Sanders investigated the Townsend coefficient (α) in the regions with different pressure and distance levels in his two papers. For the former one, the author criticized the early measurements of Townsend, Bradbury and Paavola to give an explanation to the current values for higher X/p (the ratio of electric field strength [V/cm] to the pressure [mm]). Townsend explained the energetic ions cause more ions and introduced value which represents new pairs of ions per centimeter. However, his experiments were conducted in a low pressure, small distance, high electric field environment and the results failed to explain lower X/p values in high pressure conditions. The researchers proposed that at higher pressure level, the number of the created ions increases due to electron impact ionization in the dark current just before the breakdown. The distorted fields with space charge satisfy the Townsend equation with new values.
For the lower X/p, a sensitive test setup should be designed. Until then, the values could not be measured correctly but with an advanced experimental arrangement, the breakdown conditions were explored on the papers in detail. The plates were placed as parallel and the distance between them was varied with a rod-magnet assembly. Air was dried perfectly and electrons were produced by an ultraviolet radiation source. The photoelectric current was measured carefully by eliminating errors. The log of current with respect to the distance was sketched and the values were calculated by the method of least squares. As a result, a pure exponential solution was observed. Townsend thought α is independent of the current for the distances below 1 cm which was verified in the study. The experiments showed the lower X/P’s around 20 could be measured successfully. No saturation was observed as stated from the previous works asserting that saturation is not the case at the field strength before sparking. Also, current was detected below the sparking without excitation when X/p is around 36.5. The possible reasons were space charge build up and a corona discharge. If those assumptions are correct, positive ion ionization should occur at fields closer to the breakdown voltage than 2 percent. It was claimed that before the breakdown, ionization is caused by the electrons and the distortion in the field due to the space charges.
In the second paper, the experimental data was supplied more clearly. After X/p passes 120, the linearity in log graph turns into the curved line, implying that there should be a secondary process increasing the ionization by electrons or ions in high field strength. A new parameter was introduced and included in the equations. γ is the number of electrons liberated by one positive ion on impact with the cathode. The values for α, β, and γ were compared with the other studies. As a result, breakdown mechanism in air was investigated from low X/p to high X/p by extending Townsend’s work. These experiments paved the way for further studies in ionization.
Reference:
Sanders, F. “The value of the Townsend coefficient for ionization by collision at large plate distances and near atmospheric pressure,” Physical Review, 1932, 41, 667-77.
Sanders, F. “Measurement of the Townsend Coefficients for Ionization by Collision,” Physical Review, 1933, 44, 1020-1024.
Course: AME 60637