Excitation Of Lower Hybrid Waves Research Articles

Excitation of lower-hybrid waves by a conductor moving in a cosmic plasma

Excitation of lower-hybrid waves by a conductor moving in a cosmic plasma

Parametric excitation of lower hybrid waves by helicon waves in magnetoactive semiconductors

A theoretical investigation has been done on the parametric excitation of the electrostatic lower hybrid wave when an electromagnetic helicon wave propagates in a magnetoactive semiconductor. Fluid equations of homogeneous plasmas have been employed to find the nonlinear response of electrons. The low frequency nonlinearity arises through the ponderomotive force on electrons, whereas the high frequency nonlinearity arises through the current density associated with the scattered helicon wave. For the typical plasma parameters in n-type indium antimonide and for the considerable power density (∼7.6 kW cm−2) of the incident helicon wave, the growth rate of the parametric instability turns out to be quite large (∼108 rad s−1). The presence of the linear damping of the decay waves in the semiconductor plasma reduces the growth rate of the instability by two orders of magnitude.

Alfvén’s Critical Velocity Effect Tested in Space

A sounding rocket experiment involving the injection of a barium gas jet in the upper ionosphere provided an opportunity of investigating quantitatively several aspects of the beam-plasma interaction that is the substance of Alfven’s critical velocity effect. Whereas the experimental data are presented elsewhere, this paper contains some theoretical considerations of (1) the limiting neutral density for which the ionization process can operate, (2) the interaction of the freshly injected ions with the background plasma, and (3) the microprocess which leads to collisionless electron tail formation. The observed distribution of injected ions is consistent with the Townsend condition on the beam-plasma discharge. The mass loading on the ambient plasma, although locally high, has a weak effect on the dynamics of the involved flux-tube because of the limited extent of the beam. The most likely process by which energy is transferred from the freshly generated ions to the electrons is an ion beam instability leading to the excitation of lower hybrid waves

Excitation of lower-hybrid waves by the rf heating of Elmo Bumpy Torus Scale

The substantial ion heating of toroidal plasmas at rf frequencies between the ioncyclotron and lower-hybrid frequencies suggests that some additional process or processes must supplement conventional harmonic ion-cyclotron coupling of wave energy to particles. It is proposed in this paper that superthermal lower-hybrid turbulence excited because of the harmonic oscillating two-stream instability could contribute in a significant way to this observed heating. The threshold field strength for such a process to occur in the Elmo Bumpy Torus Scale is calculated. Specific tests of the theory, measurement of an enhanced lower-hybrid and quasimode spectrum, particularly at harmonics of the pump frequency, are indicated. The impact of the harmonic oscillating two-stream instability as allowing rf heating at frequencies a factor of 2 or 3 below the lower-hybrid frequency is described.

Helical slow wave antenna for launching lower hybrid waves in a plasma

The characteristics of a helical antenna employed for excitation of lower hybrid waves (LHW) in a plasma are investigated. The helical antenna is constructed by winding a cable on a glass discharge tube. It is shown experimentally that the helical antenna can excite LHW in a wide frequency range and with arbitrary refractive index parallel to the magnetic field. An increase of the longitudinal refractive index due to plasma loading is observed and analyzed by transmission line theory. Qualitative agreement is obtained between theory and experiment.

Nonlinear Penetration of Upper-Hybrid Waves Induced by Parametric Instabilities of a Plasma in an Inhomogeneous Magnetic Field

It is observed that with a sufficiently large incident extraordinary wave amplitude, the electrostatic upper hybrid wave resulting from the mode conversion process at the resonant layer can scatter off the lower hybrid wave and penetrate through its original cutoff to a distance proportional to the product of the scale length and the square root of the mass ratio. A substantial number of energetic ions also appear in the resonant region which is well inside the plasma slab as a result of lower hybrid wave excitation.

