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Re: [TCML] Rectifying A Tesla Coil: Point-Plane Collector Gaps in sparks



I have a lot of pics with gaps in normal sparks here:
http://tesladownunder.com/HVsparks.htm#Spark%20structure
Peter

----- Original Message ----- From: "Bert Hickman" <bert.hickman@xxxxxxxxxx>


Hi Jeff,

Gaps and bright regions within sparks seem to show up within relatively low energy HV systems - i.e., where terminal capacitances and follow-through currents are limited. They are easily observed within low energy electrostatic discharges, with lower power Tesla Coils, or within sparks from pulsed induction coils.

The root causes appear to due to fundamental differences between how sparks propagate across the gap from positive or negative electrodes and (in the case of Tesla coils or other repetitive discharges) as previous channels become reignited. These subtle effects tend not to be as readily observed within high energy sparks or arcs since the high. arc-like follow through currents brilliantly light the bridged gap. Best observations seem to require using a low energy/high impedance power source, low parasitic capacitance across the gap, and adjusting the gap so that the discharges can just barely bridge the distance.

In a positive discharge (where the discharge begins propagating from the more anode to the cathode), the leading edge of the propagating positive leader, and the leader channel that connects back to the anode, are bright and filamentary/spark-like. However, the streamer region ahead of the leader tip is diffuse, dimmer region that looks like a directed cone of corona pointing towards the cathode. If the HV source does not have sufficient energy to completely bridge the gap, or if the HV pulse is of very short duration, you get a miniature Tesla Coil-like air discharge: a bright length of spark that only goes part way across the gap, with a diffuse glow between the sharply defined leader tip and the cathode electrode.

A negative discharge ((where the discharge propagates from the more negative electrode to the anode) is much more complex. A bright, seemingly isolated, length of spark (called a space stem or space leader) appears in the space ahead of the main leader. Corona-streamers form on BOTH ends of this isolated space leader, and the space leader then grows backwards toward the main leader, eventually joining, and lengthening it. Negative leaders progress though a series of steps or jumps. This contrasts with positive leaders, where the propagation path tends to smoothly progress (assuming sufficient gap voltage). These negative propagation steps can be relatively small for low energy systems, or 100's of feet in the case of negative lightning (stepped leaders). If the growth process is terminated before the spark can fully bridge the gap, you'll see the bright filamentary main leader from the negative electrode, a diffuse corona filled gap, another isolated bright space leader segment, and then another diffuse corona region pointing towards the anode. Isolate bright regions within a spark may be space leader segments.

In systems where discharges simultaneously propagate from both electrodes, or where previous channels are being reignited, you may get a very complex interplay of both negative and positive discharges with various bright regions being illuminated.

You may also see another interesting effect where a leader has branched. These are points where the spark makes a very abrupt change in direction. These sudden changes cause the "tortuosity" of long air sparks. Only the "winning" (higher current) leader is usually observed. However, if you look closely at some spark photos, you can often see a short segment that went to the losing leader branch. The branch between the two leaders forms a small "V". The "branching angle" are similar for most discharges, and are centered at around 37 degrees for sparks at virtually all scales. This is seen for TC sparks, 2D and 3D Lichtenberg Figures, and lightning. The only exception that I'm aware of is the strange branching (sometimes almost 90 degrees!) seen within high power staccato VTTC discharges.

There's a virtual treasure trove of information lurking within your pictures, Jeff!

Bert
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