E. O. JOHNSON AND L. MALTER mined primarily by electron mobility considerations current id must be zero since no net potential acts in and are thus less than 10- sec. for this particular tube. the current loop. This condition corresponds to point o on the curve of fig. 7 IV. THE DOUBLE PROBE METHOD (DPM The double probe method makes use of two probes, (b)Va=small negative voltage( Fig. 6b) each similar to the single probe of the SPM. They are The probe potentials with respect to the plasma must interconnected as shown in the circuit of Fig. 5. The adjust themselves so that the basic current relations are potential Va is termed the differential voltage, and its still satisfied. The consideration of a few possibilities associated current, id the circuit current. The positive will show that the only way in which the system can sense of these quantities is established by the arrow satisfy all conditions is that it assume the potentials directions where we define positive current as the rate of shown in Fig. 6b. Probe No. 1 moves closer to plasma flow of positive charge. Unless otherwise noted the potential and collects more electrons, and probe No. 2 circuit of Fig. 5 and its polarity convention will apply moves away from plasma potential and collects fewer to all of the discussion which follows. In brief, the elec- electrons. The extra electrons flowing to probe No. 1 tron temperature will be determined from the way in pass through the circuit to make up the deficiency at which ig varies with v probe No. 2. All conditions are again satisfied and the As with the sPm the dpm is based on the boltzmann Dint b on Fig. 7. lation and the plasma-sheath properties of a gas discharge. In addition it is based on an application of application of (c)va=somewhat larger negative voltage(Fig. 6c) Kirchhoff's current law which requires in this case that Probe No. 1 moves still closer to space potential and electny instant the total net current of positive ions and collects the entire electron current to the system since ons flowing to the system from the plasma must probe 2 is now so highly negative with respect to he plasma that no electrons can reach it. Half of the Qualitative Treatment electrons reaching probe No. 1 now pass through the external circuit to probe No. 2. All conditions are satis- As an aid in understanding the mathematical formu- fied and the system locates at some point y on Fig. 7 lation, let us consider qualitatively how the system acts for several different values of vd. For sim- plicity, let us first assume that both probes are equal in plasma potential from point to point exist. Further- more we assume that v a has no effect on the ion current system.This is very closely approximated in a)Va=0(Fig 6a) plasma and will ride at the same floating potential. The SCoPE FOR R OScTIAL Each probe will collect zero net current from the FIG. 5. Ba ble probe cir Further increase in the negative value of Va can cause lo further change in the current distributions because probe No. 1 already collects a sufficient electron current to balance the entire positive ion current flowing to the system. Consequently probe No. 1 remains fixed with respect to the plasma and probe No. 2 goes negative along with V d. We can speak of the latter probe as EP VOLTS being saturated with respect to positive ions as the system moves along the fat porte of Fig. 7. In practice one finds that this fat portion has a slight lope as shown by the dotted portion yx. This slow ncrease is due to an expansion of sheath thickness as the probe goes increasingly negative with respect to the plasma. This will be discussed in greater detail FIG. 4. Probe floating potential f anode potential The symmetry of the system will cause it to reverse the previous results when V d is positive, giving the