Bondar D. Study of excitation of fields in plasma and dielectric by powerful laser pulses and relativistic electron bunches for acceleration, focusing and heating of electrons and positrons

The dissertation is devoted to the theoretical (by numerical simulation) research of a number of problems related to the excitation of wakefield, to the investigation of processes of accelerating and focusing bunches of charged particles. The paper deals with the study of excitation of wakefield in plasma, the density of which is equal to the electron density in metals by an X-ray laser pulse (powerful electromagnetic wave); study of the formation, properties and role of electron soliton cavities in inertial fusion, as well as smoothing of transverse inhomogeneities at a critical point under the interaction of a laser pulse with an inhomogeneous plasma in inertial fusion; in addition, parameters were found for a plasma lens that would allow the sequences of relativistic positron bunches to focus uniformly. An important issue was the study of the amplitude of the wakefield and the transformer ratio for excitation of wakefield by the sequence of charged particle bunches (beams) in plasma and dielectric. Excitation of the field in the plasma by a sequence of electron bunches in the non-resonant case was investigated. The process of combined laser-plasma acceleration is considered, thanks to which it is possible to ensure energy transfer between self-injected bunches and the wake wave. The method of restoring the phase synchronization of laser pulses and the wake wave is studied, as well as the influence of the external magnetic field on the electron bunches that excite the wakefield. Together with the use of X-ray laser pulses, the use of plasma with such parameters allows the excitation of wakefields with an amplitude of several teravolts per meter. It was shown that under these conditions the wakefield process is accompanied by the formation of self-injected electron bunches in the region of significant negative space charge as well as regions with high ions density as well as formation of regions with increased ion density, which provides a significant acceleration field. In addition, the so-called combined laser-plasma acceleration mode is observed. The mechanism of “adjustment” was investigated by the authors and presented in the results of the study. Thanks to it, it was possible to partially restore the mechanism of coherent addition in the nonlinear case. The use of inhomogeneous plasma to support the acceleration process of the self-injected bunch and increase the accelerating gradient was studied. The author of dissertation considers a method of increasing the density of plasma electrons, which leads to a dynamic decrease in the size of the wakefield bubble along which the electron beam moves. In the current work, a plasma lens for focusing beams is studied, which allows to focus sequences of relativistic positron bunches uniformly. The study of the amplitude of the wakefield and the transformer ratio for the excitation of wakefield by the sequence of charged particles bunches in plasma and dielectric were performed. The excitation of the wakefield by a sequence of bunches of charged particles in a dielectric resonator is investigated in this work. Numerical simulation of injection of bunches of charged particles (electrons) into a dielectric resonator (a dielectric rod with a metal casing) and excitation of a wakefield is performed. It was shown that in the case of injection of a sequence of bunches with certain parameters, in particular, with a length equal to 0.5λ of the wavelength, it is possible to obtain the value of the transformer ratio TR=2N, where N is the number of bunches. By using two-dimensional numerical simulation, the evolution of focusing force acting on electron bunches during their propagation in plasma has been investigated in dependence on the bunch length and distance between bunches for various current profiles of the bunch. The mechanism for the sequence of long relativistic electron bunches, that leads to resonant excitation of the wakefield even in cases where the frequency of injection of bunches differs from the plasma frequency, has been studied. The dependence of the transformer ratio and the maximum accelerating field on the length of the bunch with a constant charge of the bunch is investigated. The dependence of the radial force on the length of the bunches and the distance between the bunches was also studied.