Particle simulations of efficient fast electron generation near the cutoff layer of an electrostatic wave

Fast electron generation near the cutoff of an electrostatic plasma wave is investigated by particle-in-cell simulations and test particle calculations. Intense electron plasma waves which are excited in an underdense plasma region propagate up the density gradient until they are reflected from the cutoff layer. The density gradient affects the fast electron generation by the wave considerably. At low densities, the phase velocity is fairly close to the thermal distribution, which leads to wave-particle interactions with a large electron population. The trapped electrons are accelerated by the electron plasma wave with increasing phase velocity resulting in a very large and energetic population behind the cutoff layer. Since the accelerating electrons receive energy, the wave must be damped. A simple model based on the conservation of the energy of the wave and the trapped electrons is developed to describe the damping mechanism.