Theory And Simulation Of Laser Ionization Injection For High Quality Electron Beams In Laser Wakefield Accelerators

The laser plasma accelerator, or called the laser wakefield accelerator (LWFA), is the equipment that utilizes the plasma acceleration structures created by the high energy density, good pointing quality lasers interacting with gases to accelerate elec-tron beams to relativistic velocities in a millimeter or centimeter scale. Compared with the conventional radio-frequency (RF) accelerators, the LWFA have the advantages of higher acceleration gradients without the material breakdown limits, and thus have the advantages of smaller sizes and higher flexibilities. Although the LWFA has many advantages, the obtained particle beam qualities are not as good as that from the RF accelerators due to the limitation of laser facilities and the uncontrollable tiny accel-eration structures of a LWFA. In this thesis, I would like to focus on the femtosecond relativistic intensity laser interaction with plasmas and the LWFA, and emphasis on a few original methods on output electron energy spread control to overcome the low quality problems of LWFAs. With these methods, we expect the applicable LWFAs finally come true in the near future.The contents are organized as follows. In Chapter 1 we discuss the very basic theories of femtosecond relativistic intensity laser interaction with dilute plasmas, the bubble wakefield theory, the beam injection and trapping theory, the injection meth-ods that has been proposed in the past few decades together with their advantages and drawbacks, and the reason we choose ionization-induced injections. In Chapter 2 we first discuss the main problem in the ionization-induced injections, that is the large en-ergy spread, and point out the the solution is reducing the effective injection length. Through PIC simulations, I propose the plasma profile tailoring method, especially the injector density up-ramp tailoring, to minimize the output electron beam energy spread to the range of 1% to 2% when the up-ramp length is between 90μm to 150μm. The mechanism of this method is that the injection is controlled by the up-ramp gradien-t. If the gradient is properly chosen, a large number of electrons can be injected into the wake at the end of the up-ramp, and the effected injection length is just around 50μm. In Chapter 3 we show a method utilizing the laser beam self-focusing to limit the injection region in a few hundred micrometers, and minimize the output electron beam energy spread to 3%. The key point of this method is to use a larger laser waist size than the so-called matched spot size for laser beam and plasmas, so that the laser is greatly self-focused in the first few hundred micrometers in the plasma. When the laser beam reaches its minimal size due to self-focusing, the wake deforms and the injection is suppressed automatically. By calculating the self-focusing length, we can estimate the effective injection length, which is positively correlated with the initial laser waist radius in a given parameter range. We can optimize the output electron beam energy spread by choosing a relatively small laser waist while keeping the initial injection con-dition been satisfied. This is verified by simulations. In Chapter 4 we present a even more exciting method which uses dual-color lasers to trigger ionization-induced injec-tions in separated regions. Since the phase velocities of a base frequency laser and its harmonic are different, if we adjust their amplitude so that the high-order ionizations are triggered on when their peaks overlaps, and triggered off when one peak meats the valley of the other. The effective ionization-induced injection regions are separated with each length of 100μm to 200μm. We also point out that the optimal combination is the first two Fourier components of a square wave. With this method, single or mul-tiple sub percent energy spread electron beams can be produced in a single shoot. This is the first time in LWFA studies to obtain controllable energy gaps of electron beams, with each electron beam quality among the best in the world. Chapter 5 concludes the whole thesis, and provides practical guides for the experimental realizations.