Strong electric double layers are produced in a low-density plasma column confined by an axial magnetic field and maintained by single-ended inflow of plasma along the magnetic field. The double layer evolves from an anode sheath, and ionisation within the sheath is shown to be a significant process in this conversion. Once the double layer has been formed, its axial position can be controlled by the external electric circuit. The layer exhibits an axial motion back and forth with amplitudes somewhat larger than the layer thickness.
In this Letter an exact solution for nonlinear cold-electron-plasma oscillations against a periodic ion background is found. The main difference with respect to the known solution corresponding to a uniform ion background is that, after a while, electron density peaks will appear. This phenomenon, which was foreseen in general terms by Dawson, has possible practical implications which are indicated briefly. A thermal limitation on the peaks is given. PACS numbers: 52.35.Kt, 47.20.Ky, 52.35.Py Periodic ion distributions are assumed in several plasma contexts. For example, anomalous resistivity in turbulent plasmas is not well understood. 1,2 When the weak-turbulence collision frequency is less than the electron cyclotron frequency, the electron motion is essentially one dimensional. For this case it proves difficult to obtain appreciable anomalous resistivity from the weakturbulence theory. 3 To resolve this difficulty, a strongturbulence theory was presented in Ref.4. An array of ion density cavities was assumed.In the auroral region, electrostatic ion cyclotron waves may also give rise to density cavities. Computer simulations have been looked at in this context. 5 In computer simulations for plasmas with periodic ion density cavities it is usually assumed that the ion distribution remains constant in time. Simulations indicate the formation of strong electric field gradients. These have also recently been observed in the aurora. 6 In this Letter we look at a simple exact solution for cold-electron-plasma oscillations against a periodic ion background, assumed constant on the time scale of the electron motion. We find the salient features of the above-mentioned observations and simulations, 5,6 most importantly a steepening of the electric field gradient caused by the presence of the ion cavities.The problem of nonlinear cold-electron-plasma oscillations against a uniform ion background was solved many years ago by several authors simultaneously. 7 " 9 They found oscillations with the electron plasma frequency cop = (4xnoe 2 /m e ) 1/2 . Thus the frequency was amplitude independent. In general (unless the initial conditions were especially tailored such that at the onset the amplitude of the oscillations was at least y of the background density) the electron density was not found to exhibit explosive behavior. A good reference for this solution is Chapter 3 of Davidson's book. 9 However, Ref. 7 gives a heuristic argument for wave breaking (infinite densities) when the ion background is not uniform.Plasma oscillations under the influence of an applied sinusoidal field have been considered by several au- thors.The forcing term is usually assumed small. Enhanced electron density was obtained. The situation beyond breaking was considered by mathematical methods that are not universally accepted (such as altering Poisson's equation, changing the sign of the density when it becomes negative, etc.). Nevertheless, a theory of pump energy conversion to the plasma is obtained in the first of these references.In a recen...
Wave phenomena at a strong electric double layer are investigated experimentally in a magnetised plasma column where the electron gyro frequency is smaller than the plasma frequency. Fluctuations with frequencies below the ion plasma frequency and high frequency fluctuations, peaked close to the electron plasma frequency, are found to be dominating. The low and high frequency fields are shown to have characteristic spatial distributions with the maximum fields occurring in localised regions at the double layer. Spatial correlation measurements show that the high frequency field is associated with axially propagating waves. The phase velocity is nearly constant over a wide range of frequencies and 10 to 20 % smaller than the velocity of the electron beam that is formed in the layer. The double layer is characterised by a steep potential gradient in a central region with a thickness of about 20 Debye lengths. This region is surrounded by 'presheaths' where free electrons and ions are accelerated towards the layer. The role of the ionisation in the anode plasma is also discussed.
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