40MHz Spark Gap Behavior

From:  D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
Sent:  Monday, April 13, 1998 10:15 PM
To:  Tesla List
Subject:  Re: 40MHz Spark Gap Behavior

to: Terry

What is the value of the capacitor you are using for these tests?  I
presume this is a low inductance pulse duty cap?


> From: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-pupman-dot-com>
> Subject: 40MHz Spark Gap Behavior
> Date: Monday, April 13, 1998 3:07 AM
> ----------
> From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
> Sent:  Sunday, April 12, 1998 10:52 PM
> To:  tesla-at-pupman-dot-com
> Subject:  40MHz Spark Gap Behavior
> Hi All,
> 	I have shielded my voltage probe against the radiation from the current
> spikes and it is now working properly.  I can now see the actual voltage
> across the gap during the firing cycle which has given me a much better
> understanding of what I have been observing.  The voltage probe and the
> current transmitter are now in die-cast boxes that are sealed with copper
> tape (that stuff really does work).  The voltage probe now only shows
> glitches at the current spikes (these probably are real voltage
> The current spikes still remain very difficult to quantify.  Their energy
> enormous and not only do they have a heavy 50MHz content but I now
> they have much higher frequency components that also contain enormous
> energy.  This is playing havoc with the frequency response of the current
> sensor.  At very high frequencies, the skin and other effects raise the
> impedance of the current shunt and make the spikes appear larger than
> are.  How much larger?  I really don't know.  Here is a summary of what I
> now believe is happening:
> Just before the gap fires (~50ns),  The voltage across the gap fluctuates
> and seems to ring at about 50MHz for a few cycles.  Then the current
> high to an enormous level.  This spike, in my low power testing (2000
> 17nF, 120uH), appears to be around 4000 amps.  It seems to have an ~50MHz
> ring but it probably has frequency components well into the GHz.  This
> returns to lower levels after about 25ns and then rings at ~50MHz for a
> variable length of time (~~300ns).  At this point the voltage across the
> remains low and current is passing through the gap much as if it were
> shorted (I was wrong before in believing that there was not current flow
> during this time).  The current is around the level expected given the
> reactances of the inductor and capacitor.  The current continues to flow
> until it reaches the first zero current crossing (this coincides with the
> voltage of the primary cap being at it's peak).  At this zero crossing
> gap is actually opening and the voltage across the gap rises in a small
> spike.  Then the current shows enormous ringing and again the high
> energies seen in the first spike reappear but at considerably lower
> This second burst lasts for about 100ns.  This process continues for the
> rest of the firing cycle with a current burst occurring at every zero
> current crossing until the gap opens.  Often the last current burst is
> long.  
> 	Here is my best guess as to the nature of the bursts.  The first burst
> contains much energy due to the charges around and in the gap just before
> fires.  When the air ionizes and brakes down, the charges are allowed to
> transfer to a state of semi-equilibrium in an extremely short time.  This
> accounting for the enormous currents and high frequencies seen.  I
> the ~50MHz signal I see is related to some transition time.  Probably the
> air ionization time.  During this time the impedance is very unstable and
> the system oscillates for some time before the current is established and
> stability is achieved.  Everything is fine until the current drops to
> or, more importantly, must change direction.  It is easy for current to
do a
> 180 degree turn in wire, but I suspect it is much harder to do in air. 
> charges and ionized gases must completely reverse themselves.  In this
> process I believe that the initial oscillations and instabilities are
> again established and the burst is again created.  
> 	I have examined the energy and lengths of the burst and have concluded
> following:
> 	I believe the gap is dissipating the vast majority of it's power during
> these bursts.  The voltage and current clearly are in phase and energy is
> being lost.  Heat, light, and RF radiation must be huge during these
> To take a guess at the numbers for the initial spike, if the current is
> 4000A at 2000V for 25ns the energy lost is very roughly 0.2 joule!!  This
> a pretty high rough calculation for a system that only has 0.034 joule to
> begin with.  The energy being lost in the gap at other times seems very
> insignificant!  This would explain why primary systems seem to loose so
> energy on the first oscillation.  Obviously, my number of 0.2 joules is
> correct but it does give an indication that the energy lost in these
> could be very high.
> 	I also believe my initial thought, that these high frequency bursts
> account for the sensitivity of coils to good RF design, is true.  It is
> known that a sloppy primary system with thin wire and long connections
> looses much more energy than a thick copper primary system with short
> leads.  This does not make sense given the few hundred kHz frequencies
> run at.  The primaries should have milliohms or resistance instead of the
> ohms of resistance they often test at.  I have found that when poor
> is used, the current bursts change dramatically.  They loose their sharp
> fast appearance and become long and painful looking.  The energy being
> appears to be much much higher.  You can see the voltage across the gap
> start to rise and the burst can last a few microseconds which is a
> substantial part of the cycle.  The current and voltages in the non burst
> areas seem to still be efficient.  I do not know if the ~50 MHz signal is
> making its way around the inductor but the effect of poor wiring on the
> current burst is very obviously wasting much power.  
> 	I'll try to write up a nice paper on this as time permits.  It is hard
> write when the experiments are calling!
> 	There are still many questions and details that have not been answered.
> Here is my list of unknowns:
> Why do the bursts oscillate near 50MHz?
> How can I get the shunt to work well at high frequencies?  (I think I
> to add some capacitance to kill the 200MHz and above signals.  They do
> fiber-optics up to 5GHz. :-))
> Does the ~50MHz travel through the whole primary system?
> What are the factors involved in the first giant current spike?
> How will this affect the secondary system?
> Can the spark gap be made more efficient in light of all this?
> Does a rotary gap work better?  (I did a little testing and it looked
> the same but more careful testing is needed.)
> How does placing the gap in a vacuum or under pressure affect this?
> Etc. etc.......
> Thanks again to all who have sent ideas and comments!
> 	Terry
> terryf-at-verinet-dot-com
> or
> terryf-at-peakpeak-dot-com