Parallel Alfvénic shock with particles injected at a fixed rigidity of R(0) = 10^-2. Shown are the cases without (left panels) and with (right panels) turbulent electric fields. As shown in the upper panels, a strong shock efficiently accelerates the particle ensemble. The acceleration is strongest at the beginning, when all particles are close to the shock front. Due to parallel diffusion, many particles will not encounter the shock again after the initial boost, whereas others continue to get accelerated. This is apparent from the lower panels, which also show that some particles initially lose energy or never cross the shock to get accelerated.