P. Y. Cheung

Nonlinear evolution of electron-beam-plasma interactions

The nonlinear evolution of the electron‐beam–plasma instability is investigated experimentally in two stages in a large, uniform, and unmagnetized plasma. In the initial stage, the beam‐driven, linearly unstable wave saturates due to changes in the beam distribution. At sufficiently intense wave amplitude, the instability evolves into a second stage where nonlinear effects due to the ponderomotive force dominate and the wave rapidly undergoes spatial collapse and contracts to very small scale lengths in the coexisting density cavity. The transverse contraction of the wave agrees with theories on three‐dimensional Langmuir collapse. This second stage can further be classified according to the beam strength. At low beam density, a single collapsing wave packet is formed and the electron beam decouples from the collapsing wave and propagates relatively unperturbed through the plasma. At higher beam density, multiple field spikes are formed both along and across the beam path and the beam distribution is broa...