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Re: [TCML] Spark gap loss

Hi Steve,

Thank you for posting the link to this interesting paper.
take aways that I found are:
1) Losses were compared for gaps using air, nitrogen, and SF6.  SF6 had the
highest losses, and air had the lowest.
2) Losses were compared for electrode materials Copper-Tungsten, Graphite,
and Stainless Steel.  It was determined that the electrode material has no
statistically significant effect on the arc resistance.
3) Losses were compared for gas pressures of 1, 2, and 3 atmospheres.  Low
pressure (one atmosphere) results in higher losses than high pressures (two
or three atmospheres).

Re 1), I'm pleased to hear that using air is the best of these 3 options,
although I recall hearing that using hydrogen was used in early gaps, and I
see references to hydrogen in contemporary abstracts for high rep-rate
gaps, no doubt for good reason.  But we'll all probably live longer if we
stick with air.  Hydrogen and sparks - what could possibly go wrong?

Re 2), Also good to know that losses were unaffected by materials in these
3 trials, but durability and quench time weren't addressed.

Re 3), Good to know that I was on the right track using higher than ambient
pressure in my vortex gap!  I have no idea though HOW much higher than 1
ATM was achieved, probably not a lot.

Minor correction to earlier posts - *Trigger* energy was not ~1K joule.  I
don't think this was a triggered gap at all.  The paper states:
The charging voltage for the capacitor was chosen to be 35 kV to ensure
that the total energy was greater than 1 kJ. A 35 kV charging voltage
resulted in 1.15 kilojoules of energy per shot.

Regards, Gary Lau


On Wed, Nov 15, 2017 at 1:56 PM, Bert Hickman <bert@xxxxxxxxxxxxxxxxxxxxx>

> Hi Steve,
> Thanks for the link - very interesting paper.
> I really didn't address your question of energy loss or efficiency in my
> earlier post. IN the late 1990's I did a series of experiments on my static
> spark gap coil. These were designed to measure energy losses during
> primary-secondary energy transfers.
> As you are aware, a full energy transfer from Pri -> Sec (or vice versa)
> for a SG coil typically takes 2.5 to 4 RF cycles to complete. By setting
> the coupling coefficient (k) to one of the "magic" values (such as 0.133,
> 0.153, 0.18, or 0.22), complete Pri -> Sec  or Sec -> Pri energy transfers
> can be done within an integral number of RF half-cycles. Using a magic
> value avoids stranding some energy in either the primary or secondary,
> simplifying primary V or I measurements and resulting energy calculations.
> k:     RF cycles:   RF Half-cycles (N):
> 0.133    4               8
> 0.153    3.5             7
> 0.18     3               6
> 0.22     2.5             5
> My test coil was set up with a k of 0.18. During testing, I purposely
> reduced the bang size so that the secondary did NOT break out to prevent
> streamer losses from introducing errors.
> Spark gap coils never "ideally" quench at the first primary notch (single
> Pri -> Sec) energy transfer), especially when the secondary is prevented
> from breaking out. So, after all the system energy has been transferred to
> the secondary, the gap continues to conduct, and energy in the ringing
> secondary now transfer "backwards", ringing up the primary tank circuit.
> After another "N" RF half-cycles, all remaining system energy now resides
> back in the primary circuit. By comparing the initial peak voltage in the
> primary circuit versus the peak voltage (or current) returned to the
> primary circuit after a full Pri -> Sec -> Pri energy transfer, I was able
> to calculate the total energy lost during a round-trip cycle. A wideband
> current transformer could used to measure primary current to make similar
> energy calculations.
> For my coil, the portion of system energy lost when making a primary to
> secondary energy transfer (from ALL causes) was about 15%. Most of the
> losses likely came from the spark gaps, since my system used a series
> vacuum gap with 8-12 static gaps. Because of the large number of gaps, my
> losses may be higher than for a typical rotary gap. If you have a HV
> divider or wideband CT to measure primary capacitor voltage or tank circuit
> current and an oscilloscope, you can duplicate these measurements for your
> system.
> Although my coil transferred 85% of primary bang energy to the secondary,
> a well designed SG coil with fewer gaps might hit 90%, perhaps more...
> Bert
> Steve White wrote:
>> I was reading a thesis that studied spark gap losses recently. Although
>> the test apparatus (electrode material, diameter, and gap spacing) does not
>> match exactly what you would find in a typical tesla coil, I found the
>> results very interesting. The closest to the tesla coil scenario was the
>> following from the paper.
>> 1. Electrode material: copper-tungsten
>> 2. Electrode diameter: 2.5 cm
>> 3. Gap spacing: 1.4 cm
>> 4. Trigger voltage: ~30K volts
>> 5. Trigger energy: ~1K joule
>> 6. Air at 1 atmosphere
>> The test results at these conditions measured an energy loss of about 7%.
>> If I extrapolate these results to my 4800 watt coil, I am losing about 336
>> watts in the rotary spark gap. This is less than I imagined since I have
>> always been lead to believe that the spark gap was very lossy.
>> Does anyone else have any other data which shows the loss caused by a
>> spark gap?
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