| dc.description.abstract | Laser wakefield acceleration (LWFA) has been a well-recognized technique to generate a huge acceleration gradient to produce ultrashort and energetic electron bunches that leads to the development of future laser-driven electron accelerators. To date, most of the LWFA experiments are conducted with high-peak-power, high-energy Ti:Sapphire-based lasers. However, these 800-nm pulses of requisite pulse energies are typically operated at a relatively low repetition rate <= 10 Hz, which can inhibit the attractiveness of LWFA when applying it in advanced x-ray radiography in which a sufficiently high radiation flux is desired. Therefore, this thesis reports our efforts and accomplishments on the development of novel laser-driven acceleration methods that can be conducted with a relatively low laser peak power.
One of the promising methods is quasi-phase matched direct laser acceleration (DLA) of electrons, which can be achieved by utilizing the axial field of a well-guided, radially polarized laser pulse in a density-modulated plasma waveguide. When a laser pulse of a few TW peak power is applied, however, the laser ponderomotive force perturbs plasma electrons to concentrate in the center, such that the generated electrostatic fields can significantly defocus the externally injected electron witness bunch and considerably deteriorate the acceleration efficiency. To improve the performance of DLA, a leading electron bunch, which acts as a precursor, can be introduced in DLA to effectively confine the witness bunch. Three-dimensional particle-in-cell simulations have been conducted to demonstrate that the transverse properties of the witness bunch can be significantly improved when a precursor bunch is used. Selected bunch transverse sizes, bunch charges, and axial separation from the witness bunch have been assigned to the precursor in a series of DLA simulations. Since a favorable ion-focusing force is provided by the precursor, the transverse properties of witness bunch can be maintained when a relatively high-power (~2 TW) laser pulse is used in DLA, and an improved overall acceleration efficiency can be achieved.
In addition to DLA, LWFA can be accomplished by introducing a sub-terawatt (TW) laser pulse into a thin, high-density gas target. In this way, the self-focusing effect and the self-modulation happened on the laser pulse produce a greatly enhanced laser peak intensity that can drive a nonlinear plasma wave to accelerate electrons. A particle-in-cell model is developed to study the sub-TW LWFA when a 0.6-TW, 800-nm laser pulse interacts with a dense hydrogen plasma. Gas targets having a Gaussian density profile or a flat-top distribution are defined for investigating the properties of sub-TW LWFA when conducting with a gas jet or a gas cell. The peak density which allows the laser peak power PL~2Pcr of self-focusing critical power is favourable for conducting sub-TW LWFA. Otherwise, an excessively high peak density can induce undesired filament effect which rapidly disintegrates the laser field envelope and violates the process of plasma wave excitation. The plateau region of a flat-top density distribution allows the self-focusing and the self-modulation of the laser pulse to develop, from which well-established plasma bubbles can be produced to accelerate electrons. The process of the electron injection is complicated in such a high-density plasma condition; however, increasing the length of the plateau region represents a straightforward method to realize the injection and acceleration of electrons within the first bubble, such that an improved LWFA performance can be accomplished.
In addition to using 800-nm laser pulses, simulations are performed to study the scheme in which 1030-nm pulses produced from a diode-pumped laser system are introduced into a gas cell with a flat-top density profile, allowing the LWFA to be operated at high frequencies. Because 1030-nm lasers are typically produced with a long duration > 200 fs, a spectral broadening technique can be applied to reduce the pulse duration, from which a greater ponderomotive force is acquired to drive LWFA. To understand the dependence of LWFA performance on the driving pulse duration, selected durations, ranging from 200 fs to 10 fs, are assigned for 0.5-TW, 1030-nm pulses in a series of simulations. Results show that a duration around 50 fs can provide the optimal LWFA results, as a compromise between the weak ponderomotive force available from a long pulse > 100 fs and the depletion effect which can rapidly diminish a short pulse < 25 fs in a dense plasma. When a low laser peak power of 0.25-TW is available, the pulse depletion can be significant at a high target density and render LWFA ineffective. Using a laser pulse with a longer wavelength > 2 micrometer represents a viable route to realize the LWFA with a low laser peak power; in this way, an appropriately selected target density which allows the laser peak power PL ~ 1.25 Pcr of self-focusing critical power is favourable for realizing an efficient LWFA process. | en_US |