Particle-in-cell Simulation Of Electromagnetic Pulse Generated By High-power Laser-target Interaction

Intensive electromagnetic pulses(EMP)can be generated when a high power laser strikes a target.The transient electromagnetic field can have an intensity up to several hundred kilovolts per meter with a broad frequency up to gigahertz,which may interfere or even damage electronic equipment.With the development of high power laser facilities(SG-III,NIF,LMJ/PETAL,et al.),the challenges regarding EMP become even more severe.Researchers have been trying to reveal the characteristics and mechanisms of EMP for effective shielding and better physical insight in laser plasma.It's widely accepted that hot electrons which eject from the target play an essential role.EMP can be related to the transient current of escaped electrons or target polarization.In this paper,EMP related to electrons emission is studied,and the numerical simulation is investigated.The finite-domain time-domain(FDTD)and particle-in-cell(PIC)method are used in the simulation.The PIC scheme involves calculation of electromagnetic fields,the motion of charged particles and coupling of fields and particles.The FDTD method is used for calculation of electromagnetic field.The differential expressions of Maxwell equations in Cartesian coordinate systems and cylindrical coordinate systems are given.Time-bias FDTD algorithm is studied and implemented to filter high frequency noise.The algorithm formulas for motion equations of charged particles are derived.The interactions between particles and fields are analysed.The generation of EMP in laser-target interaction is a complex process of multiphysics and multi-scale.Considering main physical effects,the whole process is divided into three stages for analysis and simulation.First hot electrons generated in the interaction of a laser pulse with a target are calculated by a two dimension PIC code LPIC-2D.Then hot electrons diffuse and collide on the target surface.A fraction of electrons are energetic enough to overcome the charge separation potential on the target surface,escape from the target and propagate to the chamber.According to the dynamics model of target charging established by the research team at the French Atomic Energy Commission(CEA),the total ejection charge is calculated.Finally,EMP radiated by the electrons that escape the target and flow in the vacuum chamber is simulated using the PIC method together with the time-bias FDTD method.A 2D cylinder chamber model where escaped electrons are ejected from the chamber centre is constructed.The characteristics of EMP are obtained.And the impacts of laser-target conditions,such as laser intensity,laser duration and target size,on EMP generation are discussed.The simulation results are consistent with the measurements.The study can provide basis and ideas for EMP protection in laser facilities.