Simulation Research On Monoenergetic Proton Acceleration Based On Ultra-short Ultra-intense Laser Pulse And Multi-component Plasma Interaction

With the development of the laser technology and the Fast Ignition research, the interaction between ultra-short and ultra-intense laser pulse and plasma has drawn more and more attentions. High intensity and high contrast laser in-teracting with plasma can generate a electric field as high as 1012 V/m, which provides a good opportunity to develop the new table-top compact laser particle accelerator. Protons with ultra-high energy have valuable applications, such as cancer therapy, high resolution radiographing, fast ignition and modeling of as-trophysics, etc. In this thesis, the generation of high-quality proton beam with higher energy and better monochromaticity based on the interaction between ultra-short ultra-intense laser pulse and multi-component plasma has been sys-temically discussed using theoretically analysis and numerical simulations. The generation of energetic protons from laser target interaction, the excitation of wakefield from laser propagating in underdense plasma and the capture and ac-celeration of pre-accelerated protons by laser wakefield have been specifically researched. The main content includes the following three parts:In the first part, the interaction between ultra-short ultra-intense laser and hydrocarbon target is discussed. Under proper laser and target parameters, the radiation pressure proton acceleration mechanism plays the leading role. Because of the high charge-to-mass ratio of electrons (about 1836 times larger than that of the protons), the radiation pressure of the laser pulse will first push the electrons forward, while the protons remain, creating a large charge separation field. Under the balance between the radiation pressure and the electrostatic field, a double layer structure of the electron and proton layers will be stably formed. Thus, protons will get accelerated by the strong electric field within the double layer. However, with the use of a hydrocarbon target, a three layer structure will be formed instead, due to three different charge-to-mass ratios. As a result, protons will be pulled by the front electron layer via the electric field and be pushed by the back carbon layer, too. On the other hand, when multi-dimensional effects are taken into account, due to the transverse instabilities (such as Rayleigh-Taylor-Like instability and Weibel-Like instability), the double layer is no longer stable, resulting in a limited acceleration length of protons and a limited maxi-mum energy. However, using a hydrocarbon target can effectively suppress those instabilities due to the back carbon layer. So the acceleration length and the maximum energy of the protons in the hydrocarbon target is improved, and the proton energy spectrum is also optimized.In the second part, the trapping condition of the pre-accelerated protons into the fast-moving wakefield is analysed. When laser propagating in the un-derdense plasma, a wakefield will be generated at the back of the laser front. Commonly, this wakefield moves near the speed of light with the laser in the underdense plasma and only electrons can catch up with it, i.e. only electrons can get trapped in the wakefield and further accelerated. However, by adding a target before the the underdense plasma, the laser will interacts with the target before propagating in the underdense plasma and exciting the wakefield. As the laser interacts with the target, protons in it will get pre-accelerated through radi-ation pressure dominated acceleration mechanism. Some of these pre-accelerated protons, with a certain initial velocity, can be trapped by the wakefield and get further accelerated through laser wakefield acceleration to GeV level. This com-bined acceleration regime solves the acceleration length problem of the radiation pressure dominated acceleration mechanism and the injection problem of the laser wakefield acceleration mechanism. The key achievement of our research is the optimization of this combined acceleration regime with a multi-component target scheme. When mixing the traditionally used hydrogen target with a cer-tain ratio of carbons, the improvement of the pre-acceleration stage will lead to a better proton injection process into the wakefield, resulting in an improvement of the proton beam quality with higher energy and better monochromaticity.In the third part, parameter scalings of laser, target and underdense plasma are studied for their influences on the combined proton acceleration regime. The scaling of laser intensity shows that using a hydrocarbon target can reduce the high requirement of the laser intensity, compared to the use of a hydrogen target in order to get the same energetic proton beams. Additionally, the scaling of theCH number density ratio shows that with a larger ratio of carbon to hydrogen density, a smaller energy spread and a higher maximum energy of the proton beam can be realized. Moreover, proton energy will be further increased by employing a longitudinally negative gradient background plasma density.