Direct Laser Acceleration of Electron by Intense and Short-pulsed Laser in Plasma
|Course||Atomic and Molecular Physics|
|Keywords||Ponderomotive force Acceleration mechanism Laser wavelength Laser pulse Plasma Electron bunch Accelerated electrons Laser intensity Pulse edge Maxwell distribution Wakefield Laser Technology Ultrashort pulse Number of particles TEM ponderomotive Beam quality Electron density Laser induced Laser Particle|
With the rapid advance of laser technology and the realization of TW laser and fs laser pulse, the laser-plasma interaction becomes one of the most interesting topics in physics. Along this line, laser-particle acceleration in plasma has been one highlight area. Usually, laser acceleration in plasma can be divided into two kinds:one is the laser Wakefield acceleration(LWFA) scheme, by which electron gains energy from the charge-separated field excited by laser; another is the radiation pressure acceleration, by which electron absorbs energy from the laser directly. When an ultra-intense laser pulse propagates through under-dense plasma, electrons at the ascending front of pulse are expelled forward by the longitudinal ponderomotive force of laser, and reach to high energy promptly with ultra-high acceleration gradient. Unlike LWFA, DLA is a linear process with no threshold intensity and offers an attractive alternative for producing high energy electrons. When laser intensity is up to 1022 W/cm-2, DLA phenomenon becomes more and more significant21 and plays an important role in many processes. Over the past decade, relevant researches both on theoretic and experimentation have made evolutional breakthrough. In this paper, By theoretical analysis along with PIC (Particle-In-Cell) numerical simulation results, we discussed about the main physical process and characteristics of this acceleration mechanism, and researched the relationship of the DLA acceleration and laser&plasma parameters.When an intense laser pulse propagates through plasma, it can produce a longitudinal ponderomotive about 100GeV/cm, which accelerates the electrons in the ascending front of pulse to GeV order in 300fs, which we carried out a serial of numerical simulation, and analysis this process with ponderomotive model. We found that with laser pulse propagating, the number of electron accelerated by DLA scheme increases gradually, i.e. there exits an electron accumulating effect, which made the number of high-energy electron up to tens of pC/μm. The electrons accumulated formed a electron beam in front of laser pulse and bubble, and nearly synchronized with laser pulse. By analysis, we found the angle spectrum of the DLA beam presents doublet structure, and the energy spectrum is Maxwell-like, the maximum energy reaches to 3GeV. Besides, we found the electrons with high energy are modulated by laser.Usually, electrons accelerated by DLA are limited to research just in plasma, in other words, DLA acceleration in plasma and in vacuum is relatively independent physical phenomenon. But in our research, we found a two-stage acceleration phenomenon first time, i.e., accelerated in vacuum by the laser directly soon after a DLA process in plasma, which is explained in the paper.We found just under proper initial laser intensity, laser radius, pulse duration, plasma density, there would be obvious DLA phenomenon, There are four main factors to influence direct laser acceleration:laser intensity, laser radius, pulse duration, plasma density. Besides, we discuss how the four factors influence on DLA beam in detail, and the bubble shape and electron beam in bubble in brief. In addition, by numerical simulation for the laser intensity of 1022 W/cm-2 with different kinds of initial plasma density, we get the density range for forming electron beam with high-energy and high-charge, and provide some viable experimental parameters.Based on 2.5D (2D3V) PIC program computation, we carefully investigated characteristics of the DLA beam by TEM(0,0) mode laser pulse in plasma, and compared the characteristics of the DLA beam by TEM(1,0) mode laser pulse in brief. We found a special phenomenon of electron reflux, which links up the DLA beam and bubble beam and bisects the electron bubble extruded by laser ponderomotive force. By anglicizing the energy spectrum of bubble beam in the two cases, we found that reflux electron beam affects on bubble acceleration. The direct laser acceleration by TEM(1,0) mode laser is an development and supplement of laser-plasma acceleration mechanism for its’ special ponderomotive mode and special reflux electron.