The Studies On Some Important Subjects Of Vacuum Laser Acceleration

With the rapid development of the ultra-intense laser technology, there has been growing research interest to use the extremely strong electromagnetic field of intense lasers to accelerate charged particles, especially electrons. We have proposed a new scheme of laser-driven electron acceleration in vacuum, namely Capture and Acceleration Scenario (CAS). It has been found that relativistic electrons of several MeV injected into a focused laser beam propagating in vacuum, can be violently accelerated to energies up to GeV by this scheme. And optical components are not needed near the laser focus, which remove constraints due to laser damage. Based on the previous works, this thesis has studied deeply on some significant advanced problems of vacuum laser acceleration. The conclusions of this work not only develop the CAS theory, but provide the forthcoming experiment on CAS scheme with useful theoretical background as well.In most of our previous work, the linearly polarized (LP) field is employed to interact with electrons. In this thesis, based on large amount of calculations and simulations, it has been confirmed that the CAS acceleration can be achieved in circularly polarized (CP) focusing laser filed, and discovered that the CAS acceleration channel occupies relatively broader incident phase space, owing to its highly axisymmetric field, which brings about better acceleration efficiency. Comparing with the electron dynamics in LP field, the advantages of CAS acceleration in CP field include: the axisymmetric distribution of the longitudinal electric component or acceleration channel; the maximum electron final energyγ_fm is not sensitive with the incident polarization azimuth angleα; the gentle variation ofγ_fm with the impact parameter |b₀|; more energetic CAS electrons distributed in the space p_j-θ_j ; the gentle variation ofγ_f with the laser initial phase, etc. Therefore, as for a real electron bunch incident with initial transverse emittance and energy divergence, more high-energy CAS electrons with lower energy divergence can be acquired in CP field.It has also been found that there is a noticeable disadvantage in using CP field for CAS acceleration: the 'energy saturation' phenomenon, that the maximum electron final energyγ_fm rises much more slowly as the laser intensity a₀ (a₀= eE₀/m_cωc) increasing when a₀ is large enough. While in LP filed,γ_fm keeps to nearly linearly increase with laser intensity a₀．This phenomenon stems form an additional magnetic field force in CP field, which weakens the total longitudinal acceleration force at rather high laser intensity. And it restricts the application of CAS acceleration in CP field when a₀ is large enough. In this thesis, the detailed physical explanation on the "energy saturation" effect is presented, and the laser intensity range applicable for CAS acceleration in CP field is recommended.The above results provide us with reliable theoretical background for choosing an appropriate polarized field in CAS experiment.It was noticed that there are two acceleration channels distributed in LP field, and the study on CAS acceleration scheme in our previous work was limited to using the near channel (CASI), which locates on the electron incident side. In order to fit the presently available experimental condition, the CAS acceleration by the other channel, which locates on the other side of the electron incident direction (CASH), is investigated for the first time. Based on calculations and simulations. it has been found that relativistic electrons with higher initial momentum can go through the axial region of the laser beam entering the CASII channel to gain energy. And the CASH scheme has a number of advantages comparing with the CASI scheme such as it allows the incident electrons with broader range of the electron initial momentum p_j; and the CASII acceleration scheme can be beneficial with less energy divergence of outgoing electrons due toγ_fm is not sensitive to p_t. The new results extend both the CAS theory and its application. Actually, this study was initiated by the requiring to perform CAS experiment under the conditions of BNL-ATF (Accelerator Test Facilities) in USA.The essential point of the CAS acceleration scheme is that the relativistic electrons are injected into the subluminous phase velocity regions (the acceleration channels) and accelerated quasi-synchronously by the laser field. We have also studied the vacuum laser "direct acceleration" scheme, where the acceleration process mainly takes place outside the acceleration channel of the laser field. The basic point of the "direct acceleration" is that owing to the relatively slower phase slippage of the fast electrons in the laser intense field, some electrons can be trapped in the acceleration phase and remain in it for sufficient long times to obtain considerable energy; while they slip into the deceleration phase, they already locate in the week field region. Therefore, these electrons can gain net energy from the laser field finally. Through calculations and simulations, the direct acceleration phenomenon has been confirmed theoretically. As it always exists in the interaction process between laser and electrons, the direct acceleration can be considered as a kind of supplement of CAS acceleration. Both acceleration schemes show that the so-called Lawson-Woodward theorem is failed to explain many acceleration phenomena.