Motoichi Kawaguchi

Modes of the Screw Instability in a Positive Column with Longitudinal Magnetic Field

The modes of screw instability in a positive column of a nitrogen gas discharge have been studied. Under a certain condition, the critical magnetic field for the m=2 mode has been found to be smaller than that for the m=1 mode. In addition, the regular oscillations have been observed at the magnetic field larger than the critical one. These phenomena can not be described by the results of Kadomtsev and Nedospasov which are calculated from the small‐amplitude theory. In this paper a theoretical explanation for these phenomena is proposed based on the conception that only the instability which has the maximum growth rate can grow while other instabilities are masked.

A Theoretical Investigation of the Collective Acceleration of Cluster Ions with Accelerated Potential Waves

A theoretical treatment of the acceleration of cluster ions for additional heating of fusion plasma using the trapping effect in an accelerated potential wave is described. The conceptual design of the accelerator is the same as that by Enjoji,1,2) and the potential wave used is sinusoidal. For simplicity, collisions among cluster ions and the resulting breakups are neglected. The masses of the cluster ions are specified to range from 100 mD to 1000 mD (mD: mass of a deuterium atom). Theoretical treatment is carried out only for the injection velocity which coincides with the phase velocity of the applied wave at the entrance of the accelerator. An equation describing the rate for successful acceleration of ions with a certain mass is deduced for the continuous injection of cluster ions. Computation for a typical mass distribution shows that more than 70% of the injected particles are effectively accelerated.

RADIAL DENSITY PROFILE IN A POSITIVE COLUMN WITH A LONGITUDINAL MAGNETIC FIELD.

The radial density profile is investigated experimentally and compared with the theoretical results of Holter and Johnson. The experimental results agree qualitatively with the theoretical ones under some experimental conditions.

Trapping criterion for deuterium cluster ions in an accelerator using an accelerating potential wave

The behavior of cluster ions in an accelerating potential well is discussed in terms of a collective accelerator of cluster ions. By studying the time variation of the potential energy and the energy-like quantity of cluster ions, the trapping criterion in the accelerating potential well is obtained in an analytic form, which makes a simple calculation of acceleration efficiency possible for arbitrary distributions of the mass and initial injection phase of cluster ions. For a Gaussian mass distribution and continuous uniform injection, the efficiency amounts to 70%.

Density Accumulation of a Magnetized Current-Carrying Plasma Caused by a Metal Disk

Experiments on the stationary disturbance of a weakly ionized current-carrying plasma caused by a floating metal disk in a longitudinal magnetic field are described. Electron and ions in the plasma form longitudinal counter-streams. Experimental results are presented for the upstream region of electrons relative to the disk disturber, where the disturbance is characterized by accumulation of plasma particles. The transverse and longitudinal lengths of the plasma accumulalion are measured. The peak value of the plasma density increases with B/p (B: the magnetic field, p: pressure of neutral helium gas). In the disturbed region the potential is lower than in the surrounding area. The mechanism for the disturbance is discussed as related to the suppression of the electron current caused by the disk with the flow patterns of the magnetized plasma particles.

A Potential Disturbance of a Magnetized Current-Carrying Plasma Caused by a Floating Disturber

The potential disturbance caused by a floating disturber in a current-carrying magnetized plasma is analyzed theoretically. The equations used are the Boltzmann relation for electrons and the continuity equation for a collisional flow of ions, which lead to a Laplaces equation by using a function transformation. The solution shows that a potential overshoot appears, and the criterion for this overshoot is given. The peak value of the overshoot and the position of the peak are compared with experimental results. The mechanism for the overshoot is discussed, and it is concluded that it originates from the rarefaction effect of the electron density in the shadow region of the disturber in the E-driven flow of electrons.

Disturbance Caused by Probes in the Electric Field Measurement in a Magnetized DC Discharge Plasma

The potential difference between two floating probes placed in a dc discharge in a magnetic field was measured as a function of the distance between them. When the two probes approach closer to each other, the potential difference behaves as if there were an electric field in the inverse direction. Its dependence on the pressure, the magnetic field and the dc discharge current is studied and the experimental region where this anomaly occurs is shown. A discussion is provided on the relation between the apparently anomalous electric field and the disturbance of the plasma column by the probes.

The Modes of the Screw Instability in a Positive Column for Several Gases

The mode, frequency and wavelength of screw instability are experimentally observed for several gases. These experimental results are compared with the theory modified from that of Kadomtsev and Nedospasov. The modified theory shows good agreement with the experiment. The experimental results are divided into two cases: one is obtained in rare gas and the other is in diatomic gas discharge. For rare gas discharges the mode coincides with the Kadomtsev-Nedospasovs theory, while for diatomic gas discharges the column shows a peculiar mode transformation with increase of longitudinal magnetic field. These experimental results are explained by the modified theory.