Plasmoid Generation from Water Discharge
This series is quickly turning into a "reading a paper" series, which is fine enough for me. At the end, hopefully I will have a somewhat detailed understanding on the highly specific topics of said papers, and the papers need to be read either way. For this time, we're starting with a few papers featured by the IPP in Munich. In no particular order, I'm starting with "Generation of an atmospheric plasmoid from a water discharge" [https://doi.org/10.1063/1.4816311], just because it shows up first in my system. All four(-ish) papers are based on the works of one A. D. Shabanov, which I'm assuming will show up in this series sooner rather than later. The gist of it is an experimental setup that yields a spherical plasmoid about 20 cm above the surface of water discharge (at atmospheric pressure). I'm not sure how this will be relevant to me specifically, but that's what I'm trying to figure out.
The experimental setup features a water container with electrodes that stick through the feedthrough in the bottom plate of the acryl glass container. The central electrode is a tungsten rod along the center z-axis of the cylindrical container. The choice of material is less prone to evaporation than copper. The rod is set into a ceramic coat, in a way that a small amount of water can fill the space between, to avoid contact with the other electrode, which is a copper ring surrounding the rod. The tip of the tungsten electrode is sharpened, presumably for more controlled discharge. The central electrode is grounded out and the plate electrode is powered with 4.8 kV. The setup allows for variable distance between electrodes. The geometry of the cylinder is assumed to be relevant to the results. The energy supply is behind a capacitor bank with a total of 2.475 mF, and outputs 5 kV with up to 500 mA. A switch is installed between the power supply and the central electrode contact, which can be pulsed open to generate currents up to 100 A. The autonomous phase of the plasmoid occurs on the open switch position. Voltage and currents are essentially measured by an oscilloscope, meaning more of less directly. By parallel switching, the capacitance of the capacitor bank can be varied from 0.55 mF to 2.475 mF. The conductivity of the water can be varied by salinity. Notably, the temperature of the water rises after several discharges. The measurements of the plasmoid's spatial dimensions are taken using a high-speed camera with 600 fps and 1/125 s exposure time, observing the plasmoid from the side, focused on an area of 15-20 cm above the water surface. The distance is arranged, so that the plasma should be visible during its whole lifetime. The size estimation are helped along with markers behind the setup. The dimensions of the plasmoid are of central interest in this case. The steady, near 1 correlation between height and width, indicates a spherical form of the plasmoid. The width is slightly larger than its height. There is a slight expansion of the plasmoid as energy transferred into it increases. The ascent velocities are in the low m/s areas, with a linear relationship to the water's salinity. During the autonomous phase, the plasmoid's energy, as well as lifetime increases with voltage-on time. Rising height and velocity also scale with the energy deposited into the plasmoid. Almost all these relationships are linear.
In the initial phase, the energy forms plasma streamers in within the first 20 ms over the water's surface, which originate from the inner electrode. They move outward symmetrically at random, until the entire surface is covered with a thin plasma layer. This establishes conductive channels to transport greater amounts of energy to the plasmoid. This process does also interact with the water, as it produces small waves on the surface. The light reflects the spectra of calcium, which is likely that naturally present in the tap water, hydrogen, and sodium from the added salinity.
During the later stages of the on-voltage time of the central electrode, a spherical plasmoid with a diameter of about 20 cm has already formed on the tip of the tungsten rod. So far, no plasma channel to the rod is visible. When the switch is opened, and energy supply is cut off, the plasmoid begins moving upward with 2m/s, vanishing at around 400 ms. The lifetime scales with the total capacitance of the charge.
The lifetime of the plasmoid is divided into the ignition phase, the formation phase, and the autonomous phase. In the ignition phase, the lightning-typical charge breakdown the streamers begins to develop, and after 25 ms, the entire surface is covered with plasma, forming a highly conductive channel for the current flow.
In the formation phase, usually reaching from 70 ms to the end of the voltage-on time (150 ms), the plasma detaches from the surface and the spherical geometry arises. The plasmoid is still connected to the electrode by a plasma channel. The autonomous phase begins with the time of detachment from the electrode. Whether it is due to the opening of the switch, or charge balance is irrelevant for the categorization.
During the voltage-on time, the voltage drops more or less steadily. The current jumps quickly to a maximum, then decays stronger, but similarly. The drop is associated with the development of the lightning streamers from the central electrode to the water surface. At standard, the maximum current reaches about 55 A. The energy dissipated into the experiment is at 19.09 kJ, and the maximal energy stored in the capacitor is 28.5 kJ.
The dissipated energy is determined primarily by external parameters of the experiment. At time zero, when the switch is closed, the air is broken down and the resistance drops quickly. The resistance of the water reservoir helps calculate the power dissipated into the water.
The main part of the power is dissipated into the water reservoir, even though the power of the plasmoid peaks at the initial streamer formation. From this, one reads from the data, that only about 22% even reaches the plasmoid.
The electric resistance of the system is a sum of the experiment's component, along with the non-constant plasma resistances. The resistance of the water can be tuned to be the only effectively relevant component. The streamers, and the plasma at both electrodes have their own resistances. Using Kirchhoff, this can be used to determine the current at each element, and most importantly for the development of the plasma.
The electrode gap and area will see the a variable effective area, somewhere in between that of the central electrode and the plate electrode. This wraps up the known parameters of the experiment. The best fit according to expectations is found at an effective area of 200 cm² (at about 58% of the plate electrode). The average resistance of 132 Ω, which is higher than the minimum resistance 88 Ω, derived from current and voltage. The largest increase in dissipated energy is achieved through increase in the voltage and capacitance, which in turn tends to increase the damping effect.
The analysis based in the total dissipated energy of 19.1 kJ, 4.2 kJ is dissipated into the plasmoid, and 14.9 kJ into the water reservoir, which will result in a temperature change. For the plasmoid itself, conduction, convection and radiation are the relevant loss mechanisms. The heat conductivity of air is poor, and a distinct boundary between plasmoid and surrounding air is visible. Spectroscopy yields the radiation loss component. The total power radiated power is estimated to 50 W, and the projection to the typical lifetime will place this at 0.5 of the plasmoid energy. The remainder will likely be convection losses, though reabsorption processes are neglected for the lack of data.