Nonuniform Plasma Column Research Articles

Microwave propagation along nonuniform plasma column as surface plasmon

Formation and propagation of surface plasmons (SPs) of microwaves (MWs) along a dense non-uniform plasma column were investigated by employing the finite-difference time-domain method and evidenced by experiment of argon helicon plasma. The effects of plasma density, incident frequency, and radius of plasma column on the SP features were discussed. Results showed that when the MW frequency and the density distribution of plasma column satisfy suitable conditions, SPs can be excited on the surface of a uniform plasma column when the frequency is 2–5 times of the MW, while inside a nonuniform plasma column when the plasma frequency is only a little higher (about 5%–20%) than the MW. The SP wavelength depends on the plasma density for a given incident MW frequency. The forward scatter MW field will be enhanced significantly when SP appears along the nonuniform plasma column.

Low-frequency sheath instability stimulated by an energetic ion component

Spontaneous low-frequency oscillations have been observed in the circuit of a positively biased electrode immersed in a non-Maxwellian laboratory plasma containing an energetic ion component produced by the resonant absorption of a short microwave pulse in a nonuniform plasma column. The oscillations are found to be due to an instability of the electron-rich sheath. The instability with its characteristic frequency below the ion plasma frequency is driven by the energetic ion component reflected in the sheath area. A qualitative model of the instability is suggested.

Magnetic fluctuations of a large nonuniform plasma column

A broad experimental survey is made of the properties of spontaneously generated magnetic fluctuations in a large linear device in which the plasma density has different cross-field gradient scales. It includes steep gradients at the plasma–wall edge as well as at an interior plasma–plasma interface, thus phenomena of interest to magnetic fusion research as well as to space plasma physics is illustrated. Fluctuations in uniformly magnetized columns and plasmas with an axial gradient in the confining magnetic field are studied. Some of the highlights include the identification of a universal spectrum for drift-Alfvén turbulence and the role of partial reflections in shaping the spectra.

Study of lower-hybrid wave propagation in the presence of low-frequency fluctuations

Phase-sensitive probe diagnostics as well as two-point-correlation analysis of radio-frequency probe signals are applied to investigate small-amplitude lower-hybrid (LH) waves launched into a linear nonuniform plasma column by a slow-wave antenna. The LH wave propagation in the inhomogeneous plasma is described by a model taking into account the wavenumber spectrum of the antenna. Special attention is focused on the interaction of the LH waves with the low-frequency density fluctuations. The LH frequency and wavenumber spectra observed can be explained by scattering of the LH waves from the (incoherent) low-frequency density fluctuations. Moreover, the enhanced damping, which is found to be much higher than expected from collisional and electron Landau dampings, is correlated with the level of low-frequency fluctuations.

Multigroup representation of fusion product orbits in a plasma column

A method is derived for describing the time-dependent behavior of α particles produced in a radially nonuniform slender plasma column as a distribution function among the possible orbits. A multigroup numerical approximation is introduced to analyze the development of the distribution function and its moments. Results are presented of calculations of the time-dependent α-particle energy spectrum and radial density, energy, and electron heating profiles in plasma columns with radii comparable to the α Larmor radius. This technique allows calculation of the α particle history at much more rapid rates than allowed by Monte Carlo techniques: The characteristic time scale is the α-electron slowing-down time rather than the cyclotron period.

Determination of electron density and electron collision frequency in a nonuniform plasma column by rf coil probe

A nonimmersive-coil-probe method is presented for determining the electron density and electron collision frequency for momentum transfer in a cylindrical plasma column with a Bessel-type density distribution along the radius, the plasma being either isotropic or embedded in an axial static magnetic field. The method is based on the relation between the plasma parameters and the impedance of an rf coil placed coaxially around the plasma. Simplified expressions relating the parameters to the coil impedance are found to be valid under suitable conditions in the isotropic case.

Measurements of electromagnetic field modifications in turbulent plasma

An experimental technique is developed to measure the modification of an electric field in a turbulent plasma. The method is suitable for use up to near critical density of the plasma. Measurements have been performed under three plasma conditions, namely, underdense, intermediate density, and near critical density. The results show more rapid attenuation of the coherent wave than predicted by exponential decrease, which is attributed to refraction in our nonuniform plasma column. The data indicate an initial buildup of the incoherent wave (both direct and cross‐polarized components) and then a decrease to a certain level depending on the equivalent thickness as the wave penetrates into the plasma.

Scattering of a Beam Wave from a Nonuniform Plasma Column in the Born Approximation

The scattering of an electromagnetic wave with a Gaussian amplitude distribution from a nonuniform and nonmagnetized plasma column is theoretically investigated with the Born approximation. The incident Gaussian beam wave is assumed to be normal to the plasma column axis, and both TM and TE modes are treated. For the electron density profile with a Gaussian distribution in a plasma column, the expressions that determine the scattering patterns are obtained. The computed results indicate that the smaller the dimension of the spot size of incident beam wave is, the broader is the width of scattering angle.

