Propagation of a Strong Circularly Polarized Electromagnetic Wave through Afterglow Plasmas near the Electron Cyclotron Frequency

1971 ◽  
Vol 49 (22) ◽  
pp. 2797-2824 ◽  
Author(s):  
I. P. Shkarofsky
Keyword(s):  
Distribution Function ◽  
Electric Field ◽  
Electron Density ◽  
Cyclotron Resonance ◽  
Collision Frequency ◽  
Strong Field ◽  
Velocity Distribution Function ◽  
Decay Rates ◽  
Relaxation Processes ◽  
Experimental Conditions

We seek to explain the experimental observation of enhanced transmission of a strong right-hand circular wave near cyclotron resonance in an afterglow helium slab plasma occurring at earlier times than for a corresponding weak electromagnetic field. A strong field heats the electrons and alters the collision frequency and the velocity distribution function. For our experimental conditions, electron–electron relaxation processes can influence the distribution function. Furthermore, because of the field dependent absorption coefficient, the temperature and field strength can vary within the plasma slab. Although the collision frequency increase with temperature predicts an earlier time for stronger-field transmission at cyclotron resonance, none of the above processes is sufficient to explain the experimentally observed large spread in times of transmission for various electric field strengths. The explanation is ascribed to the fact that the electron density decay constant due to diffusion also increases with temperature, so that the electron density decay rates vary with electric field resulting in a larger spread in times of transmission. Experimental results from double probes, although operating under low sensitivity, suggest a decrease in density near the front edge of the plasma subject to strong fields.

Translational freezing in free jet expansions, with special reference to molecular beam formation

1969 ◽  
Vol 47 (12) ◽  
pp. 2161-2165 ◽  
Author(s):  
Rodney L. LeRoy ◽  
Jacques M. Deckers
Keyword(s):  
Distribution Function ◽  
Transition Region ◽  
Molecular Beam ◽  
Collision Frequency ◽  
Collision Cross Section ◽  
Experimental Studies ◽  
Velocity Distribution Function ◽  
Free Jet ◽  
Beam Formation ◽  
Good Agreement

A theoretical model is described which can be used to calculate the velocity distribution function in the transition region of a free jet expansion. It makes use of a simple mechanism to account for the way in which collisions "perturb" the distribution function which would apply in the absence of collisions. The results are compared with experimental studies of argon beams isolated from free jet sources. If the collision frequency is calculated using a hard sphere collision cross section of (25 ± 5) Å2, good agreement with experimental beam intensity profiles is obtained. In the transition region computed values of the bulk mass velocity, and of the parallel and perpendicular temperatures, are intermediate between the values which would be found if the flow were collision-dominated or free molecular. In particular the perpendicular temperature varies slowly from a dependence on the −4/3 power of the distance from the source, approaching monotonically a −2 power dependence at large distances.

New kinetic cyclotron instability for electron beam in time-changing magnetic fields

2020 ◽  
Vol 86 (3) ◽  
Author(s):  
Irena Vorgul ◽  
M. Ayling ◽  
C. R. Straub ◽  
D. M. MacKay ◽  
J. D. Houghton ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Distribution Function ◽  
Velocity Distribution ◽  
Cyclotron Resonance ◽  
Cyclotron Frequency ◽  
Resonance Condition ◽  
Velocity Distribution Function ◽  
Spatial Gradient ◽  
Total Change ◽  
The Magnetic Field

This paper examines the velocity distribution function and cyclotron resonance conditions for a beam of electrons moving in a magnetic field which gradually changes with time. A spatial gradient of magnetic field is known to result in an unstable horseshoe distribution of electrons. The field gradient in time adds additional effects due to an induced electric field. The resultant anisotropic velocity distribution function, which we call a Luvdisk distribution, has some distinctive properties when compared to the horseshoe. Fitting the cyclotron resonance condition circle shows that the frequency of the resultant emission is under the local cyclotron frequency. While the spatial gradient results in the emission coming almost perpendicularly to the field, the direction of the radiation under a time-changing field has more variability. The Luvdisk distribution also arises when the magnetic field has a gradient both in space and time. The beam can be unstable if those gradients are added or subtracted from each other (if the gradients are of equal or different sign), which occurs even when the total change of magnetic field is negative. While the frequency of the emission is related to the final magnetic field value, its direction is indicative of the field’s history which produced the instability.

scholarly journals On relaxation processes in a completely ionized plasma

2020 ◽  
Keyword(s):  
Distribution Function ◽  
Electric Field ◽  
External Electric Field ◽  
Electron Distribution ◽  
Electron Distribution Function ◽  
Relaxation Times ◽  
Relaxation Processes ◽  
Energy And Momentum ◽  
Energy And Momentum Densities ◽  
Relaxation Coefficients

Relaxation of the electron energy and momentum densities is investigated in spatially uniform states of completely ionized plasma in the presence of small constant and spatially homogeneous external electric field. The plasma is considered in a generalized Lorentz model which contrary to standard one assumes that ions form an equilibrium system. Following to Lorentz it is neglected by electron-electron and ion-ion interactions. The investigation is based on linear kinetic equation obtained by us early from the Landau kinetic equation. Therefore long-range electron-ion Coulomb interaction is consequentially described. The research of the model is based on spectral theory of the collision integral operator. This operator is symmetric and positively defined one. Its eigenvectors are chosen in the form of symmetric irreducible tensors which describe kinetic modes of the system. The corresponding eigenvalues are relaxation coefficients and define the relaxation times of the system. It is established that scalar and vector eigenfunctions describe evolution of electron energy and momentum densities (vector and scalar system modes). By this way in the present paper exact close set of equations for the densities valid for all times is obtained. Further, it is assumed that their relaxation times are much more than relaxation times of all other modes. In this case there exists a characteristic time such that at corresponding larger times the evolution of the system is reduced described by asymptotic values of the densities. At the reduced description electron distribution function depends on time only through asymptotic densities and they satisfy a closed set of equations. In our previous paper this result was proved in the absence of an external electric field and exact nonequilibrium distribution function was found. Here it is proved that this reduced description takes also place for small homogeneous external electric field. This can be considered as a justification of the Bogolyubov idea of the functional hypothesis for the relaxation processes in the plasma. The proof is done in the first approximation of the perturbation theory in the field. However, its idea is true in all orders in the field. Electron mobility in the plasma, its conductivity and phenomenon of equilibrium temperature difference of electrons and ions are discussed in exact theory and approximately analyzed. With this end in view, following our previous paper, approximate solution of the spectral problem is discussed by the method of truncated expansion of the eigenfunctions in series of the Sonine polynomials. In one-polynomial approximation it is shown that nonequilibrium electron distribution function at the end of relaxation processes can be approximated by the Maxwell distribution function. This result is a justification of Lorentz–Landau assumption in their theory of nonequilibrium processes in plasma. The temperature and velocity relaxation coefficients were calculated by us early in one- and two-polynomial approximation.

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