The effect of gas pressure on the characteristics of a short-gap lightning discharge in air was investigated. For the tests, 70 ns front lightning pulses were applied to a short (11 cm) point-plane gap under variable pressure. The diagnostics employed included electric current and field measurements, spectroscopy in the visible and fast-frame photography. We found that the pressure has a clear effect on the electric field at the plane. For low pressures, the high fields measured (∼7 kV cm−1) are comparable to the Laplacian field, indicating that very little ionization takes place in the gap at this pressure; at higher pressures the space charge contributes substantially to the field magnitude. The effect of pressure on the current pulse was, in contrast, minimal; its peak amplitude and shape remained largely unaffected by pressure. Time-resolved spectroscopy allowed the determination of the instantaneous electron density and temperature to be made; the latter, for example, was found to reach 33 000 K at t ∼ 1 µs for most of the pressures employed. Using the measured temperature and radius we made estimations of the arc's resistance. We found that the Spitzer resistivity model gives values of resistance that are compatible with the experimental data obtained.
The properties of the impulse discharge in carbon dioxide under variable pressure in the range 26.7 kPa -0.16 MPa (200-1200 Torr) were investigated using fast photography and electrical measurements. In the experiments, fast lightning impulses (70 ns front) were applied to a point/plane gap with gap separation of 6.5 and 11 cm. It was found that the peak current was not affected by pressure; the electric field, on the contrary, did respond to it. The computer simulations and electrical field measurements performed show that the space charge had a dipolar structure. This structure might be responsible for the diminished negative breakdown level observed in CO 2 . It was found that the arc expansion in the first 3 μs is very fast and slows down afterwards following a logarithmic curve in time. The pressure of the gas had a small, inverse effect on the arc's diameter.
The effect of gas pressure on fast impulse discharges (70 ns) in a short point-plane gap was investigated. Fast-frame photography allowed the tracking of the development of the discharge with high temporal resolution. At low pressures, the gap is initially bridged by a bright glow that can carry significant amounts of current. At higher pressures, the gap is initially filled with numerous filaments that transport little current. In this case, the current takes off only when the arc bridges the gap. In the interval t = 1−10 µs, the arc exhibited a braided structure that rotated rapidly.Index Terms-Effect of pressure, lightning discharge.A IR DENSITY affects the development and propagation of electrical discharges; a reduction in density, for example, diminishes the threshold field for ionization in air [1]. Several researchers have found it useful to simulate the lightning discharge in the laboratory, and valuable physical information can be garnered from impulse experiments [2]. One of the goals of this paper is to experimentally simulate the effect of altitude on a lightning discharge. Accordingly, we varied the pressure from 760 torr for ground level to 200 torr corresponding to the altitude of thunderclouds. In order to extrapolate the effect of pressure, we also used 1200 torr.The experiments were carried out by using a set of pointplane electrodes inside a sealed discharge chamber, and the pressure was varied from 200 to 1200 torr. The gap separation in all the tests was 11 cm. Fast negative-polarity lightning pulses were applied to the rod electrode. The voltage pulse had a front of 70 ns when applied to a 100-pF load, and its tail had a length of 60 µs. The electric field at the plane and the current were monitored during the tests. An intensified high-speed chargecoupled-device camera (PI-MAX from Princeton Instruments) was employed to take fast frames of the discharge. The camera's gate width employed varied from 2 ns to 1 µs depending on the brightness of the subject. A built-in delay generator controlled the timing of the photo after trigger reception. This was provided by the electric field signal that initially follows the application of the HV pulse.The results obtained at different pressures are shown in Fig. 1. The frames were taken at separate shots, and the time elapsed after application (t = 0) of the pulse is shown in the labels. For air pressures 200 and 400 torr (top rows in Fig. 1), the gap is initially bridged by a bright conical glow while a short cathode stem can be seen at the point cathode. The frames at t ∼ 300 ns show that the cathode stem has elongated and that the glow continues to fill the gap. By t = 400 ns, the gap has already been bridged by a discharge channel. For the higher pressures, the discharge initially consists of numerous filaments filling the entire gap, as can be seen in the third and fourth rows in Fig. 1. At intermediate times, a leader was seen departing from both electrodes and meeting midgap, as shown in the frame for 420 ns in the bottom row.For all the pressu...
[1] At a concentration of 10-50 ppbv, methane is suggested to be a trace element in the Martian atmosphere. The sharp variations in its concentration observed are difficult to explain using current theories for sources and sinks (be it biotic or abiotic). Here we propose, and demonstrate with a lab simulation, a new production mechanism for methane based on the effect of electrical discharges over iced surfaces. The discharges, caused by electrification of dust devils and sand storms, ionize gaseous CO 2 and water molecules and their byproducts recombine to produce methane. Our experimental results show that pulsed electrical discharges over ice samples in a synthetic Martian atmosphere produce about 1.41Â1016 molecules of methane per joule of applied energy. The results of the electrical discharge experiment were compared with photolysis induced with UV laser radiation and it was found that both produce methane although the efficiency of photolysis is one-third of that of the discharge.Citation: Robledo-Martinez, A., H. Sobral, and A. Ruiz-Meza (2012), Electrical discharges as a possible source of methane on Mars: Lab simulation, Geophys.
The properties, as a function of pressure, of an arc produced by fast impulses in a carbon dioxide (CO2) atmosphere were investigated. A time-resolved spectroscopy technique consisting of a spectograph coupled to a fast camera was implemented to analyse the light emitted by the arc. Using this technique the arc's spectrum could be obtained at different moments of the development of the discharge. The use of a multi-element Saha–Boltzmann method allowed both the temperature T and the electron density ne of the arc to be obtained. These were found to have peak values of T = 28 000 K and ne ∼ 9 × 1017 cm−3 at the time of maximum current. The measured temperatures were incorporated into a variation of the Spitzer model to obtain the plasma resistivity. With it, the arc resistances were calculated and found to be consistent with the measured currents (∼1.85 kA). Mass spectroscopy sampling of the spent gas shows that CO2 degraded very little after several dozen kiloampere shots.
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