V.I. Sotnikov

Excitation of electron acoustic waves near the electron plasma frequency and at twice the plasma frequency

In this paper we investigate the nonlinear development of the electron acoustic instability that can lead to the transfer of wave energy to frequencies just above the electron plasma frequency (ωpe) and to waves with approximately twice the electron plasma frequency (2ωpe). Using plasma conditions in the upstream electron foreshock region based on data from the AMPTE-UKS spacecraft, an electron beam is considered in plasma containing a background of hot and cold electrons. This leads to the linear excitation of large-amplitude electron acoustic waves at frequencies between about 0.8 and 1.0 ωpe. A modified decay instability then excites waves in the spectrum just above ωpe. This is followed by a second nonlinear coalescence process that causes the excitation of waves at frequencies just below 2ωpe. The linear and nonlinear properties of the electron acoustic instability are examined for observed conditions using analytical theory, particle-in-cell simulations, and Vlasov simulations. These results have application to observations made inside the electron foreshock region, as well as the polar cap and auroral zone, where plasma oscillations and waves at 2ωpe are observed.

Linear analysis of sheared flow stabilization of global magnetohydrodynamic instabilities based on the Hall fluid model

A systematic study of the linear stage of sheared flow stabilization of Z-pinch plasmas based on the Hall fluid model with equilibrium that contains sheared flow and an axial magnetic field is presented. In the study we begin with the derivation of a general set of equations that permits the evaluation of the combined effect of sheared flow and axial magnetic field on the development of the azimuthal mode number m=0 sausage and m=1 kink magnetohydrodynamic (MHD) instabilities, with the Hall term included in the model. The incorporation of sheared flow, axial magnetic field, and the Hall term allows the Z-pinch system to be taken away from the region in parameter space where ideal MHD is applicable to a regime where nonideal effects tend to govern stability. The problem is then treated numerically by following the linear development in time of an initial perturbation. The numerical results for linear growth rates as a function of axial sheared flow, an axial magnetic field, and the Hall term are reported.

Investigation of Magnetic Fields in 1-MA Wire Arrays and

A Faraday rotation diagnostic was applied for the investigation of magnetic fields in plasma of 1-MA wire arrays and X-pinches. Laser-probing diagnostics at the Zebra generator include a four-channel polarointerferometer and a four-frame shadowgraphy. The Faraday rotation diagnostic consists of shadow and Faraday channels, shearing air-wedge interferometer, and an additional schlieren channel. The implosion dynamics of the wire arrays were studied. A current in the plasma column of Al low-wire number arrays was found by the Faraday rotation diagnostic. Optical diagnostics showed a turbulent plasma and bubblelike objects in the plasma column of Al wire arrays. The Faraday rotation diagnostic demonstrated a complicated structure of magnetic fields in X-pinch plasma

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In this paper the generation of electrostatic wake field and electron acceleration in this field by intense laser pulse propagating in plasma in the presence of external magnetic field is investigated. It is shown that if the change in the laser pulse wave amplitude (approximation of the constant laser pulse wave field) is not taken into account, it is possible by switching to Lagrangian variables to reduce the system of equations describing excitation of the wake field and electron acceleration to a simplified system of ordinary differential equations (ODE), which can be easily integrated. This makes it possible to find the energy, transferred to electron oscillations in the wake field for different angles between the direction of laser pulse propagation and magnetic field. Stability of the laser pulse propagating with arbitrary angle to external magnetic field towards excitation of parametric and decay instabilities with involvement of upper hybrid and lower hybrid waves is investigated, as well. Equati...

-Pinches

The excitation of electron acoustic waves by a gyrating electron beam in a plasma with cold and hot electron components has been examined. Different instabilities are possible due to Cerenkov, normal, and anomalous Doppler resonances. The Cerenkov and anomalous Doppler resonances lead to the excitation of intermediate- and short-wavelength parts of the electron cyclotron sound dispersion branch with frequencies well above the electron cyclotron frequency but below the electron plasma frequency, i.e., Ωe ≪ ω < ωpe. The instability due to the normal Doppler resonance leads to the excitation of intermediate- and short-wavelength parts of this branch as well. In addition, due to the normal Doppler resonance, waves with much larger wavelengths and frequencies ω ≳ Ωe can also be excited. These results are applied to active beam injection in the Earths low-altitude ionosphere during the CHARGE 2B rocket experiment, but also have applications to other regions of the magnetosphere where waves are observed in the frequency range between the electron gyrofrequency and electron plasma frequency.