Simulation of lower-hybrid heating in a nonuniform plasma slab

A computer simulation study of the excitation of lower hybrid waves by external sources and the associated plasma heating is presented. A plasma slab with a nonuniform density profile is modeled with a two and one-half dimensional electrostatic particle code. Both a finite-length electrostatic antenna and a single wave exciter are considered. The frequency is chosen so that a wave conversion layer exists in the plasma. It is found that a narrow sheath is formed at the plasma edge and the dominant effect is surface electron heating within that region. Ponderomotive force effects as well as the nonlinear generation of harmonics and subharmonics also occur. A low amplitude lower hybrid wave is observed to be excited by the sources and propatates to the wave conversion layer where it is absorbed.

Formation of Thermal Eddies during rf Heating of Plasma

Moderate-power (\ensuremath{\sim}1 kW) excitation of lower-hybrid waves in a linear plasma column is found to increase the reflectivity of the phased waveguide exciter and to change the vertical position of the resonance cone. Probing of the plasma near the mouth of the waveguide reveals that the increased reflection results from an undulation in the plasma surface. This Letter presents evidence that this surface distortion is driven by thermal eddies associated with asymmetrical electron heating.

Excitation of lower hybrid waves in a finite plasma

A computer simulation study of the launching of lower hybrid waves by external sources and their propagation in a finite but homogeneous plasma slab is presented. The study makes use of a two and one-half dimensional electrostatic particle code developed for simulation of bounded plasma. A number of antenna configurations are considered: a point source, two finite-length capacitor plates, and a phased array of capacitor plates. When the oscillating frequency does not match a bounded plasma resonance, one can observe resonance cones, energy absorption at the plasma surface, ion cyclotron modulation of the source, and energetic ions, depending on the parameters chosen. The excitation of a bounded plasma resonance is also considered. The nonlinear evolution of the resonance shows that wave-particle interactions and ponderomotive force effects play an important role. The possibility of controlling the electron temperature profile is discussed.

Ion energy and plasma density increase in mirror magnetic field by lower hybrid wave excitation

Using a coaxial double plasma device we excited the lower hybrid wave in an argon plasma in mirror magnetic fields. When the lower hybrid wave was excited the perpendicular ion energy E ⊥ and the plasma density n were increased. A relation between the relative changes of the average energy ΔE ⊥/ E ⊥ and the density Δn n agrees very well with a relation expected from loss-cone considerations.

Excitation of lower hybrid waves by electron beams in finite geometry plasmas. Part 1. Body waves

The quasi-static lower hybrid eigenmodes of a plasma column in a cylindrical waveguide are determined, and their linear excitation by a small density electron beam is discussed for the cases of a hot electron beam as well as for a cold electron beam. It is shown that under certain conditions, finite geometry effects introduce important quantitative and qualitative differences with respect to the results obtained in an infinite geometry.

Excitation of lower hybrid waves by electron beams in finite geometry plasmas. Part 2. Surface waves

The dispersion relations for the quasi-static lower hybrid surface waves are derived. Conditions for their existence and their linear excitation by a small density electron beam are discussed. Instabilities appearing in low-frequency surface waves are also discussed.

Ion Energy and Plasma Density Increases in Mirror Magnetic Field by Lower Hybrid Wave Excitation

Ion Energy and Plasma Density Increases in Mirror Magnetic Field by Lower Hybrid Wave Excitation

Excitation of lower hybrid waves in a plasma by electron beams

The linear theory of excitation of electrostatic lower hybrid waves by electron beams is presented. The importance of such waves in tokamaks with runaway electrons is discussed.

Excitation of Lower Hybrid Waves in a Coaxial Type Double Plasma

Lower hybrid waves were excited in a coaxial type double plasma device, where their wave vectors were almost perpendicular to the magnetic field. The dispersion relation of the excited waves agrees fairly well with the theoretical one. The waves are effectively excited by the sinusoidal potential differences between the driver and target plasmas without insertion of metal electrodes. The method may be applicable to the plasma heating due to lower hybrid waves in high temperature plasmas.