A systematic investigation of the effects of a parallel velocity shear and negative ions on the collisionless drift wave instability has for the first time been realized by simultaneously using a segmented tungsten hot plate of a Q-machine and sulfur hexafluoride (SF 6 ) gas in a magnetized potassium plasma. The parallel velocity shear of the positive ion flow tends to decrease the fluctuation level of the drift wave. The introduction of negative ions first increases the fluctuation level and then starts to decrease it at the negative ion exchange fraction of around 10%, while keeping the above-mentioned shear effect qualitatively. In addition, a simple dispersion relation based on the local model has been calculated to show that it can predict wave characteristics similar to the experimental results. Our findings provide a potential for gaining a more profound insight into the physics of space/circumterrestrial plasmas.

An electron temperature gradient (ETG) is formed perpendicular to the magnetic field lines by superimposing low-temperature thermionic electrons emitted from a tungsten hot plate upon high-temperature electrons of an electron cyclotron resonance plasma, which pass through two different-shaped mesh grids. The radial profile of the plasma space potential can be controlled independent of the ETG by changing the bias voltages of the hot plate.

The specific history of collisionless drift waves is marked by focusing upon current-driven, shear-modified, and electron-temperature-gradient modes. Studies of current-driven collisionless drift waves started in 1977 using the Innsbruck Q machine and was continued over 30 years until 2009 with topics such as plasma heating by drift waves in fusion-oriented confinement and space/astrophysical plasmas. Superposition of perpendicular flow velocity shear on parallel shear intensively modifies the drift wave characteristics through the variation of its azimuthal structure, where the parallel-shear driven instability is suppressed for strong perpendicular shears, while hybrid-ion velocity shear cause unexpected stabilization of the parallel-shear-modified drift wave. An electron temperature gradient can be formed easily by control of thermionic electron superimposed on ECR plasma, and is found to excite low-frequency fluctuation in the range of drift waves.

Institute of Pnysics and P e r g " JanmiLs Ltd.

Plasma flow velocity shears parallel and perpendicular to magnetic field lines are independently controlled and superimposed by a modified plasma-synthesis method with concentrically three-segmented electron and ion emitters. The fluctuation amplitude of a drift wave with an azimuthal mode number m = 3 is observed to increase with increasing parallel shear strength in the absence of a perpendicular shear. When the perpendicular shear is superimposed on the parallel shear, the drift wave with m = 3 is found to transform into that with m = 2. Furthermore, the parallel shear strength required for excitation of the drift wave increases with a decrease in the azimuthal mode number. Based on these results, superposition of the parallel and perpendicular shears can affect characteristics of the drift wave through variation of the azimuthal mode number. These phenomena can be verified by theoretical calculations of the growth rate of the drift wave using an eigenmode analysis.