1

I've been trying to design a 1 MV hydrogen gas spark gap.

I found the resistance of such to be in the range of 7 Ω/cm, not suitable for a target 40 kA current. Can I make the spark gap shorter without affecting the breakdown voltage? The electric field should theoretically be distributed along the surface if two round plates are used.

Given the round plates, do breakdown voltage and gap area increase linearly?

winny
  • 13,064
  • 6
  • 46
  • 63
Alessandro Mini
  • 43
  • 5
  • I wonder if the plasma is becoming saturated (fully ionized). In that case, higher pressure, or a higher-Z gas, might be suitable. Pressure also raises the breakdown field strength. Z can only go up so far before ionization time (and perhaps ionic drift velocity, to whatever extent that's a concern) becomes too long, assuming that's the driving choice for hydrogen in the first place. (I can't imagine one would choose a box of explosive gas for no other reason..!) – Tim Williams Aug 24 '23 at 22:53
  • Just interested into where your 1 MV is coming from. And for a fraction of a second what are you going to do with your 40 GW? – D Duck Aug 25 '23 at 16:06
  • I have a little homemade radiotelescope dish and i wanted to add a resonant cavity to it , purely for testing . Since i dont have very fast sampling equipment , i decided to use a variant of the marx gen in order to prolong the wave permanence inside the cavity . it's basically for a half wave (microwave) generator . – Alessandro Mini Aug 25 '23 at 18:22
  • Well my idea is to use a Bernoulli's constriction filled with Co2 , with two close electrodes . I would inject the H2 , accelerating it in the strict section and lowering sensibly its pressure, which would lower its breakdown voltage (if done with the correct magnitude) according to Peschens curves . One drawback could be the lower ion count , but im still working on it . – Alessandro Mini Aug 25 '23 at 18:48

1 Answers1

3

The field strength is not related to gap area, rather just the gap.

With two infinite plates, the field strength is V/d, so halving the gap will double the field strength and halve the trigger voltage for a given dielectric.

As you move down to finite electrode radius, the field strength goes up as the radius goes down. However, to a first order you will still get the direct relationship between breakdown voltage and gap.

Having said that, I wonder about your stated "on" resistance for the gap. On resistance is generally inversely related to current, because the plasma channel tends to expand as the current/heating goes up. At the currents in question, I am surprised with your quoted value.

To provide more analysis will require more details on the gap design, including dimensions, gas pressure and source inductance/capacitance. However, you might find this paper informative ARC CURRENT, VOLTAGE, AND RESISTANCE IN A HIGH ENERGY, GAS-FILLED SPARK GAP

Finally, I assume you know that to get to 1MV trigger, you are likely to need very large radius electrode, and insulator structures? Have a look at this answer for more info Spark gap minimum breakdown voltage vs air gap

Additional context

It's worth noting that gaps in this voltage range are often implemented in a stacked multi-stage configuration. The bottom gap commonly has a trigger system, and the upper gaps are designed to trigger as soon as the lower gap fires because of the increased overvoltage. There is also the potential for sync using the UV light from the lower gap (as common in Marx generators), or with external lasers.

Balance across the gaps can be maintained with grading electrodes and corona stabilizers, which leak charge across any intermediate gaps with excessive voltage difference.

These kind of setups can be very compact, and I've seen past descriptions of a pressurized gas gap with 5 stages, which fitted in an 11" gap between two large electrodes, all in a SF6 insulating tank.

colintd
  • 2,018
  • 2
  • 17