PHYSICAL REVIEW VOLUME 80, NUMBER I OCTOBER 1, 1950 A Floating Double Probe Method for Measurements in Gas Discharges E.O. JOHNSON AND L MALTER RCA Laboratories Division, Radio Corporation of America, Princeton, New Jersey Received March 14, 1950) easurements with a Langmuir probe in varying or decaying plasma potential in these cases follows that of the most positive t can contact. The difficulties can be obviated by the use of a pair of probes joined by a variable potential source. The double probe system"floats "with respect to the discharge system. densities can be determined. The method is also applicable to"going discharges where it has the advantage over the single probe of exerting a negligible influence on the discharge. a plot of the logarithm of the electron current in this discharges either of the stationary or time- region us probe potential yields a value for the electron varying type it is generally the case that the elec- temperature Te,(3)A region of electron current only trons present in the plasma regions have a Maxwellian in which the current increases slowly with increasingly distribution. If this is so the concept of temperature can positive potentials be associated with the electrons. The electron tempera In region 2, the current can be expressed as: ture is denoted by Te Knowledge of the electron temperature is of im Nie=-(e/kTe)V +InAjo, portance in the determination of such quantities as the where i, is the electron probe current, jo is the random ambipolar diffusion coefficients. Langmuir and Mott- electron current density, A is the probe area, Te is the Smith have described a single probe technique for electron temperature. measuring electron temperatures as well as of other In Eq. (1)V is the plasma potential measured with quantities such as electron density and wall and space respect to the probe. This infers a knowledge of the potentials. Their method can be used for stationary and plasma potential. In practice this need not be known for certain types of time-varying discharges. However, since it can be replaced by its equivalent Vs-Vp in any case, unless its area is extremely small, the probe where V s is the cathode-plasma potential and vp is the may draw sufficient electron current when operated cathode-probe potential. Equation(1) then becomes lose to space potential to disturb the discharge cor ditions which it is designed to measure. As will be Inie=(e/kTevp+InAjo-ev s/kTe). made clear below, the single probe method(SPM), is Thus it is merely necessary to plot Ini, s.vn quite unsuited for such cases as the decaying which is present following the interruption of to secure a value for Te. An examination of (2) charge. a double probe method (DPM)has some possible difficulties with the SPM. Equa- tion(2)is significant only if Vs, Te, and jo do not developed which exerts a negligible influence on a dis- change with V. In actual practice, as i, increases one charge and which seems to yield accurate temperature often observes changes in the discharge patterns, par- data in all types of discharges, including a decaying ticularly if ie is an appreciable fraction of the discharge plasma. Reifman and Dow have described a double probe method for measurements in the ionosphere. current. Under these circumstances, the probe is dis- intended to measure. It would definitely be more satis II. CONSIDERATIONS REGARDING THE SINGLE fying if measurements could be made in a manner less PROBE M likely to disturb the quantities it is intended to measure In the langmuir As will be seen, in the double probe method the cylindrical, or spherical electrode is immersed in a current to the probe can never exceed the positive ion discharge plasma and the current to it measured as a hundreds of times smaller than the electron current to In this region the current increases slowly with in- this respec, e the DPM appears to be advantageous in are observed:(1)A region of positive ion current only. Langmuir and Mott-Smith have shown how in ad- creasingly negative potential on the probe. (2)A region dition to electron temperature, the probe data can e current passes increases rapidly with increasingly positive potenti yield values for electron and ion densities; space and wall potentials and for random electron currents. The BA Reimo ndH, M. Mott-Smith, Gen Elec. Rev. 27, 449, objections cited above for the use of the SPM in electron in and W. G. Dow, Phys. Rev. 76, 987(1949) temperature determinations are equally applicable here