Acta Technica 65. (1969)
1-2. szám - Bitó, J. F.: The Time and Pressure Dependence of Cathode Properties
J. F. BITÓ a phase angle of 180 degrees; during that period, probe measurements were performed at intervals of 10 degrees, and the cathode fall was established for each test point by the extrapolation of the plasma potential values obtained from the probe measurements. The cathode-fall characteristic shown in Fig. 1 was plotted with reference to those values. This means that the measured values are instantaneous ones and, to facilitate reproducibility, the 0-degree phase angle was established in such a way as to coincide with the typical peakvoltage point of the tube-voltage characteristic (shown in the insert of Fig. 1). As a result, the starting moment of the measurement coincided with that of the discharge. In this instance, the half cycle begins at about 0,55 msec (10 degrees of phase) before the breakdown point; however, since no discharge occurs during this period of time (the transient phase), no cathode fall can be measured either. Therefore, it would have been impractical to regard the hardly determinable starting point of the half cycle (in the strict sense) as the starting point of cathode-fall measurements. As could be pointed out, the discharge is discontinued at about 0,55 msec prior to the end of the half cycle. Of course, no cathode fall could be established for that period of time either; accordingly, the tests gave the cathode fall of the discharge taking place for a duration of 160 degrees (about 8,9 msec) during a half cycle of 180 degrees (10 msec). As a result, the curve representing the time dependence of the cathode fall reaches the time base at a phase angle of 160 degrees (taking the shift corresponding to the location of the 0-degree point into account). As will he seen, the cathode-fall characteristic follows variations of the burning voltage in time. The cathode fall is by far the greatest at the instant of beginning of the discharge; afterwards, it settles at about 10 to 15 Y. To facilitate the interpretation of the various time dependences, in the following only the mean values of cathode fall obtained from the instantaneous values Fig. 1. Time dependence of cathode fall Vk and variations in the voltage drop Ve of the discharge tube throughout an entire cycle Acta Technica Academiae Scientiarum Hungaricae 65, 1969