Re: Spark Gap Sustaining Current (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 29 Sep 2007 20:24:19 +0000
From: David Rieben <drieben@xxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Cc: drieben@xxxxxxxxxxx
Subject: Re: Spark Gap Sustaining Current (fwd)

Hi Bert,

All that I can say is, "WOW" ! That's a really superb
explanation of this subject matter for a "layman" as myself,  
although my mind is still trying to "absorb" all that you've 
said ;^) You should have been a professor, if you weren't.

David Rieben

-------------- Original message -------------- 
From: "Tesla list" <tesla@xxxxxxxxxx> 

> 
> ---------- Forwarded message ---------- 
> Date: Sat, 29 Sep 2007 14:16:17 -0500 
> From: Bert Hickman 
> To: Tesla list 
> Subject: Re: Spark Gap Sustaining Current (fwd) 
> 
> Tesla list wrote: 
> > ---------- Forwarded message ---------- 
> > Date: Thu, 27 Sep 2007 12:55:46 -0500 
> > From: Crispy 
> > 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 
> -- 
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