Expanding plasma structure and its evolution toward long wavelengths

The expansion of a plasma slab across an initially uniform magnetic field is simulated by the use of a two‐dimensional electromagnetic hybrid (particle ions, fluid electrons of nonzero mass) computer code. The expanding plasma develops magnetic‐field‐aligned structure on time scales faster than an ion gyroperiod. Through the full duration of the m i /m e =100 simulation, the structure wavelength is well predicted by the wavelength at maximum growth rate from the linear Vlasov theory of the lower hybrid drift instability modified by deceleration. At m i /m e =400, the late time structure wavelength is about 1.5 times the early time value. At m i /m e =1836, the structure wavelength at early times is close to that corresponding to the maximum growth rate of linear theory, while at later times the structure wavelength becomes about twice as long as its early time value. These three results suggest that the ratio of the late time wavelength to the early time value gradually increases with m i /m e . Extrapolation of this scaling to larger m i /m e values is consistent with structure wavelengths observed in an expanding aluminum plasma experiment [J. Appl. Phys. J. 2 0, 157 (1981)], as well as the observed wavelength in the expanding barium plasma of the AMPTE magnetotail experiment [J. Geophys. Res. 9 2, 5777 (1987)].