The helium ash exhaust function of a divertor has been experimentally demonstrated. Helium atoms accumulate in the divertor region as the electron density of the main plasma increases. With a helium concentration of ~ 1.6% of electron density in the main plasma, neutral helium pressure at the divertor region is as high as 1.0 x 10" 4 Torr. This experiment indicates the possibility of helium ash exhaust in an a-particleheated diverted tokamak with use of pumping ducts of a practical size.
Basic characteristics of a particle detection method using laser breakdown were studied for a system of polystyrene standard particles dispersed in ultrapure water. The method was able to detect 0.02 µm particles. The detection sensitivity decreased with the particle size due to size dependence of the laser breakdown threshold. The plasma emission delay time from the laser pulse decreased with particle size (9.8±0.8 ns for 0.04 µm, 5.7±0.8 ns for 3.0 µm). The above results suggested the possibility of concentration and size measurement of fine particles in liquids by the proposed method.
The successful operation of a single-null poloidal divertor in Doublet-Ill has demonstrated several new advantages of a diverted tokamak in addition to the suppression of impurity influx as demonstrated in DIVA: 1) The impurity contamination and radiation loss of the main plasma has been reduced by an open divertor geometry, i.e. without a divertor chamber; 2) The radiative cooling and formation of a dense and cold (n e >5Xl0 13 crn~3, T e <7 eV, P H , < 4 X 10' 3 torr) divertor plasma have been observed. -Up to 50% of the Ohmic input power is radiated in the divertor region, thus cooling the plasma in front of the divertor plate down to several eV. This remote radiative cooling greatly reduces the heat load on the divertor plate without cooling the main plasma. -The feasibility of remote radiative cooling in INTOR was studied by use of a volume integration technique of the radiation power along the field line.
Stable plasmas with surface elongations of up to 1.8 (aspect ratio = 3.4) have been produced in the upper lobe of Doublet III with the use of both passive and active controls. The growth rate of the vertical instability has been measured at various values of elongation by disabling the feedback circuit of the vertical position control power supply. A rigid-shift analysis of growth rates indicates that the passive stabilization effect of the field-shaping coils plays a key role in achieving a high elongation of 1.8. Experiments have demonstrated that the maximum stable elongation is determined by the strength of the passive stabilization effect even with active feedback control. The dee-shape is found to be preferable to an elliptical shape because the triangularity reduces the absolute value of the decay index required to produce a given elongation.
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