MEASUREMENTS IN GAS DISCHARGES ortion ozw or ozw. The flat portion ww or sw corre- sponds to positive ion saturation current to probe No. 1 The total positive ion current to the system is simply the sum of the positive ion currents to both probes and so can be found by adding the magnitudes of the cur rents at y and z, as symbolized by ip and ipr The electron current which flows from the plasma to probe No. 2 is simply the difference between the total space current and the positive ion current to this probe. Thus the electron current ie, to probe No. 2 is given by The value of ie, which corresponds to a voltage V d is illustrated graphically in the same figure FIG. 7. Ve aracteristic of the double where he potential diagram of Fig. 8 yields VI+vo=V2+vd or V1=V2+Va-Ve.(5) (b) Substituting (5)into(4)and rearranging, we obtain In[(2ip/ie-1]=-pVa+In o=In I,(6) where T=(Eip/ie)-1 (A1jo,/Azjo ) et Thus the plot of In r against Va should straight line whose slope is a measure of the d"FAIRLY LAR temperature. This equation is seen to be EGATIVE VOLT form to that used in the SPM except that in(6)one uses r instead of the electron current It is to be noted that the slope of (6) is essentially unaffected by any of the factors included in electron random current densities, difference in plasma potential FIG. 6. Sample potential diagrams of the double probe method. between probes, and contact potentials. For an unam biguous determination of Te, the random current den V. TEMPERATURE DETERMINATION sities should not change with probe current. This is A. Logarithmic Plot Method much more likely to be the case with the DPM than The generalized potential diagram for the system with the SPM since the current drain can be hundreds of Fig. 5 is shown in Fig 8. The potentials Vi and v, of times smaller in the former case. Another important represent the voltages of the surrounding plasmas with difference between the two cases is seen from Eas respect to the corresponding probes. The potential Ve and( 8). We note that the constant term of the latter is represents any small difference in plasma potential free from any restricting dependence on the plasma which may exist between the regions surrounding the potential. Thus we see that the DPM is inherently a probes, plus the total contact potentials acting in the more general method. It can be used during or after the system. The other symbols are defined in the figu discharge and even when the plasma potential varies Since the net current to the system must be zere with time Possible errors in this method require discussion. The ralues of ipi and ip, used were those at which the curve Substituting the equivalents for ie, and i, in terms of of Fig. 7 broke away from the saturated regions Boltzmann relation we obtain of error present. In the first place, there is Σip=A1jiof1+Ajig∈p, (4) tainty in the choice of points y and z. To see how serious