Here are presented experiments on fine particles levitating in low-pressure weakly ionized plasmas under a vertical magnetic field. The magnetic field is useful to provide a vertically long cylindrical column of fine-particle clouds, yielding even string-shaped vertically aligned fine particles, under the double-plasma configuration. Measurements show that fine-particle clouds rotate in the azimuthal direction on the horizontal plane even in such a weak magnetic field that positive ions are slightly magnetized. With an increase of the magnetic field, the rotation speed increases, being followed by subsequent saturation. The rotation speed and direction can be controlled by varying radial plasma potential and/or density profiles. The rotation is induced under the condition that the interparticle distance is small enough for the strong Coulomb coupling among fine particles. A mechanism of the rotation could be explained by effects of ion motions on fine particles, which are modified in the presence of the vertical magnetic field.

Electron-temperature control is performed on plasmas passing through a coarse mesh grid from a discharge region. By increasing a negative potential applied to the grid, the electron temperature is continuously decreased in a very wide range covering almost two orders of magnitude down to the value nearly equal to the background gas temperature in case of direct current argon or helium gas discharge. The temperature decrease is accompanied by an increase in the electron density. This method of electron-temperature control can also be applied to plasmas produced by radio-frequency and electron cyclotron resonance discharges.

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Instabilities and double-layer formation are systematically investigated on a bounded collisionless system with electron beam penetrating through a plasma. The Buneman instability is observed at low beam currents. Above a critical current, there appears a sudden nonoscillatory potential drop due to the Pierce instability, which traps ions in the potential well. This collisionless ion trapping provides a new formation mechanism of the double layer, which is controlled by changing the speed of the potential drop.The Buneman instability (electron-ion twostream instability)' has been expected to be one of the most effective heating mechanisms in a current-carrying plasma. " To our knowledge, however, no clear-cut experiment on this instability has been published, except our preliminary report. It is a strong convective instability, and boundary effects are quite important for experiments on the instability. There are two types of boundary: a periodic boundary and a boundary of electron-beam injection. The former corresponds to the torus experiment of Hamberger and Jancarik, ' in which the spectrum of fluctuations was analyzed and the anomalous resistivity was said to be attributed to the Buneman instability. The latter situation was first realized by Nezlin etal. ' They observed unstabj. e oscillations and a virtual cathode due to the Pierce instability, ' although their theory is not suitable to the experiment as will be shown later. On the other hand, laminar' and turbulent' double layers were observed under this situation, where trapped electrons were, respectively, supplied externally and produced by the turbulent electron-electron two-stream instability. Trapped ions were also pointed out to be important for the double-layer formation. The mechanism of their initial production, however, was not explained clearly. In this Letter, we identify the Buneman instability in an electron beam-plasma system under a reasonable consideration of the boundary effect, and demonstrate a subsequent potential drop due to the Pierce instability, which provides a new mechanism of double-layer formation based on collisionless ion trapping. The dispersion equation in an infinite plasma with beam electrons is given by k'[1 -~,'/((u-kV)' -(u, .'/(u']+ k D' = 0, where kD (= &u, /v, ) is the Debye wave number of plasma electrons; U and v, are the electron-beam 3 3 MbL/m U I I (b), PI BI

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A uniform plasma is produced by a new plane slotted antenna with permanent magnets for the electron cyclotron resonance. The plasma is uniform within a few percent in a diameter of 25–30 cm at a distance of 25–30 cm from the antenna. The plasma density, 1.5×1010 cm−3 with electron temperature of 1.8 eV at a microwave power of 100 W, is almost proportional to the microwave power. A magnetic distribution provided by the magnets is essential for producing such a uniform plasma.