Modeling Plasma Probes with Speed-Limited Particle-In-Cell Simulation

Langmuir probes are widely used to measure density and temperature in laboratory and space plasmas. However, interpretation of probe data is often difficult because of non-ideal plasma conditions, such as complicated geometry, non-zero electric fields due to proximity to the chamber or spacecraft, magnetic fields, moderate collisionality, secondary emission, etc. Particle-in-cell (PIC) simulations can robustly include all these effects, but often require excessive computation time. The Speed-limited PIC (SLPIC) approach is a modification of PIC that can speed up simulation by roughly the square root of the ion/electron mass ratio. In a steady state, this approach is equivalent to the numerical timestepping method, which evolves electrons and ions using different timesteps; for steady-state electrostatic simulations, this method can be realized simply by performing a simulation with electrons and positrons (reduced-mass ions), and scaling the resulting ion velocities and currents appropriately. However, SLPIC offers a way to simulate slow time-dependence as well as the steady state. We demonstrate these approaches by simulating a Langmuir probe in electron-argon plasma, reducing simulation time by more than a factor of 100.