MEASUREMENTS IN GAS DISCHARGES quation(19)above can be written as Maxwellian distribution. Then exp(ivA)/cosh(lov a). where j, is the random ion current density, but p)=iova-ln cosh (pv d).(20) A plot of Eq(20)is given in Fig. 16. The values of where ip is positive ion current to In(iea/ip) were computed for a set of experimental data sheath area and the points plotted on Fig. 16. The agreement Estimates made by the method of Langmuir and between theory and experiment is agreeably good M1ott-Smith! indicate that at the points y and a of Reifman and Dow have plotted the quantity In(ie/ip) Fig. 4, the positive ion current to the probe is space- for measurements in the ionosphere. Their double probe charge limited but that the sheath area may be appre consisted of the nose and a portion of the body of a ciably larger than the probe area rocket. From their curve they conclude that the ob- Making the substitution Cp=1.87X10-(Tp/M) we served electron distribution is not Maxwellian. Since obtain the shape of their observed curve is similar to ours, it is np=(1.34×10/A)ip(M/Tp) (24) of the In(ie/ip) should not be altered qualitatively even Let us apply this to the case of Fig. 11. For that case ,=1.35×106 amp, T X. DETERMINATION OF ELECTRON AND ION =350°K.Then DENSITIES AND OF WALL POTENTIAL p=(8.0×10/4)ions/cm3 Neither the SPM nor DPM are suited for the deter- To determine 4, we make use of the space-charge- mination of the electron density ne in decaying plasmas. limited current equation for cylindrical diodes This arises from the fact that the plasma potentia ip=14.66×10(m/M)(Lv/r6), Gi. e, in a positive direction) in potential. As a con- where v is the difference in potential between the sequence, ent to the probe unless 35 SPM it is the saturated electron 82=162V current (corresponding to the bend in the current To obtain V, we recall first that when the probe is at voltage characteristic) which is used to cor apute the wall potential(ia=o) The situation is not completely hopeless, however. varies from zero in Fig. 11 to the saturation value of kno p, it is merely necessary ipu, ien changes by a factor of about 2(this is readily to set a value on one unknown, the positive ion tem- obtained from Fig 12). The probe-space potential must perature Tp. This is an exceedingly fortunate situation, then decrease by an amount determined from the doubtedly very close to Te, the gas temperature This Boltzmann relation: nce the value of the decaying un- follows from the fact that even though the electro 0.5=exp[(11,600/950°K)△V], mperature may still (in some cases)be considerably or above gas temperature, the kinetics of the impacts of △V=-(950/11,600×0.093≤-0 the ions with electrons and gas molecules is such that Then it is the temperature of the latter which will dominate V=(0.50-0.06)=0.44volt will be seen, e and n, vary as the square root of t Thus, errors in selecting a value for Tp will hav much smaller effect on the values of n, and n. We set where ca is the average drift velocity of the ions. In the decaying plasma, where the space-charge fields extremely small, CA must be due almost entirely to the outward motion from the plasma into the sheath arising from the random motion of the ions that case CA=tCp, where Cp is the ion velocity averaged over a re figures very close those obtained by the methods here described FIG. 17. Idealized double probe characteristic