MEASUREMENTS IN GAS DISCHARGES ANOCE time Il, the grid sheath extends across the 1 mm grie PROBE ARGON AT opening thus isolating the plasmas on both sides of the grid. In Fig 3, t is the time at which the kinks in the decay curves occur. For any value of Ep, E could be set so that at some time (>li, the current to the probe was zero. Under this condition, the probe is at floating CATHODE potential E/. Ep was varied over a range of +16 volts on either side of ground potential and Er was determined FIG. 1. Section th uag e prere en dae twin ora measuring fioating in each case for 1>h The results are plotted III. POTENTIAL PROPERTIES OF A The plasma potential Es, is always slightly positive DECAYING PLASMA with respect to floating potential. The difference Consider a region enclosed by a unipotential be between the two is a function of the electron and ion temperatures. Since(as will be seen) both these tem- of any shape whatsoever and suppose that at peratures are low for I>li, it follows that E, lie close the region is filled with a plasma of arbitrary to Ey and therefore close to Ep, i. e, the space potential strout that, due to their greater velocity, electrons will begin when E,=0, E, appears to be about -0.8 volts Since to pour out into the bounding walls. However, as the contact difference of potentials between the various this occurs, the plasma potential will rise. This will, electrodes is not known, nothing can be said concerning in turn, soon prevent the further loss of all but the the significance of this value fastest electrons,(as determined by the boltzmann In order to determine how rapidly the plasma poten equal that of the rate of loss of positive ions. The plasma potential of the anode was varied by means of steep now decays while retaining its plasma-like character- wave-front signal during the period (>t1. It was found istics In low pressure discharges the losses of charged that equilibrium conditions(as observed on potential will be slightly positive with respect to that Since this is about the rise time of the pulso ishe scope) particles are by the process of diffusion. The plasma were always re-established within about i es employed of the surrounding envelope which will be at the wall it is impossible to estimate the time for equilibrium to potential. As will be seen later the difference in potential be established. Very likely the times involved are deter between the walls and the decaying plasma will not be more than of the order of tenths of a volt Now let some portion of the envelope be increased in potential with respect to the remainder. Then excess electrons will pour into that portion ing the poten- tial of the plasma to rise until once again the balance between loss of electrons and positive ions is attained The plasma will now assume a potential slightly positive with respect to the most positive electrode with which it"makes contact. "[An exception to this occurs when the area of the most positive electrode is so small that the normal electron diffusion current to FIG, 2, Circuit for measuring floating potential of probe it does not exceed the positive ion diffusion current to the entire boundary. This property of decaying plasmas vas demonstrated in a very simple fashion a tube was built in the form shown in Fig. 1. The ling, was argon at 2504. It was connected in a circuit as shown in Fig. 2. The tube was fired by impressing a negative pulse on the cathode. The current to the probe as a function of time and Eb, following the inter- rup: ion of the discharge was measured by means of the scope across Rs(Eb is the probe battery potential Two typical oscilloscope traces are shown in Fig. 3 The lower curve is for a case in which the probe is FIG 3. Decay currents to probe in tube of Fig. 1 slightly positive with respect to the anode; i.e upper curve is for the case in which Es-Ep <o. At a i. rech. Pros. 17, 6 1930).ngs is descnbed by the formation of Eb-Ep>o,(Ep is the anode supply potential). The