Producing high current spark discharge is like producing lightning but on a smaller scale.
The property that initiates a lightning strike is the formation of the leader.
Leader propagation is an important phase in the discharge breakdown of very long gap in a gas. The leader channel is characterized by a high gas temperature (1000 K at the leader tip and 5000–6000 K in the long-lived regions of the leader) which causes a rise in the lifetime of the leader plasma by many orders of magnitude. In order to maintain the plasma during the leader development it is necessary not only to slow down the loss of electrons but also to ionize neutral particles. The current density in the leader channel is governed not by the plasma conductivity but rather by the ionization processes in the streamer zone in front of the leader tip. The process that does this is the development of a corona.
Corona dischargeA corona is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules.
When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a
sharp point , the the gas around that point will be at a much higher gradient than elsewhere. Gas near the electrode can become ionized (partially conductive), while regions more distant do not.
When the gas near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less
sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.
If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric arc.
Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations, so that the gas breaks down and forms plasma there first. In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large diameter rounded shapes like balls or toruses, and corona rings are often added to insulators of high voltage transmission lines.
Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly-curved (pointy) electrode. If the curved electrode is positive with respect to the flat electrode we say we have a positive corona, if negative we say we have a negative corona.
The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.
See this for more info:
http://en.wikipedia.org/wiki/Corona_discharge
Both positive and negative leaders will form with the negative leader being longer than the positive leader. When the positive and negative leaders find each other and connect, a flow of heavy current can begin.
In our case, it is important that the time it takes these leaders to connect does not produce a shockwave. This time of heavy current flow is what determines instantaneous power and thus the formation of the shockwave. This time should be as short as possible to mazimize the shockwave.
The spark formation process should be comprised of two stages; first leader connection across the electrodes using very high voltage; and next, after leader connection has been established then a flow of high current.
To emphasize,
shark electrode tips are important in the consistant formation of the leaders.
The following video shows how long it takes leaders to form when shark tips are not present in the charged volume.
https://www.youtube.com/watch?v=6MUYsIjTKvk&feature=endscreenAn experimentAs soon as someone gets their popper setup to scientifically measure the force of gas expansion, one experiment I would be interested in conducting to determine the role that leaders play in high current discharge is as follows.
Connect a very thin wile between the anode and the cathode but leave a very small separation between them.
The thin wires simulate the leader formation process. Then trigger the high current spark.
Both these thin wires will be vaporized by the high current and form a plasma channel that the high current flow will follow.
The small separation distance will minimize the time it takes ionization and associated breakdown to occur.
If an oscilloscope could measure the difference in spark initiation time that rapid leader production produces, this time can be correlated against any change in expansion force produce by the hydrogen gas.
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