Nonlinear Effects in Near-Critical Density Laser Wakefield Acceleration*

Conventional Laser Wakefield Acceleration relies on low density plasmas so that the group velocity of the laser is near the speed of light. This allows the the laser to excite plasmon (electron langmuir) waves with a similar phase velocity. The high phase velocity is far from the thermal distribution and so the train of wakes left behind by the laser are stable from thermal instabilities and dissipation effects. However, we have recently found that at near critical densities (≥ 0.25 n_c) there are nonlinear effects that allow for the laser excite various electrostatic modes with high to low phase velocity. These nonlinear excitations then continue to beat together and cascade to excite further modes with lower phase velocity. This process allows for self injection and efficient laser to particle acceleration with the use of lower intensity (≤ 10¹⁷ W/cm²) single laser, and even lower intensities for a pair of beatwave lasers (≤ 10¹⁶ W/cm²) to obtain near MeV energies. In this presentation, we analyze the nonlinear processes (nonlinear conductivity, parametric instabilities of higher order) that allow for these processes with the use of Particle-in-Cell and hydrodynamic simulations in 1D. The use of lower intensities could allow for numerous applications such as radiation therapy and medical sterilization.