Studies Of The Properties Of A New Type Of Laser-Driven Accelerator

The Capture and Acceleration Scenario(CAS) is a new mechanism that we proposed in our far-field laser acceleration research, which is a promising scheme for a new type of laser accelerator. In this thesis, I study CAS electron behaviors beyond the paraxial approximation, and prove the existence of CAS phenomena in a more accurate description of a Gaussian laser field. Furthermore, the features of electron bunch interactions with a high intensity laser pulse and the parameters for a CAS experimental test are given.The physical principle of CAS is the following. For a focused laser beam, there exists a subluminous phase velocity region due to the diffraction effect. In this region, the phase velocity of the wave is lower than the light speed c. Combined with the strong longitudinal electric component for any focused laser field, an acceleration channel is formed, which shows similar characteristics to that of a wave guide tube of conventional linear accelerators. Relativistic electrons injected into this region can be trapped in the acceleration phase and remain in phase with the laser field for sufficiently long times, thereby receiving considerable energy from the field. The conditions under which CAS can occur are as follows. The laser intensity should exceed a threshold which depends on the laser beam width. The required energy of the incident electron is in the range of 4-15 MeV. The electron incident angle relative to the beam propagation direction should be small. By deriving the higher order corrections to describe a Gaussian laser field, and by using test particle simulation programs, the electron dynamics obtained using the paraxial approximation, the fifth-order, and the seventh-order corrections are compared. The results obtained from the three correction models qualitatively coincide with each other, but exhibit some quantitative differences. The results reveal that, when kw0&50, the paraxial approximation field(PAF) is good enough to reproduce all the electron dynamic characteristics. In the range of 40.kw0(50, the fifth-order corrected field should be used. For very tightly focused laser beams, kw0.30, one has to utilize seventh-order or higher order corrections to accurately describe the field of a Gaussian beam. As for CAS, the applicable range of PAF model can be extended to around kw0&40. The above results support strongly the conclusions on CAS from our previous work.The output properties of electrons accelerated by the vacuum laser acceleration scheme are addressed. The transport process of the electron bunch, the correlation of energy with position and scattering angle, the energy spectrum and angular distribution, the emittance of the outgoing electrons, etc. are studied. In addition, the influences of the laser intensity, beam width, and pulse duration on the properties of the output electrons are also examined. Physical explanations of the output characteristics are presented based on the mechanism behind the CAS scheme. We find that the outgoing electrons can be divided into two groups. One group is CAS electrons, which have higher energies and small scattering angle (less than) with respect to the laser beam. The other group is the inelastically scattered electrons which have low energies and spread widely in space. The emittance of the output electron pulses in the polarization direction of the laser is improved greatly after the interaction. For nearly monoenergetic CAS electrons, their scattering angle is almost unaffected by the laser intensity, which is basically consistent with the formula deduced from the Hamilton-Jacobi theory. This feature is favorable for extracting a nearly monoenergetic electron beam with a spectrometer. The energy gain is found to increase linearly with for 10≤a0≤100.The energy gain can be substantial more than. The acceleration efficiency may exceed 10% with existing laser systems . With increasing laser intensity, the final energy gain, the fraction of CAS electrons, the energy dispersion and the emittance of the outgoing CAS electron...