The influence of a negative biased electrode on the plasma parameters is investigated by using an emissive hot cathode made of LaBe in the Tohoku University Heliac. We have already succeeded in inducing the strong negative radial electric field by the negative biased hot cathode, which works as an electron source. In order to clarify the radial electric field formation in the heliac system, we attempted the following experiment; (i) radial current control with a constant current power supply, (ii) radial electric field control by the radial current. And then the measured damping force was compared with the theoretical prediction by the momentum balance equation to explain the confinement transition. Figure 1 shows typical temporal behavior of hot cathode current IE (a) and biase voltage VE (b). In order to control radial electric field, we controlled IE, which was ramped from -4.2A up to 0 A during 5 ∼ 10 ms using the constant current power supply. It is clear that during 7.75 ∼ 8 ms, VE decreases even though IE increases, which correspond to negative resistance of plasma. Poloidal rotation equilibrium equation is derived from the balance between the torque driven by the radial current and the damping forces due to viscosity and ion-neutral interaction. Pig. 2 shows the dependence of the normalized ion damping force on poloidal Mach number Mp. Three lines correspond to theoretical predictions based on Rozhansky model for ion viscosity with assuming Ti = 1, 4, 20 eV, respectively. These curves have the local maxima, which are located at |Mp| ≅ 1.5. The symbols in Fig.2 indicate damping forces measured at different radial positions and the closed symbols means the data at the negative resistance region in Fig. 1. It is clear that the measured damping forces agree well with the theoretical curves. The damping forces corresponding to the negative resistance regions become almost constant, where the theoretical curves show their local maxima.
|Number of pages
|IEEE International Conference on Plasma Science
|Published - 2003
|2003 IEEE International Conference on Plasma Science - Jeju, Korea, Republic of
Duration: 2003 Jun 2 → 2003 Jun 5