Re: Spark Gap Sustaining Current (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 29 Sep 2007 16:28:45 -0500
From: Crispy <crispy@xxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Spark Gap Sustaining Current (fwd)

Thanks a lot, that helps me greatly.  Perhaps I should elaborate
somewhat on my end goal with all of this.
I'm trying to design and make a type of disruptive discharge Tesla coil
that should be able to create very long sparks with minimal power input.
Classic TCs have a single point of disruptive discharge with the primary
spark gap being the switching device.  The large peak power in this
discharge is what causes the large sparks as compared to a CW coil.
However, with the same basic primary circuit, spark length can be
further increased by increasing the bang rate, such as with an ARSG.  My
understanding of this is that the ionized trails off the topload have
less time to dissipate.  The problem is that to have many disruptive
discharges in rapid succession, the overall power draw increases
dramatically.
When I was considering a way to fix this, I first started thinking about
DC Tesla coils.  The first idea I had (which has apparently been tried
once before) was to use a variation of an ARSG instead of the normal
ARSG with a large charging inductor.  This ARSG would essentially act
like a SPDT relay (instead of the SPST relay that normal spark gaps
operate with) with an oscillator connected to the coil.  It can be
connected in such a way that the charging supply is physically
disconnected from the primary circuit when the primary circuit starts
resonating.  Because of this, such a large charging inductor (with a
value in the Henries) is unnecessary, because the charging circuit will
never be shorted out.  A charging inductor of a much lower value can
still be used to limit charging current and to double the charging
voltage.
After I planned out a coil of this type, I started working on the power
supply for the coil (which is now finished).  It consists of a 12/30
NST, relevant filter circuit, a hv bridge rectifier made out of a total
of 120 1N4007 diodes, and a smoothing capacitor made with 50 450V 22uF
electrolytic capacitors in series.  Relevant equalizing resistors and
protection diodes were added across the components.  To test this setup
for durability and functionality, I connected it up with a simple spark
gap in parallel with the smoothing capacitor.  As expected, the gap
periodically fired as the capacitor charged to the gap's breakdown
voltage.  Interestingly, the capacitors all appear to be completely
unharmed after a number of repetitions.  While watching these
discharges, I came up with an idea - what if this same "smoothing"
capacitor was discharged in impulses into the whole primary circuit?
This way, the ARSG could be run at very high speeds to take the most
advantage of remaining ionized trails in the air, and the bang size
would be relatively constant over the short period of time that the
primary spark gap was firing.  The repetition rate for sparks visible to
the human eye would be slower than the normal ARSG speed or the 60Hz
main line feed in classic TCs, but the spark length should be
substantially longer.  In my setup (about half done so far), the
repetition rate should be very roughly around 5Hz by my calculations.
What I am currently considering is what method I should use for
discharging the "smoothing" capacitor (I put it in quotes because it no
longer really serves that purpose).  I've built a high voltage relay
based on 2 solenoids and tungsten contacts, but I was wondering if a
simple non-quenched static gap would function just as well.  The key
would be to keep the arc sustained between ARSG charging cycles -
otherwise the entire purpose is lost, and that is what my original
question was about.
I'm working on implementing this design right now, but it's going a bit
slowly since this is my first year at college and couldn't bring many of
my materials and testing systems with me.  Also, an aggravating aspect
is that the rules are so strict here that I can't even use a hacksaw
without jumping through a number of hoops.  I will of course let
everyone know how it goes.
Are there any thoughts on this design?