Interaction of powerful laser pulse with magnetized plasma

Cluster observations in the near-Earth plasma sheet boundary layer (PSBL) region have shown the presence of ion shell distributions related to velocity-dispersed ion structures (VDIS) coincident with electrostatic emissions. We have examined ion shell instabilities in the presence of a cold ion and electron background using linear theory and particle in cell simulations. Linear theory shows that the shell instability is only excited when a cold ion background is present, generating a broad range of ion cyclotron harmonics. Numerical simulations confirm that ion Bernstein modes are preferentially excited transverse to the ambient magnetic field, along with a lower level of wave power at oblique angles. The background ions are heated primarily in the transverse direction because of a linear nonstochastic ion cyclotron heating mechanism, and overall saturation of the instability occurs because of thermalization of the shell combined with heating of the background ions and electrons. Comparison with Cluster observations shows that observed electrostatic waves with a spectrum from a few Hz to several hundred Hz is in good agreement with that expected from the shell instability. The cold background ions are observed to have a temperature anisotropy with T⊥ > T∥, while electrons are observed to have T∥ > T⊥, also in qualitative agreement with the shell instability. The wave-particle effects due to the shell instability in the near-Earth PSBL could have important consequences for auroral potential structure at lower altitudes and may cause the gaps in VDIS structure leading to beamlets.

Excitation of electron acoustic waves by a gyrating electron beam

Ablation and implosion dynamics were investigated by optical probing in linear wire arrays of different geometry. Formation of ablation jets begins on the outermost wires. In the beginning of implosion plasma bubbles arise in breaks on the outer wires. Implosion bubbles move to the next wire in the array and hit the plasma column with the speed >250km∕s. Imploding plasma moves to the center of the array cascading from wire to wire. Configuration of magnetic fields in the linear array can be changed by variation of wire spacing. The regimes of ablation and implosion in the wire arrays are found to differ with different wire spacing.

Instabilities driven by ion shell distributions observed by Cluster in the midaltitude plasma sheet boundary layer

Interferometry and two-frame schlieren imaging were used to study arc discharge evolution in a small-gap, coaxial, magnetically insulated transmission line driven by a 2-TW generator with a current pulse rise time of 70 ns. Two kinds of plasma objects were observed in experiments: plasma of arc discharges and low-density peripheral plasma. Plasma fills most of the magnetically insulated transmission line (MITL) gap in the area of the arc and produces a stripe trace of evaporated metal on the surface of electrodes. Arc discharge typically arises near the cathode. Anode plasma arises in the later stage, after which, the plasma fills the gap. A scenario of plasma evolution of the arc discharge is discussed. Low-density plasma is located in thin layers near the cathode or the anode. It plays a role in the seeding of arc discharges that grow before the closure of the gap and dissipates after the closure.

Investigation of ablation and implosion dynamics in linear wire arrays

Implosion of wire arrays was investigated at the 1MA Zebra accelerator by multiframe laser probing and gated x-ray self-emission diagnostics. Different regimes of implosion were observed in Al and Cu wire arrays. Implosion of Al loads with masses of 33–37μg∕cm produces a dense pinch 1–1.5mm in diameter. Strong instabilities are observed in the Z pinch at the time of stagnation. Implosion of “overmassed” loads produces a plasma column 3–4mm in diameter with a core. The plasma column does not collapse during the x-ray pulse. The core of the plasma column is not subjected to the kink instability and transforms to a chain of dense spots in the later stage. Different regimes of implosion were observed in Al 8×15μm loads presumably due to variations in the current pulse and load conditions. Observed regimes are compared to three-dimensional hybrid simulation of ideal and nonideal magnetohydrodynamics modes of implosion.

Investigation of plasma evolution in a coaxial small-gap magnetically insulated transmission line

The development of global magnetohydrodynamic (MHD) instabilities in Z-pinch plasmas has been studied with a three-dimensional hybrid simulation model. Plasma equilibria without and with axial sheared flow, and with different values of the parameter eH, which appears as a coefficient before the Hall term in dimensionless nonideal MHD equations, have been considered. Increasing the parameter eH leads to larger simulation growth rates for both m=0 sausage and m=1 kink modes. The hybrid simulations do however show that axial sheared flow severely curtails the linear and nonlinear development of both sausage and kink instabilities. In these respects, the hybrid simulations are in qualitative agreement with linear Hall MHD results. Moreover, in the nonlinear stage, long wavelength modes dominate the excited wave spectrum when the parameter eH is small. For the larger value of the parameter eH, small-scale structures do however develop nonlinearly in the excited wave spectrum at late times.