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Wakefield excitation by structured electron bunches in hollow gaps between plasma wedges is studied using three-dimensional particle-in-cell simulations. The main part of the electron bunch has a triangular current distribution in the longitudinal direction with a smooth head and short tail. These bunches propagate stably in the hollow gap while being attached to cusps of the plasma wedges. The excited wakefield profile may have a very high transformer ratio and allows to accelerate witness bunches to energies much higher than that of the driver bunch. Unlike round hollow channels, where asymmetric wakefields are difficult to avoid, no deleterious transverse beam break-up (BBU) is observed in the gap between cusp-shaped plasma layers.

Polarized fusion has long ago be proposed as a method to strongly increase the efficiency of fusion reactors. However, the required nuclear spin-polarization conservation in fusion plasmas has never been proven experimentally. Here we report on first experimental data suggesting an increased ion flux from a polarized $^3\mkern-1mu$He target heated by a PW laser pulse as well as evidence for an almost complete persistence of their nuclear-polarization after acceleration to MeV energies. These findings also validate the concept of using pre-polarized targets for plasma acceleration of polarized beams.

We investigate Collisionless Shock Acceleration of spin-polarized ${}^3$He for laser pulses with normalized vector potentials in the range $a_0 = 100-200$. The setup utilized in the 2D-PIC simulations consists of a solid Carbon foil that is placed in front of the main Helium target. The foil is heated by the laser pulse and shields the Helium from the highly oscillating fields. In turn, a shock wave with more homogeneous fields is induced, leading to highly polarized ion beams. We observe that the inclusion of radiation reaction into our simulations leads to a higher beam charge without affecting the polarization degree to a significant extent., Comment: 13 pages, 7 figures

Strong magnetic fields at plasma-plasma interfaces can be naturally produced in laser-plasma interactions. Using theoretical analysis and fully three-dimensional particle-in-cell simulations, we demonstrate that relativistic topological waves can be generated via Cherenkov and Doppler resonances in the interaction of intense femtosecond laser pulses with near-critical-density plasmas. At the self-magnetized plasma-plasma interface, a new slow-wave branch appears. Its phase velocity is much smaller than the group velocity of the laser pulse and the electron beam velocity. Therefore, the Cherenkov resonance condition can be easily satisfied. Furthermore, since electrons undergo betatron oscillations, Doppler resonances may also occur and are responsible for the excitation of several frequency-shifted branches observed in our simulations. After the passage of the laser pulse, we observe a fast remnant mode with relativistic amplitude and frequency close to the local plasma frequency. This mode continues to accelerate electrons further for many tens of laser periods even after the laser pulse has left the plasma.

We present a setup for highly polarized proton beams using two parallel propagating laser pulses that have a carrier envelope phase difference of $\pi$. This mechanism is examined utilizing particle-in-cell simulations and compared to a single-pulse setup commonly used for magnetic vortex acceleration. We find that the use of the dual-pulse setup allows for peak energies of 124 MeV and good angular spread for two pulses with normalized laser vector potential $a_0 = 100$. Compared to a single pulse, we further observe higher polarization of the accelerated bunch., Comment: 7 pages, 5 figures

We propose a setup for positron acceleration consisting of an electron driver and a laser pulse creating a two-fold plasma column structure. The resulting wakefield is capable of accelerating positron bunches over long distances even when evolution of the driver is considered. The scheme is studied by means of particle-in-cell simulations. Further, the analytical expression for the accelerating and focusing fields are obtained, showing the equilibrium lines along which the witness bunch is accelerated., Comment: 6 pages, 5 figures

We investigate the effect of density down-ramps on the acceleration of ions via Magnetic Vortex Acceleration (MVA) in a near-critical density gas target by means of particle-in-cell simulations. The spin-polarization of the accelerated protons is robust for a variety of ramp lengths at around 80%. Significant increase of the ramp length is accompanied by collimation of low-polarization protons into the final beam and large transverse spread of the highly polarized protons with respect to the direction of laser propagation., Comment: 10 pages, 4 figures

Available analytical wakefield models for the bubble and the blow-out regime of electron-plasma acceleration perfectly describe important features like shape, fields, trapping ratio, achievable energy, energy distribution and radial emittance. As we show, for wakefields with an extremely small amplitude these models fail to describe the accelerating electric field and its divergence in the wakefield rear. Since prominent parameter regimes like the Trojan horse regime of photocathode injection exhibit this feature, it is of great importance to work out analytical models that fix this problem; one possible model is introduced in this work. Using a phenomenological theory, we are able to better describe the divergence of the electric field and the bubble shape.

We study the influence of finite emittance electron bunches in the bubble regime of laser-driven wakefield acceleration onto the microscopic structure of the bunch itself. Using resilient backpropagation (Rprop) to find the equilibrium structure, we observe that for realistic and already observed emittances the previously found crystalline structures remain intact and are only widened marginally. Higher emittances lead to larger electron displacements within the crystal and finally its breaking., Comment: 5 pages, 3 figures

We present a theory for describing the inner structure of the electron bunch in the bubble regime starting from a random distribution of electrons inside the bubble and subsequently minimizing the system's energy. Consequently, we find a filament-like structure in the direction of propagation that is surrounded by various shells consisting of further electrons. If we specify a 2D initial structure, we observe a hexagonal structure for a high number of particles, corresponding to the close-packing of spheres in two dimensions. The two-dimensional structures are in agreement with the equilibrium slice model (ESM)., Comment: 7 pages, 9 figures; version 2 with improved figures and references to similar results in conventional accelerators

The spatial structure of an ultra-low emittance electron bunch in a plasma wakefield blowout regime is studied. The full Li\'{e}nard-Wiechert potentials are considered for mutual inter-particle interactions in the framework of the equilibrium slice model (ESM). This model uses the quasi-static theory which allows to solve the Li\'{e}nard-Wiechert potentials without knowledge of the electrons' history. The equilibrium structure we find is similar to already observed hexagonal lattices but shows topological defects. Scaling laws for interparticle distances are obtained from numerical simulations and analytical estimations.