High-energy physics frontier is approaching the teraelectron-volt energy range for the study of the Higgs particle. To reach teraelectron-volt energy for charged particles, a high-gradient acceleration structure is desirable. Recent plasma wakefleld acceleration (PWFA) experiment has shown sustained acceleration gradient of 50 GeV/m in a meter-long plasma [1]. To study the relevant physics in an "afterburner" concept [2], i.e., an energy booster based on PWFA for the ILC-type parameters, we conduct 3-D particle-in-cell (PIC) simulation of the nonlinear beam-plasma interactions. A multiscale PIC model is developed using the quasi-static description of the interaction. We present images from the simulation, which illustrate the physical picture of an afterburner and the evolution of the accelerated beam