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

[Peter Terren]

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 at aquila.net>


> 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