[TCML] Rectifying A Tesla Coil: Point-Plane Collector Gaps in
pterren at iinet.net.au
Thu Sep 4 11:00:37 MDT 2008
I have a lot of pics with gaps in normal sparks here:
----- 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
> 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!
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