with high brightness not only allow us to diagnose high density object, but also further expand our research object, such as material composition, density, kind and thickness, etc. The high brightness of x-rays is required. Therefore, more and more attention is attracted in generating a bright enough x-ray source.One of the most popular methods to generate x-ray sources is using powerful laser beams to irradiate some specific targets. By this way, one can increase the energy of the laser beams to generate a highly bright x-ray source. However, in an actual experiment, the maximum of the energy of the laser beams is limited for several reasons, such as the requirement of the experiment itself and the damage threshold of the laser facility. As a result, enhancing the x-ray conversion efficiency (CE) is necessary for a bright x-ray source, which is our present interest. In this dissertation, we focus ourselves on several kinds of efficient multi-keV x-ray sources driven by nanosecond laser pulses:1. multi-layer thin foil targetA novel and efficient multi-keV x-ray source, multi-layer thin foil target, is proposed for the first time. Titanium multi-layer thin foil targets and underdense targets are simulated by using a one-dimensional Lagrangian radiation hydrodynamics code Multi ID. The results of Multi ID prove that the two targets are very similar in generating plasma and producing multi-keV x-rays. However, as an efficient multi-keV x-ray source, the multi-layer thin foil target is superior to the underdense target in two aspects. First, its fabrication is much easier. Second, more kinds of materials can be used, and that provides more choices of x-ray energy bands. Preliminary experimental results substantiate that the multi-keV x-ray emission region of titanium multi-layer thin foil target is larger than that of thick foil target. Lots of simulations are conducted to investigate the dependence of the multi-keV x-ray CE of multi-layer thin foil target on the distance between thin foils, the foil thickness and the laser pulse waveform. Besides, titanium multi-layer thin foil target is employed successfully in the backlight imaging experiment, in which a high-quality image of gold mesh is obtained. 2. underdense targetTitanium underdense targets are also simulated by using Multi ID. Simulation results give the relationship between the multi-keV x-ray CE of underdense target and the initial target density. The optimal initial target density, which is corresponding to the maximal x-ray CE, increases with increasing laser intensity, decreases with increasing duration of laser pulse and with increasing the atomic number Z. Besides, the flux limit factor used in Multi1D effects the optimal initial target density significantly for an intense laser pulse. At this moment, the optimal initial target density is increased with increasing flux limit factor. Other investigations with the same total energy of the incident laser pulse show that the underdense target irradiated by the square pulse emits more multi-keV x-ray than that irradiated by the shaping pulse which keep ablating underdense targets supersonically. A simple theoretical model is adopted to analyze the production of multi-keV x-rays from the underdense target. And it gives the optimal initial target density which is consistent with those obtained by Multi1D. The factor1.5, which is the speed ratio of heating wave and rarefaction wave, is given as a rule to determine the optimal initial target density.3. hohlraum targetThe multi-keV x-ray CE of the titanium hohlraum targets is studied on the SG-Ⅲ prototype laser device for the first time. The images obtained by the x-ray pinhole camera (XPHC) show that the plasma collision near the cavity axis enhances the multi-keV x-ray emission significantly. The effect of bottom foil in improving the multi-keV x-ray emission is also evidenced by comparing the results of titanium hohlraum target with and without the bottom titanium foil. The experimental results suggest that the optimal diameter of hohlraum is d=aCsx, where1.4<a<1.8(Cs is the ion sound speed, and τ is the laser pulse duration).In summary, we provide the physical image of improving multi-keV x-ray CE through the investigations of the three types of multi-keV x-ray sources mentioned above. Moreover, we provide key data for application of multi-keV x-ray sources through our detailed study on the dependence of multi-keV x-ray CE on laser parameters and target parameters.