Measurement Technique Based On Laser Wake Field Frequency-domain Holography

Using ultra-short ultra-intense laser pulse to excite the electron plasma wave(wakefield,Langmuir waves or electron density "bubbles") is one of main mechanism for laser-driven plasma accelerators to accelerate electrons in the low-density plasma.This accelerator with ultrahigh accelerating gradients and compact size is promising for the next-generation particle accelerators,and is one of the most attractive frontiers.In particular,it has been experimentally demonstrated that laser wake-field acceleration(LWFA) has great potential to produce ultrahigh field gradients excited by intense ultrashort laser pulses(＞100GeV/m). However,the beam quality of relativity electron beams produced by LWFA depends on the space-time structures of the wake-field,and it is important for controlling electron beams from LWFA that these structures are directly observed.Based on frequency domain holography(FDH),the single-shot real-time measurement for temporal and spatial evolution of the wake-field propagating near light speed,is investigated theoretically in this dissertation, including the following research topics:1.The development,application and general status of laser wake-field accelerator are summarized.Domestic and oversea development for measuring techniques of laser-wakefield, using Frequency domain interferometry and Frequency domain holography,are emphatically introduced,respectively.Especially,representative technical schemes and qualifications are given a presentation in detail.2.Excitation principle of laser-wakefield,mechanism of electron acceleration and four schemes for laser driven plasma accelerators are briefly depicted.The resonance conditions between the ultra-short ultra-intense pulse laser and plasma,temporal and spatial characteristics of wakefields and phase perturbation of the probe pulse imposed by wakefields are specially discussed for the appropriately choice of experiment parameters.3.Because supporting technology of the frequency-domain holography is spectral interference(SI) between two chirped pulses,FDH is an extension of Frequency domain interferometry(FDI).Fourier-transform spectral interferometry(FTSI) and the measurement of delay time between two pulses are analyzed theoretically,numerically and experimentally, respectively.Thereinto,the characteristics of spectral interferometry between two linear chirped pulses are mostly studied.4.Two methods for frequency-domain holography,direct mapping mode and full Fourier transform mode,are theoretically investigated,respectively.With the direct mapping mode, the spectral phase shifts are transformed to time-domain by time-frequency relation,while the spectral phase shifts are transformed to time-domain by Fourier transform with the full Fourier transform mode.Comparison of the two modes is given,and results show that more accuracy can be obtained through the full Fourier transform mode for the measurement of the wake-field.5.The measurement scheme for transient time-space evolution of the wakefield induced by the ultra-short ultra-intense laser pulse is presented,based on FDH using picosecond chirped pulses.The coaxial generation of the reference and probe pulses is investigated theoretically and experimentally,based on spectral characteristics of the generated pulse in the process of broadband second-harmonic generation.6.According to the theoretical analysis of FDH,the optical layout and experimental parameters of FDH are designed based on the SILEX-I,a Ti:sapphire laser system at CAEP, where the wake-field is excited in a He gas jet pumped by a pulse of～30TW/800nm/30fs,and probed by two chirped pulses of～1ps/400nm.