chemical physics of discharges - Argonne National Laboratory
chemical physics of discharges - Argonne National Laboratory chemical physics of discharges - Argonne National Laboratory
58 0 0 0 VI J > I cv . u H a
elow the saturation limit we find59 in which n is the random ion density, q’ the electronic charge and Vf thePvelocity of the ions arriving at the collector. This is given by: in which Vo is the initial ion velocity and Vacc is the potential difference between the plasma and the collector. Thus: Therefore, a plot of i+ versus Vacc1/2 should yield a straight line whose slope is + np q(?)’’2 . The positive ion currents, plotted in this manner always yielded a straight line to a break point which was assumed to be the saturation limit. A plot of this type is shown in Fig. 2 (b); the small residual ion current has been subtracted from the experimental points. From this slope we calculate np = 145 x 1013/m3 in good agreement with the electron concentration of Fig. 2 (a) for the same probe position. (3) Axial variation of ions and low energy electrons. The observed ion and electron concentrations as a function of axial distance from the cathode are given in Fig. 3 for the probe facing the anode. There is a slight displacement between the maxima for the two distributions and this may reflect a slight penetration of the field from the cathode dark space. However the results show that a true plasma is present in the negative glow, since to a gbod approximation ne = np. electron current was relatively insensitive to position of the probe and gave an average concentration of 3 ? 1 x 10 3/m3 with electron temperatures in the range of 3.5 - 4.0 eV. The rising portion of the secondary electron curve marks the anode edge of the negative glow. At this point the electric field begins to accelerate thermal electrons to the secondary electron velocities. For example at d = 7.2 cm, the secondaries account for almost 50% of the total electron current. (2) (3) The secondary (4) Residual electron current. Small residual electron currents were observed ,for all probe positions and orientations. These are summarized in Fig. 4 for pure neon. These minimum currents were ccnstant for a range of -35 to -50 V in grid potential. For grid voltages in excess of -50 V there was a gradual rise in the current, presumably as a result of grid emission by the bombardment of positive ions. We find in Fig. 4 that the residual current decreases exponentially throughout the negative glow with increasing distance from the cathode for both orientations of the probe surface. Additionally, there is a reduction by almost a factor of 5 for the probe facing the anode versus cathode position. We have also observed no variation in the residual current as a function of radial position from the center to a point only 0.6 cm from the wall. The current increases slowly with axial distance in the Faraday dark space (d > 5 cm) and reaches a plateau in the positive column.
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