Kazuo Kitao

Effects of a High-Frequency Electric Field on Micro-Instabilities Driven by Density Gradient

Drift, drift-dissipative, and drift-cyclotron instabilities in an inhomogeneous plasma are considered for the case when a uniform high-frequency electric field is applied along the magnetic field. For the applied field, the dipole approximation is made and the skin effect is neglected. Without averaging the distribution function over the period of the applied electric field, the dispersion relations are derived for both the collisionless and collisional regimes. It is found that the high-frequency field whose frequency is within some ranges increasing the frequencies of perturbations can stabilize drift instabilities and also drift-cyclotron instabilities with finite k z of the fluid-type, where k z is the component of the wave vector along the magnetic field. It is found, however, that when the unstable drift-dissipative mode is stabilized the stable mode becomes unstable.

On the Physical Picture for the Anomalous Propagation of an Ordinary Wave in a Magnetoplasma

The physical picture for the anomalous propagation of an ordinary wave at frequency near but below the electron cyclotron frequency is discussed on the basis of the single-electron equation of motion in the magnetostatic and the wave fields. It is shown that the anomalous propagation originates from the forces along and across the magnetostatic field. The perpendicular force deflects the electrons and gives the density perturbation which contributes to the anomalous propagation, as shown by Fried. It is found, however, that the parallel forces connected with the gyration of the unperturbed motion also contribute to the anomalous propagation and accompany no density perturbation. (auth)

Radial Motion of a Field-Reversed Ion Layer

Radial motion of a field-reversed ion layer is investigated theoretically in the limit of an extremely field-reversed thin layer. It is assumed that the ion layer consists of a fluid of rigidly rotating ions with finite temperature and a fluid of cold stationary electrons. It is shown that the ion layer radially vibrates as a whole with a frequency nearly equal to the betatron frequency of a single ion.

Stability of Field-Reversed Ion Rings in a Background Plasma

A two-fluid energy principle is developed that leads to sufficient conditions for the low frequency stability of an axisymmetric, field-reversed ion ring immersed in a dense, background plasma. It is assumed that the ion ring is described as the rigidly rotating fluid with finite temperature while the background plasma as the stationary fluid. The stability condition is approximately given by the potential energy W (ξ p ,ξ b )>0, where ξ p and ξ b are the displacements of the ring and the background plasma, respectively. As an application, the stability condition for kink modes of a long ion layer immersed in a cold background plasma is examined in detail in order to compare the existing theories with the present theory.

Electrostatic oscillations and feedback control of a lossfree plasma

The general features of the electrostatic oscillations in a lossless plasma are discussed from the dispersion relation and energy equation. It is shown that the unstable electrostatic waves in lossless plasma have always zero energy density which is realized as a sum of a negative kinetic energy density and a positive electrostatic energy density. The feedback stabilization for reactive instabilities is reconsidered by a simple model, and a cold electron plasma--beam system is studied as a simple example. (auth)

Ion Cyclotron Instability Driven by a Cross-Field Current

The electrostatic instabilities driven by a current perpendicular to an external magnetic field are studied for a homogeneous Vlasov plasma of cold electrons and hot ions. The ions are assumed magnetized. This assumption is allowed for a E × B rotating plasma in which the cross-field current flows for a much longer time than the ion cyclotron period. A new instability of reactive type is found to occur by the coupling of an electron acoustic mode and a Doppler-shifted ion cyclotron mode (in the electron frame).

Anomalous Penetration of an Ordinary Wave into a Magnetoplasma Slab

The penetration of an ordinary wave into an infinite magnetoplasma slab is calculated by adopting the image-metal technique in treating the electrons reflected at the plasma boundaries. After general expressions are derived, the wave frequency is assumed to be near the electron cyclotron frequency because the influence of the thermal motion of electrons is remarkable only for this case. It is found that the evanescent wave penetrates deeply into the slab compared with the case of a cold plasma, and the size effect appears due to the increase of the wave reflection at the plasma-vacuum boundary. The absorptive power due to collisions, evaluated for the limiting case of infinite thickness of the slab, is much larger than the value for a cold plasma and shows the surface effect. These two effects become more noticeable for the higher density and higher temperature of the plasma.