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A terahertz radiation enhancing scheme, in which a linearly polarized weakly relativistic laser pulse irradiates a target consisting of two parallel thin-solid layers with a certain gap, is proposed and studied by using two-dimensional particle-in-cell simulations. The radiation is known to be produced by laser-produced hot electrons via mechanisms such as coherent transition radiation at the target surfaces. Under optimized conditions, the energy conversion efficiency of terahertz radiation can be as high as 3.3%, which is nearly 1.5 times higher than that obtained with a single-layer target with the same drive laser. This is mainly due to the enhanced hot electron generation with moderate energy via multiple reflections of the laser pulse between the two target layers. The radiation has two peaks close to 30° from the target surface, which are more collimated than that with the single-layer target. The dependence of the terahertz radiation on a variety of target parameters is given, which can control the terahertz spectrum and radiation efficiency and thus provide guidance for experimental investigations. Moreover, both the coherent transition radiation and antenna radiation models are applied to explain the angular distributions of the terahertz emission found in the simulations.

We present a laser-assisted electron injection scheme for beam-driven plasma wakefield acceleration. The laser is collinear with the driver and triggers the injection of hot electrons into the plasma wake by interaction with a thin solid target. We present a baseline case using the AWAKE Run 2 parameters and then perform variations on key parameters to explore the scheme. It is found that the trapped witness electron charge may be tuned by altering laser parameters, with a strong dependence on the phase of the wake upon injection. Normalized emittance settles at the order of micrometres and varies with witness charge. The scheme is robust to misalignment, with a 1/10th plasma skin-depth offset (20 μm for the AWAKE case) having a negligible effect on the final beam. The final beam quality is better than similar existing schemes, and several avenues for further optimization are indicated. The constraints on the AWAKE experiment are very specific, but the general principles of this mechanism can be applied to future beam-driven plasma wakefield accelerator experiments.

Dark current can spoil witness bunch beam quality and acceleration efficiency in particle beam-driven plasma wakefield accelerators. In advanced schemes, hot spots generated by the drive beam or the wakefield can release electrons from higher ionization threshold levels in the plasma media. These electrons may be trapped inside the plasma wake and will then accumulate dark current, which is generally detrimental for a clear and unspoiled plasma acceleration process. Strategies for generating clean and robust, dark current free plasma wake cavities are devised and analyzed, and crucial aspects for experimental realization of such optimized scenarios are discussed.

A novel, flexible method of witness electron bunch generation in plasma wakefield accelerators is described. A quasistationary plasma region is ignited by a focused laser pulse prior to the arrival of the plasma wave. This localized, shapeable optical plasma torch causes a strong distortion of the plasma blowout during passage of the electron driver bunch, leading to collective alteration of plasma electron trajectories and to controlled injection. This optically steered injection is more flexible and faster when compared to hydrodynamically controlled gas density transition injection methods.

In radiation pressure dominated laser ion acceleration schemes, transverse target deformation and Rayleigh-Taylor (RT)-like instability always develop quickly, break the acceleration structure, limit the final accelerated ion energy, and lower the beam quality. To overcome these issues, we propose a target design named dual parabola targets consisting of a lateral thick part and a middle thin part, each with a parabolic front surface of different focus positions. By using such a target, through interactive laser and target shaping processes, the central part of the thin target will detach from the whole target and a microtarget is formed. This enables the stable acceleration of the central part of the target to high energy with high quality since usual target deformation and RT-like instabilities with planar targets are suppressed. Furthermore, this target design reduces the laser intensity required to optimize radiation pressure acceleration by more than 1 order of magnitude compared to normal flat targets with similar thickness and density. Two-dimensional particle-in-cell simulations indicate that a quasimonoenergetic proton beam with peak energy over 200 MeV and energy spread around 2% can be generated when such a solid target (with density 400n_{c} and target thickness 0.5λ_{0}) is irradiated by a 100 fs long circularly polarized laser pulse at focused intensity I_{L}∼9.2×10^{21} W/cm^{2}.

A two-stage proton acceleration scheme using present-day intense lasers and a unique target design is proposed. The target system consists of a hollow cylinder with conical inner wall, which is followed by the main target with a flat front and a dishlike flared rear surface. At the center of the latter is a tapered proton layer, which is surrounded by side proton layers at an angle to it. In the first acceleration stage, protons in both layers are accelerated by target normal sheath acceleration. The center-layer protons are accelerated forward along the axis while the side protons are accelerated and focused towards them. As a result, the side-layer protons radially compress as well as axially further accelerate the front part of the center-layer protons in the second stage. Two-dimensional (2D) particle-in-cell (PIC) simulations show that a quasimonoenergetic proton bunch with the maximum energy over 250 MeV and energy spread ∼17% can be generated when such a target is irradiated with an 80 fs laser pulse with focused intensity 3.1×10^{20} W/cm^{2}. Three-dimensional (3D) PIC simulation gives the reduced maximum energy ∼112 MeV but even smaller energy spread ∼3% under the same laser conditions due to anisotropic electron acceleration with linearly polarized lasers.

Within the framework of plane-wave angular spectrum analysis of electromagnetic fields, a solution for the field of a tightly focused radially polarized (RP) chirped laser pulse is presented. With this solution, direct laser acceleration of protons by this kind of RP laser pulses is investigated numerically. It is found that a RP laser pulse with proper negative frequency chirps can lead to efficient proton acceleration, reaching sub-GeV at the laser intensity of 10^{22} W/cm^{2} from its injection energy of 45 MeV.

Controlled electron injection into a laser-driven wakefield at a well defined space and time is reported based on particle-in-cell simulations. Key novel ingredients are an underdense plasma target with an up-ramp density profile followed by a plateau and a fairly large laser focus diameter that leads to an essentially one-dimensional (1D) regime of laser wakefield, which is different from the bubble (complete blowout) regime occurring for tightly focused drive beams. The up-ramp profile causes 1D wave breaking to occur sharply at the up-ramp-plateau transition. As a result, it generates an ultrathin (few nanometer, corresponding to attosecond duration), strongly overdense relativistic electron sheet that is injected and accelerated in the wakefield. A peaked electron energy spectrum and high charge (~nC) distinguish the final sheet. [ABSTRACT FROM AUTHOR]