A multi-dimensional particle-in-cell code KLAP is introduced. Field ionization, impact ionization and two body collision are considered in a one-dimensional code. Moving window technology is used in a three-dimensional code to study laser wakefield acceleration in GeV region. Terahertz radiation, laser propagation in neutral gas medium, surface electron acceleration as well as GeV electron generation in laser wakefield acceleration are studied.

A charge-conserving current assignment algorithm based on current conservation equation proposed by Villasenor and Buneman is introduced.It is found that the algorithm is not available as particle boundary conditions are adopted. With typical boundary conditions amendment methods are proposed in which the simulation satifies charge conservation.