Nonlinear electron magnetohydrodynamic physics. VII. Magnetic loop antenna in a field-free plasma

Stenzel, R. L.; Urrutia, J. M.; Strohmaier, K. D.
February 2009
Physics of Plasmas;Feb2009, Vol. 16 Issue 2, pN.PAG
Academic Journal
Nonlinear whistler phenomena near a magnetic loop antenna in a field-free plasma have been investigated experimentally. The loop field oscillates at a frequency far below the electron plasma frequency, hence all linear electromagnetic modes are cut off. However, the peak antenna field is so large that the electrons become magnetized allowing whistler modes to exist in the near zone of the antenna. The shielding magnetic field propagates at a speed which increases with magnetic field strength and decays slower than the rf period, resulting in a remnant field when the antenna field vanishes. A field-reversed configuration (FRC) is produced when the antenna field reverses direction. The FRC expands into the magnetized plasma and produces self-consistent magnetic helicity consistent with that of whistler modes. Thus, the new field penetrates in the whistler mode in a background field left over from the previous half-cycle. The electrons become unmagnetized at large distances, and the field convection goes over into field diffusion. Observations of light emission indicate electron energization. Current-driven instabilities produce magnetic oscillations with frequencies much higher than the applied one. Small amplitude, high-frequency whistlers propagate in the large amplitude fields of the low-frequency whistler, but become absorbed at larger distances where the cyclotron frequency decreases below the wave frequency. The nonlinear phenomena observed may be relevant to inductively coupled plasma sources, laser-plasma interactions, plasma opening switches, and active experiments in space.


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