Numerical studies of driven, chirped Bernstein, Greene, and Kruskal modes

Abstract

Recent experiments showed the possibility of creating long-lived, nonlinear kinetic structures in a pure-electron plasma. These structures, responsible for large-amplitude periodic density fluctuations, were induced by driving the plasma with a weak oscillating drive, whose frequency was adiabatically decreased in time [W. Bertsche, J. Fajans, and L. Friedland, Phys. Rev. Lett. 91, 265003 (2003)]. A one-dimensional analytical model of the system was developed [L. Friedland, F. Peinetti, W. Bertsche, J. Fajans, and J. Wurtele, Phys. Plasmas 11, 4305 (2004)], which pointed out the phenomenon responsible for the modifications induced by the weak drive in the phase-space distribution of the plasma (initially Maxwellian). In order to validate the theory and to perform quantitative comparisons with the experiments, a more accurate description of the system is developed and presented here. The new detailed analysis of the geometry under consideration allows for more precise simulations of the excitation process, in which important physical and geometrical parameters (such as the length of the plasma column) are evaluated accurately. The numerical investigations probe properties and features of the modes not accessible to direct measurement. Due to the presence of two distinct time scales (because of the adiabatic chirp of the drive frequency), a fully two-dimensional numerical study of the system is expected to be rather time consuming. This becomes particularly important when, as here, a large number of comparisons (covering a wide range of drive parameters) are performed. For this reason, a coupled one-dimensional, radially averaged model is derived and implemented in a particle-in-cell code.