An intermediate frequency (fci≤f≤fce) electrostatic instability has been observed in an electron beam produced, cylindrical plasma column. This instability has been identified as a new instability, the modified Simon–Hoh instability (MSHI), which has an instability mechanism similar to the Simon–Hoh instability (SHI). This instability can occur in a cylindrical collisionless plasma if a radial dc electric field exists and if this radial dc electric field and the radial density gradient are in the same direction. The origin of the dc electric field is found to be the difference between the ion and the electron radial density profiles. In such a plasma if the ions are essentially unmagnetized but the electrons are magnetized, a velocity difference in the θ direction can arise because of the finite ion Larmor radius effect. This leads to a space charge separation between the electron and ion density perturbations in the θ direction. The consequent perturbed azimuthal electric field Eθ1 and the enhancement of the density perturbation by the Eθ1×B0 velocity occur in the same manner as in the SHI. The instability frequency is decided by the ion azimuthal drift velocity. This new instability has been investigated through experiments and theory.

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The modified Simon-Hoh instability is observed in a collisionless cylindrical plasma, produced by a weak electron beam, in which electrons are strongly magnetized and the ions are essentially unmagnetized. The nonlinear evolution of this instability occurs through a sequence of sideband instabilities, thought to be induced by trapped ions, and can lead to a chaotic state.PACS numbers: 52.35.-g Turbulence abounds in plasmas [1] as is evident from measurements on laboratory, fusion, space, and astrophysical plasmas. However, it is extremely difficult to study the evolution of turbulence in a plasma because, in addition to having a rich variety of collective modes a of oscillation, a plasma also has many nonlinear coupling mechanisms. In this Letter we document the nonlinear evolution of a single coherent mode (a modified Simon-Hoh instability) in a plasma produced by a weak electron beam via a sequence of sideband instabilities that are thought to arise because of ion trapping effects. While the eventual frequency-locked states observed in these experiments appear to be of the generic period-doubling type [2], the transitions are not. As in some fluid experiments [2] each transition begins with the appearance of an apparently new oscillation mode at a very low frequency after which the frequency rises to half the value of the previous lowest frequency where it meets and locks with the decreasing-frequency local lower sideband frequency. We believe that such a sequence of successive sideband instabilities and locking may be common to plasma systems in which large ion orbit dynamics are important and may lead to understanding their transition to turbulence.To be able to exhibit the fine details of the transitions such as those shown here, the plasma must be extremely controllable. The low electron density (n e~~ 10 6 -10 9 cm -3 ) plasma is generated by ionization of background gas by a very-low-density electron beam («^ -10 4 -10 7 cm -3 ). A Gaussian, temperature-limited, electron beam (7^ -1 M-l mA, 250 V) with 5 mm (FWHM) diameter is injected axially from one end of a 10-cm-diam, 180cm-long stainless-steel tube immersed in a dc axial magnetic field (5 -50-320 G). The beam and the plasma are terminated by a grounded collector located at 80 cm distance from the gun. The plasma density can be controlled by varying the ionization rate either by changing the gas fill pressure (5xl0~6 to 5xl0~5 torr) or by varying the beam current over its range. At these low values of pressure and current, the system is always below the beam-plasma discharge [3] threshold and thus can be kept remarkably free of noise.In this experiment (5 = 160 G, Ar plasma) the electrons are strongly magnetized (ri e =0.04 cm for T e -4 eV) whereas the ions are essentially unmagnetized (r Li -5.6-17.7 cm for Tj±~~\-\0 eV). Here ru and ru are

Drift waves in a cylindrical plasma column with an exponential density profile have a Bessel function mode structure and are linearly unstable because of finite Larmor radius effects. In the presence of a lower hybrid eigenmode pump, these modes suffer a nonlinear frequency upshift because of the ponderomotive force and tend to be stabilized when k0z>kz.

Experiments have been conducted to find the variation of the velocity of an acoustic wave propagating through air plasma. It is found that the velocity of sound increases with increasing charged particle density in the plasma. Also it has been observed that as the frequency of the acoustic wave through the plasma increases, the perturbed variation in charged particle density and the electron-neutral atom elastic collision frequency remain constant as long as the frequency of the sound wave is less than the ionisation frequency of the plasma. For further increase in the wave frequency, equal to, or greater than, the ionisation frequency of the plasma, the change in the number density reduces to a minimum value while the elastic collision frequency of the plasma increases.