The Experimental Study Of Electron Acceleration By A Laser Wakefield

The charged particle accelerator was one of the greatest inventions in the twentieth century. It is very important for promoting the development of the high energy physics and social economics. However, for the radio-frequency acceleration used in the conventional accelerator, the accelerating gradient is limited to 100MV/m because of the electrical breakdown of the material. Larger scale (multi-km) and higher cost (multi-hundred million dollars) would be necessary for the higher energy accelerator, and restrict the development of the conventional accelerator. The plasma is a ideal medium for acceleration because the plasma itself is ionized and there is no breakdown. As the development of the Chirped Pulse Amplification, ultra-short and ultra-high intensity laser pulse has been invented in the last twenty years. When such a laser pulse interacts with the underdense plasma, it can drive high-amplitude plasma wave behind its tail. This plasma wave can trap electrons and accelerate them to GeV in the cm scale, which supplies a new approach for the electron acceleration.The accelerating gradient of the laser wakefield could be higher than 100GV/m, and the duration of the generated electron beam is shorter than 50fs and the spot size is usually several micrometers, all of these characters are better than that of the conventional accelerator. The laser wakefield acceleration (LWFA) has been researched for more than twenty years all over the world. Experimental results of the LWFA in the SILEX-â… laser facility are introduced. The main results in this thesis are presented in the follows:1 Quasi-monoenergetic electron beam with 58MeV was obtained by a nozzle jet of 2.7mm diameter, and the matched gas pressure was 2.5MPa for the condition of the laser facility. PIC simulations were conducted to explore the detailed physics process of the LWFA under different plasma densities. Simulations demonstrated that the interaction plasma density was deduced by the pre-pulse, which was validated by the forward Raman spectrum. For the deduced plasma density, the self-evolution length of the laser pulse was about 2.3mm, and the accelerating length of the electron beam was just 0.4mm, which resulted in the lower energy of the electron beam.2 Electron bunch with total charge up to 20nC was obtained. Analysis of the energy distribution of the laser pulse show that the large spot size and the multiple pulses are in charge of the large charge electron bunch. The generated electron bunch consists of several beamlets. Detailed calibration of the ICT demonstrates that the charge measurement was reliable.3 The Bernoulli equations are used to calculate the gas flow of the gas-filled capillary. MHD simulations are used to study the process of the plasma channel generation of the discharge capillary. The simulations also demonstrate that there are three stages for the channel generation in the discharge capillary, which are gas ionization, channel formation and channel stable. Detailed simulations show that the stable plasma channel can be generated in different discharge currents and filled gas pressures.4 The gas filled capillary discharge waveguide system was constructed in LFRC, which consists of discharge source, capillary and the gas filled system. The discharge source can supply a voltage of 10kV-30kV steadily. There are three different capacitors in the discharge source for generating different discharge current, the duration of which is between 200ns and 500ns, and the current amplitude is between 200A and 500A. Such discharge current can breakdown the capillary and generate plasma channel successfully.5 Based on the gas filled capillary discharge waveguide, the plasma density is measured by the Stark broaden of plasma using a spectrometer. The scaling law of plasma density and gas filled pressure in capillary is obtained. This scaling law can be used to control the plasma density in the following laser guiding and LWFA experiments.6 The laser guiding and electron acceleration experiments by the gas filled capillary system were conducted in LFRC. In the experiments, high energy electron bunch with Maxwellian distribution was obtained and the laser pulse was guided about 15mm in the capillary without discharge. Analysis of the laser pulse's contrast ratio shows that the pre-pulse condition played an important role in the guiding of the laser pulse.2D simulations are done to explore the reasons of the much lower plasma density generated in the capillary. The simulations demonstrate that the smaller gas inlets of the capillary are in charge of the lower density in the capillary. Due to these simulations, the improvements for the next generation capillary are proposed in the end.