Negative Current-Voltage Characteristics in Hydrogen at High Pressure Using Plane Parallel Electrodes

Abstract

In conjunction with measurements of current-voltage characteristics in hydrogen, a few characteristics have been obtained which include a region with negative slope. The latter characteristics were obtained with plane parallel electrodes at an electrode separation of 2 cm at a pressure of 400 mm Hg and with three values of externally initiated cathode current. The initial currents ranged from about 10~n to 10"^ amp, and the amplified currents reached values as high as 10~4 amp. The characteristics corresponding to the larger initial currents become negative at large currents (~10~6 amp). The voltage at which a characteristic becomes negative, i.e., the maximum attainable voltage across the electrodes, decreases slightly with increasing initial current. The circuit included a series resistor of 20 megohms.These characteristics can be explained quantitatively on the basis of the first and second Townsend coefficients (previously measured with the same apparatus) acting in conjunction with space charge, if a not unreasonable discharge area is assumed. These calculations were carried out on an IBM 704 computer. INTRODUCTIONW HILE measuring static dc breakdown potentials in hydrogen near atmospheric pressure with plane parallel electrodes, Lessin, 1 using apparatus previously described, 2 observed the following effects. With ultraviolet illumination of the cathode, it was noticed that as the generator voltage is gradually raised, the voltage directly across the electrodes initially increases, reaches a maximum value, and then decreases somewhat (by as much as 30 volts), still remaining stable. Further increase in the generator voltage results in the familiar cataclysmic irreversible spark breakdown, with a drastic reduction in the potential across the electrodes. It was also noticed in a rough way that the extent of the above effects depends on the intensity of ultraviolet illumination. The above observations were made with a resistance of 20 megohms in series with the electrodes. This resistance took up the difference between generator and gap voltages. These results imply the existence of a stable negative slope region of the current-voltage characteristics, heretofore unobserved at high pressures.A number of authors have discussed the existence of a negative characteristic theoretically on the basis of space charge along with a concomitant lowering of the breakdown potential by ultraviolet light. One of the earliest attempts to formulate this problem quantitatively was made by von Engel and Steenbeck. 3 More exact calculations subsequently were made by Crowe,

Oscillations of low-current electrical discharges between parallel-plane electrodes. III. Models

“…[44] A. Local-equilibrium model of differential resistance (39) where E~is the electric field at the cathode. This same Experiments have shown that in H2 discharges at low currents for both low pressures [2,9,16,19] and high pressures [18] the discharge voltage decreases linearly with increasing current for small voltage changes. This behavior corresponds to a constant negative differential resistance.…”

“…The predictions of our model of the normalized negative differential resistance are compared with published experimental data [9,18,19] and data from papers I and II in Fig. 7 for low-current hydrogen discharges.…”

“…The second topic of this paper is the extension of models for the effects of space-charge distortion on breakdown [15 -19] to the calculation of the differential resistance of low-current discharges [9,18,19]. This negative resistance plays a crucial role in the theory of the observed instabilities and oscillations [3 -5,9, 15].…”

Laser discharge initiation for gas-fed pulsed plasma thrusters

“…This statement has been contradicted numerous times in the literature, as researchers have found the sparking voltage to be lowered by high levels of photocurrent [2,3,8]. Since it is our goal to use a burst of photocurrent to produce a spark while at an undervoltage, we need to further examine the reported dependence of sparking voltage on photocurrent.…”

“…Previous analyses of the problem of photoelectric induced breakdown have concentrated on steadystate photocurrents lowering the applied breakdown voltage of a gap [2,3,4,5]. Using a photocurrent pulse as an initiation method from an undervoltage is less well documented.…”