On the (Apparent) Paradox between Space-Charge Forces and Space-Charge Effects

International audienceWith the advent of high-intensity linacs, space charge forces are now well known as a major issue causing undesirable effects on particle beam qualities like emittance growth or sudden losses. They should be stronger when there are more particles or when the latter are contained in a smaller volume. But a detailed examination of the beam along an accelerator show that space charge effects are weaker where the beam size is smaller. This article clarifies this paradox and revisits the recommendations on beam sizes in view of mitigating space charge effects

Preserving emittance by matching out and matching in plasma wakefield acceleration stage

In more than four decades, particle acceleration by plasma wakefield has demonstrated its feasibility and efficiency. This acceleration technique is now starting to be planned for providing high-quality beams to well-defined user communities. High beam energy is also considered by piling successive plasma acceleration stages. In this context, avoiding beam degradation, on top of all emittance degradation, is the main concern when transferring the accelerated beam to the users or to the following acceleration stage. After examining the behavior of the trace and the phase emittances when crossing through a conventional transfer line, we are able to determine the criteria to be achieved in the plasma ramps so as to minimize emittance growth. Then the optimal density profile is studied for these ramps at the entrance and exit of a plasma stage accelerating electrons from the energy of 150 MeV to 5 GeV. Finally, the design of an optimal transfer line allows showing that the emittance growth can be contained to less than 10% in realistic conditions when transferring a beam to a free-electron laser

Slice Energy Spread Optimization for a 5 GeV Laser-Plasma Accelerator

International audienceGeV-scale laser-plasma accelerating modules can be integrated into a multi-staged plasma linac for driving compact X-ray light sources or future colliders. Such a plasma module, operating in the quasi-linear regime, has been designed for the 5 GeV laser plasma acceleration stage (LPAS) of the EuPRAXIA project. Although it can be employed to optimize the total energy spread, the beam loading effect introduces an non-negligible slice energy spread to the beam. In this paper, we study the slice energy spread from linear theory, establishing a relationship between it and the laser-plasma parameters. To reduce the slice energy spread, simulations have been carried out for various plasma densities and laser strengths. The results will be discussed and compared with the theory

Design of a 5 GeV laser–plasma accelerating module in the quasi-linear regime

International audienceMulti-GeV-class laser–plasma accelerating modules are key components of laser–plasma accelerators, because they can be used as a booster of an upstream plasma or conventional injector or as modular acceleration sections of a multi-staged high energy plasma linac. Such a plasma module, operating in the quasi-linear regime, has been designed for the 5 GeV laser–plasma accelerator stage (LPAS) of the EuPRAXIA project. The laser pulse ( ∼ 150 TW, ∼ 15 J) is quasi-matched into a plasma channel ( np=1.5×1017 cm −3 , L∼ 30 cm) and the bi-Gaussian electron beam is externally injected into the wakefield. The beam emittance is preserved through the acceleration by matching the beam size to the transverse focusing fields. And a final energy spread of < 1% has been achieved by optimizing the beam loading effect. Several methods have been proposed to reduce the slice energy spread and are found to be effective. The simulations were conducted with the 3D PIC code Warp in the Lorentz boosted frame

Toward low energy spread in plasma accelerators in quasilinear regime

In this paper, we address the energy spread and slice energy spread of an externally injected electron beam in plasma wakefield accelerators operating in the linear or quasilinear regime. The energy spread is first derived taking into account the phase dependence of the wakefield along the finite-length bunch together with the dephasing during acceleration and found to be strongly dependent on the bunch length. This could be compensated by the beam loading effect, the energy spread from which is then derived and found to be nearly independent of the bunch length. However, the transverse dependence of the beam loading effect also makes the particles at the same longitudinal position experience different accelerating fields, introducing a significant slice energy spread. To estimate the slice energy spread, a theoretical analysis was conducted by taking the transverse betatron motion into account. As a study case, 3D simulations for the 5 GeV laser-plasma acceleration stage of the European Plasma Research Accelerator with eXcellence in Applications project have been performed. Careful optimization of the parameters allows one to obtain an energy spread of ≤1% and a slice energy spread of ≤0.1%, with good agreement between theories and simulations

Avoiding Emittance Degradation When Transferring the Beam From and to a Plasma-Wakefield Stage

International audienceThe plasma-wakefield acceleration technique is known to provide a very strong accelerating gradient (GV/m), up to three orders of magnitude higher than the conventional RF acceleration technique. The drawback is a relatively higher energy spread and especially a huge beam divergence at the plasma exit, leading to an irremediable and strong emittance degradation right after its extraction from the plasma for transferring it to an application or another plasma stage. In this article, we determine the criteria to be achieved so as to minimize this emittance growth after pointing out all the parameters involved in its mechanism. Then the plasma down ramp profile is studied in a typical configuration of the EuPRAXIA project at 5 GeV. It turns out that no specific profile is needed. For minimizing emittance growth at beam extraction, it is enough to optimize the ramp length so that the Twiss parameter γ is minimized. Finally the design of an optimal transfer line allows showing that the emittance growth can be contained to less than 10% in realistic conditions when transferring the beam to a free electron laser

Beam Dynamic Studies for the SARAF MEBT and SC Linac

International audienceThe SARAF MEBT and Super Conducting Linac (SCL) transport and accelerate deuterons or protons from the RFQ to the final energy. In this report, beam dynamics studies for this section are described. A rational distribution of the different roles of the MEBT leads to defining its necessary quadrupole/rebuncher composition. This allows easy beam re-tuning following changes from the RFQ or the SC Linac. After observing evidences of beam losses mainly due to phase unhooking, efforts have been dedicated to enlarge the SCL longitudinal acceptance. A combination of cavity field phases is found so that the required final beam energy is also fulfilled

Basic Relations of Laser-Plasma Interaction in the 3D Relativistic, Non-Linear Regime

International audienceIn the approximation where the plasma is considered as a fluid, basic relations are derived to describe the plasma wave driven by an ultra-intense laser pulse. A set of partial differential equations is obtained. It is then numerically solved to calculate the resulting 3D electric field structure that can be used as accelerating cavities for electrons. The laser strength parameter is varied to investigate regimes from weakly nonlinear up to total cavitation where all the initial electrons of the plasma are expelled

Core-halo issues for a very high intensity beam

The relevance of classical parameters like beam emittance and envelope used to describe a particle beam is questioned in case of a high intensity accelerator. In the presence of strong space charge effects that affect the beam differently following its density, the much less dense halo part behaves differently from the much denser core part. A method for precisely determining the core-halo limit is proposed, that allows characterizing the halo and the core independently. Results in 1D case are given and discussed. Expected developments extending the method to 2D, 4D, or 6D phase spaces are examined