Christopher Breneman

On Sat, 2007-09-29 at 13:11 -0600, Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sat, 29 Sep 2007 14:16:17 -0500
> From: Bert Hickman <bert.hickman@xxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Spark Gap Sustaining Current (fwd)
> 
> Tesla list wrote:
> > ---------- Forwarded message ----------
> > Date: Thu, 27 Sep 2007 12:55:46 -0500
> > From: Crispy <crispy@xxxxxxxxxxx>
> > To: tesla@xxxxxxxxxx
> > Subject: Spark Gap Sustaining Current
> > 
> > Hello,
> > 
> > I have a quick question about spark gaps.  How much current is required
> > to sustain an established arc in a spark gap in "dead air"?  Let's say
> > that there are two tungsten contacts about an inch apart, and an arc is
> > ignited by a voltage of a little over 20kV.  Say that the ambient
> > temperature is room temperature and that there is not significant
> > airflow through the gap other than that which is generate by the gap's
> > heat itself.  Is the sustainability of the arc purely a function of
> > current through it?  
> 
> Hi Chris,
> 
> The short answer to your first question is YES.
> 
> A somewhat longer answer:
> There's nothing simple about sparks, arcs, or plasma in general. The 
> answer depends on current limiting by the external circuit, whether you 
> have AC or DC and, if AC, the frequency. AC and RF arcs must be 
> reignited after each current zero crossing. This makes low frequency AC 
> arcs easier to extinguish than DC arcs. It also depends on the electrode 
> shape and material, and the orientation of the gap (vertical arcs behave 
> somewhat differently than horizontal ones).
> 
> Arcs require the formation of a "cathode spot" - a small incandescent 
> region at the cathode root of the arc - which injects large numbers of 
> electrons to help sustain the arc. For most metals, a true arc occurs 
> when the current reaches a level of amperes or, at most, tens of 
> amperes. The actual electron injection mechanisms into an arc differs 
> significantly between electrodes made from refractory metals versus non 
> refractory metals. A refractory metal, such as tungsten, develops a 
> stable incandescent cathode spot which liberates large numbers of 
> electrons via thermionic emission. Non-refractory metals such as copper 
> (with boiling temperatures below the point of substantial thermionic 
> emission, ~ 3000 degrees C), are "cold cathode" materials. The cathode 
> spots of these metals are in constant motion, with electrons being 
> liberated via field emission. The high E-field is created between 
> between the cathode and positive ions just above the cathode (called the 
> cathode sheath).
> 
> If you're able to limit short circuit current to tens of milliamperes, 
> the voltage across the gap climbs sharply. Gap voltage vs current begins 
> to follow a curve that is heading towards a low current glow (or corona 
> discharge) or to the initial spark-over voltage of the gap. In an AC 
> arc, the reignition voltage also begins to climb, ultimately approaching 
> the initial breakdown voltage for the gap or a stable glow/corona 
> discharge point. This phenomenon can be seen for low current AC arcs 
> (such as from a low current NST's), where the arc really can't be drawn 
> out much further than the point of initial breakdown before being 
> extinguished.
> 
> If the short circuit current from the power source is even further 
> decreased, once the gap breaks down, the low impedance of the spark 
> drops the voltage below the gap's instantaneous sustaining voltage. 
> Without using a careful low capacitance design approach, this usually 
> leads to unstable operation when short circuit current is limited to a 
> few milliamperes or less. The result is usually repetitive spark-like 
> discharges as the circuit operates as a relaxation oscillator. The 
> discharge tries to follow the sharp negative resistance curve heading 
> towards an arc, but the voltage across the gap collapses and kills the 
> arc, so the circuit cannot achieve stable "arc-like" operating point.
> 
>  > If so, in such a theoretical gap, how much
>  > sustaining current would normally be required?
>  >
>  > Thanks,
>  > Chris
>  >
> 
> Again, it depends on the external circuit and whether you apply AC or DC.
> 
> For a HV DC source, with little capacitance, resistively current 
> limited, you can actually follow the progression of the gap breakdown 
> process from the glow discharge, abnormal glow (for refractory metals), 
> down to a full fledged arc discharge. Once you have exceeded the 
> breakdown threshold for the gap, the "sustaining current" simply is 
> either the stable or unstable operating point(s) defined by the external 
> circuit. Simply stated - for a resistively current limited low 
> capacitance DC HV source, there is no minimum sustaining current once 
> you've successfully bridged the gap. Practically speaking, the discharge 
> ceases being "arc-like" in air once you go below a few mA and, for most 
> circuits, the discharge is no longer stable.
> 
> For a low frequency HV AC source, reignition after each current zero 
> becomes increasingly difficult as you begin lowering shirt circuit 
> current, and a single failure to reignite after a zero crossing leads to 
> almost complete dielectric recovery of the gap. Again this is with 
> milliamperes of current - higher than for the DC case with an identical 
> gap. Because of the shorter zero crossing intervals and capacitive 
> effects, an RF arc is somewhat harder to extinguish, with "sustaining 
> current" falling somewhere between low frequency AC and DC case.
> 
> As with all things arc-ish and spark-ish, YMMV... :^)
> 
> Bert