Experimental studies of three-dimensional Langmuir collapse and strong turbulence effects in nonlinear electron beam/plasma interactions
The nonlinear evolution of the electron beam plasma instability is investigated in detail experimentally in a large, uniform, unmagnetized plasma using a cold, finite size electron beam. At sufficiently intense field strength, this nonlinear evolution can be divided into two stages. In the initial stage, beam nonlinearities dominate and the beam driven narrowband, unstable wave saturates after a period of linear growth. The possible saturation mechanism is observed to be beam trapping. In the second stage, nonlinearities due to the ponderomotive force dominate and the wave rapidly undergoes spatial collapse and contracts to very small scale lengths in a co-existing density cavity. The transverse contraction of the wave agrees with theories on three-dimensional Langmuir collapse. The collapse process depends on both the beam strength and the duration of the beam pulse. The number of intense field spikes increases as a function of beam density. At high beam density, a large density cavity trough is created and the collapsing waves dissipate their energy to the background electrons. At long beam pulse, the presence of the density cavity strongly influences the evolution of the instability as lower frequency, trapped waves are being created inside the density cavity and relaxation oscillations are observed. The growth and decay of the waves are modulated by the growth and relaxation of the density cavity and the waves can undergo several cycles of spatial collapse, forming intense wave bursts in time. A steady state was not observed in these experiments.
- Research Organization:
- California Univ., Los Angeles (USA)
- OSTI ID:
- 5475996
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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