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Spark gaps -- making and triggering

T

Tim Williams

Jan 1, 1970
0
Robert Latest said:
BTW, a setup like this is really more like a relay, not a spark gap.
It's a relay that happens to spark a lot ;-)

Eh, not really, consider the whole thing is over in a hundred microseconds,
while a relay might take a whopping ten miliseconds to engage. For it to be
a "relay that sparks a lot" in any meaningful way, you would have to close
the gap before the sparks are done with.

Hum... figure it has to close the gap (sufficient to hold off the voltage
mind you!) in 10us or so. Let's say a mass 0.l kg, distance 0.02 m and time
= 10^-5 s. Average velocity must be 0.02/10^-5 = 2 x 10^-2 x 10^5 = 2000
m/s, breaking the sound barrier I believe! Assuming constant acceleration
from an initial velocity = 0 to final velocity = 4000 m/s, that's dv/dt =
4000 m/s / 10^-5 s = 4 x 10^3 x 10^5 m/s^2 = um, 0.4 gig m/s^2, or roughly
40 million G's? F = ma so that's around 4 MN (mega newtons), or for N = G *
1 kg ~= 10 on Earth, roughly 400k kg, that is to say, 400 metric tons,
sorry, tonnes. This force has to be applied with a rise time under say, 5
us, so inflating a wide piston with pressurized air isn't going to cut it
(speed of sound as it is). A light gas piston might, particularly if
pressurized greater, but it still begets fast risetime elsewhere to do it.
An ignited H2/O2 gas mixture might do it. Other propellants may also work,
considering the speed of sound is much faster with higher gas temperatures.

Oh, and a final kinetic energy of 1/2 * 0.1 kg * (4000 m/s)^2 = 800kJ ;)

The force and energy would of course be much smaller given a lighter
contact, but that doesn't matter, your biggest concern (if it were a
concern) is really getting things to actually move in the first place.

Heh.

Fortunate that gasses can ionize so quickly and completely, huh?

Though on the other hand, if you pressurize or vacuum the spark gap chamber,
you can get shorter distances, requiring less robust drive train. 'Course,
if you pressurize the chamber, you can also get a lot more ions in the
spark! Come to think of it, that might be a good reason to use something
easily vaporized, like zinc or aluminum...

Tim
 
I

Ignoramus26024

Jan 1, 1970
0
Thanks... I went to farm supply and bought some 1/4" copper tubing, 1"
by 1/8" aluminum flat to make a bus connecting several caps, 3/4"
copper pipe and corresponding pipe caps. I will start putting a plan
together.

i
 
W

Winfield Hill

Jan 1, 1970
0
Bert Hickman wrote...
You do need HV relays, which can be expensive. I get many of
mine from eBay sellers. Manufacturers are Kilovac, Gigavac,
and Jennings, which you can use in your search terms. You can
expect to pay $75 to $250 for a 30kV vacuum relay, and perhaps
more if I'm bidding against you. :) I just won three Kilovac
KC-38 25kV spst relays for $60 each, a better price than usual.
http://cgi1.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7603579946

Here's a nice 35kV spdt relay with cables and HV connectors.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7604592239
Win,

Are you using high energy bulk resistors (such as Kanthal, or
HVR Advanced Power) to rapidly absorb the energy? I've seen high
power wirewounds fail spectacularly under similar conditions.

At modest power levels I use series stacks of many 2W carbon-
composition resistors. These dissipate the energy throughout
their bulk, rather than just in a thin wire, etc. For higher
power levels I'm building up a collection of Advanced Power
resistors, mostly taken from high-power IGBT snubbers, etc.,
but quite a few of these will be required in a series stack.
BTW, HVR has a nice thermal modeling tool for designing rapid
energy dump solutions:
http://www.hvrapc.com/thermalapplicationpage.asp

Thanks, Bert, that's a good lead, for repetitive applications.
I've created simple heat-spreading spice models of transient-
energy absorption in the 0.1us to 10ms timeframe, which I've
vetted against microsecond-scale measurements. You know, cells
of thermal-mass with thermal-conductivity connections. With
this I was able to see how well-designed TVS parts are - I was
impressed. I've also studied MOSFET avalanche with these spice
models. We have a site-license for a good FEA program at the
Institute, but I haven't taken the time to learn how to use it.
 
I

Ignoramus30285

Jan 1, 1970
0
Bert Hickman wrote...

You do need HV relays, which can be expensive. I get many of
mine from eBay sellers. Manufacturers are Kilovac, Gigavac,
and Jennings, which you can use in your search terms. You can
expect to pay $75 to $250 for a 30kV vacuum relay, and perhaps
more if I'm bidding against you. :) I just won three Kilovac
KC-38 25kV spst relays for $60 each, a better price than usual.
http://cgi1.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7603579946

Here's a nice 35kV spdt relay with cables and HV connectors.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7604592239

Thanks Win, I will keep an eye on eBay for HV relays, in fact I set up
a snipe for one.

i
 
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