E.O. JOHNSON AND L, MALTER Along the saturated portions ab (or cd) a change in Va results in an identical change in the value of Va(the potential of probe No. 2 with respect to the plasma as illustrated in Fig. 8), since we move beyond b into the electron collection region. We are interested in how ht of b region is given by the well-known space-charge limited equation for cylindrical diodes however. A long series of computations fae any caae effct of A? here c is a constant, Tp is the probe radius. Equation (27) nd voltage of interest showed that for the cases we are concerne with ries linearly with Va to the right as well as to the left of b. Thus if changes in V2 were equal to those of va, the i simply extending the line ab through e and f as shown in Fig. 17. Actually since V2 changes less rapidly than Va in this region, the determine the value of i.at e If in Eq.(6)we substitute for ia its equivalent value A jo e-9 --anToe -va +114ojos Now let I'a take on two values Vd and vd as shown in Fig 17 Let the values of Va corresponding to these two values of vd b Then FIG. 18. Relation between space potential and (29) 2=1.62×0.29=0.47 Zip/ie=Zip/[Anjose-9v2]=F, The and similarly let Zip/i, corresponding to I'2 and vd be denoted r'/To=1.75, by G, Equation(29) then becomes △V2=-(1/4)ln(G/F) where I, and Ip are respectively the sheath and probe From Ea. ( 6) and the definitions of F and G we can obtain radii. Then d=-(1/φ)ln[G-1)/(F-1)] A,=1.75Ap=1.75×0.16=0.28cm2 Combining Substituting this value in(25) there results n(F/G) From the equality of electron and ion density, we have to the point d of Fig l7 where detectable electron current fows. Experimentally it is observed that a definite deviation from the 29×10 electrons/cm3 straight line region of the V d-ia characteristic is observable when The random electron and ion current densities can now n/i42<100 be obtained from Eq.(21)using the proper value of c As a consequence, the value of ava/ava was in each case (32)for F=50, and F=100. The results are plotted in Fig. 18. For the case of a symmetrical vd-id characterist Variation of Positive Ion Current 1/G=i2ip=0.5 to Probes with Probe potential Av2/Avd08. Thus in practice, to find the value of in corre e merely extends the region ab of Fig. 1 mination it was necessary to know the value of the ion linearly to a point 0.8 of point a in the 'aid characteristic. We shall now present a crude showing how the desired quantity may be calculated Consider an idealized DPM Vd-id characteristic as shown in Fig. 18 to determine how far beyond the linear on ab one must ig. 17 with purposely exaggerated slopes for the ion saturation extrapolate in order to determine i egions. These slopes arise from the change in sheath thickness In the curve of Fig. 11, which is very close to being symmetrical, th change in voltage across the sheath the figure 0.8 was used