The voltage drop across gas bubbles increases with diminishing current over each period of pulsations (Fig. 3). When the electric current diminishes to a critical value, it nearly
extinguishes. The voltage drop across the bubble at this moment quickly increases to the breakdown magnitude, and breakdown occurs again. These pulses are correlated with periodic extension and shrinking of gas bubble in its length (Fig. 3). For a bubble located near the anode, the bubble shrinks to its initial size. A bubble located in the middle of the tube does not shrink to initial size. The initial bubble can be fragmented after breakdown into several smaller bubbles [Fig. 2(d)]. The average current (i.e., averaged over high-frequency noise) decreases with an increase in length L of the bubble. The average voltage
drop U across the bubble grows with increasing length of the bubble.
The free-running pulsations in the length of gas bubble likely occur due to an electrical breakdown and gas discharge inside the bubble. After electrical breakdown, evaporation of
liquid begins that results in a growing gas pressure inside the bubble and its expansion. The water vapor is an electronattaching gas. Therefore, the increase in length of a bubble
(i.e., increase in total intensity of electron-attaching processes) results in decreasing the electron number density, and the current drops. On extinction of the discharge inside the bubble, water vapor condenses rapidly. The bubble length diminishes again to its initial size, and a new breakdown in sequence repeats.