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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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