Charge plasma erosion for short-range partially and totally stripped ions stopped in silicon radiation detectors

Calculations are made on the erosion by an applied electric field of the charge carrier plasma formed by an ion stopped in a silicon radiation detector. The recent work on this by Seibt, Sundström and Tove is extended to make allowance for the strong variation of linear carrier density with distance along the plasma as produced by partially and totally stripped ions. The published plasma times for 22 MeV 16O ions and 252Cf light fission fragments, both of about 15 μm range, can then be accounted for if transfer from slow cylindrical to fast spherical erosion geometry occurs when a plasma length of about 6 μm remains uneroded. Evidence that this length may be the same for all ions of different ranges is discussed. It is suggested that erosion into the length of a non-uniform plasma column occurs primarily from the end at which the linear carrier density deposited is smaller, although this awaits experimental confirmation.

Electric field driven flute instabilities in a nonuniform collisional plasma

The stability of a magnetically confined nonuniform plasma column with a radial electric field is examined with a linear single-fluid theory including the effects of rotation, ion-neutral collisions and finite Larmor radius stabilization. The transition from the Rayleigh-Taylor instability in the collisionless limit to the Simon neutral-drag instability in the collision-dominated limit, and the effect of collisions on finite Larmor radius stabilization, is studied in detail. A numerical example is given and the application of the theory to experiments is discussed.

Magnetohydrodynamic waves in a non-uniform current-carrying plasma column

An effect of an axial current on the propagation of low-frequency axisymmetric magnetohydroclynamic waves in a radially non-uniform plasma column was investigated theoretically and experimentally. It was found that the axial current and the density gradient cause a coupling between the torsional and compressional waves.

Propagation of fast electromagnetic waves along a non-uniform annular plasma column

The propagation of the lowest order, circularly symmetric, fast wave mode along a coaxial line completely filled with a non–uniform annular plasma is considered. The complete field analysis has been applied, solutions being obtained with the aid of a digital computer. Particular attention is paid to discussing the main features of the propagation in terms of the non-uniform electron number density within the plasma. Field profiles ore presented and the behaviour of the axial power flow is considered as a function of frequency. The solutions are compared with those obtained by recourse to an approximate ‘equivalent’ transmission line analysis.

Propagation of an Ion Rarefaction Wave from a Growing Sheath

The structure of an ion rarefaction wave propagating from a sheath growing into a nonuniform plasma column is determined using the method of characteristics. Ionization and collisions both tend to damp out the wave. The interaction between two identical ion rarefaction waves from opposite sides of a plasma column produces a secondary ion rarefaction wave which spreads outward from the center of the column. The density profile in the column relaxes toward a new equilibrium profile, corresponding to that for the new (i.e., contracted) column width.

The resonance frequency of a cylindrical microwave cavity partially containing a cold, nonuniform plasma column enclosed by a quartz tube

Calculations of the resonance frequency are performed for a cavity containing a quartz tube which may be filled with a nonuniform plasma. Three methods to calculate the resonance frequency are compared. Identical results are obtained from the two exact methods: the multistrata method and a solution in a series. The third method applying perturbation theory gives results whicg agree with the other methods for a low-density plasma in a thin-walled quartz tube.

Resonances (TEmn, TMlm) of a Cylindrical Cavity Containing a Nonuniform Plasma Column

Solutions of the wave equation are obtained for TE0mn and TMlm0 modes in cylindrical cavities containing a coaxial, annular plasma column. The radial electron density distribution is assumed to be of the form ne=n0[1−γ1 (r/a)2−γ2(a/r)2], where n0, γ1, and γ2 are constants and a is the outer radius of the plasma. The results can be used to determine the effects of density gradients on measurements of electron densities and collision frequencies by the cavity method. Some illustrative examples are presented.

On the Point of Emergence of a Conical Microwave Beam Entering a Particular Nonuniform Plasma Column

A circularly symmetrical conical wave, in which E., is the only component of the electric vector, is incident upon a circularly symmetrical plasma column of radius a in which the electron density is proportional to (a2/r2)‐1 for a radial distance r≤a, and is zero for r>a. The change in phase on reflection is determined as a function of the angle of incidence. A circularly symmetrical beam of radiation can be synthesized as an angular spectrum of conical waves, and the corresponding reflected beam is easily obtained in integral form. The position of maximum intensity in the reflected beam is found by making use of the principle of stationary phase.The results obtained are compared with ray theory and with a previous calculation for a linear gradient of electron density in a planar geometry.

Wave Propagation along a Warm Non-uniform Plasma Column†

ABSTRACT Tha dipolar resonances that occur when an axially uniform plasma column is subjected to a transverse r.f. electric field have received much study, and can be predicted accurately from a hydrodynamic description of the plasma perturbation and knowledge of the transverse olectron density profile. These frequencies may be interpreted aa the cut-off's of series of modes propagating along the column. In this paper such propagation is studied in infinite planar geometry for a number of basic cases. Dispersion curves, and descriptions of the plasma parameters at cut-off are given for modes with axially symmetric and axially asymmetric potential distributions in three different geometries : first for plasma filling the plates, and then for plasma separated from the plates either by one or two dielectric layers. The feasibility of observing these waves experimentally is discussed, and experimental evidence for their propagation is cited.

Resonance Oscillations in a Hot Nonuniform Plasma Column

This Letter reports new observations of resonances in a low-density plasma column and a quantitative theory of the frequency spectrum which is in excellent agreement with these measurements. Other measurements have recently been reported by Dattner. (1)