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1 m x 1 m Parallel Plate Capacitor at 300 kV

09/18/2020 9:36 AM

An experimental setup has a parallel plate capacitor 1 m x 1 m that is to be fully charged to 300 kV. One is a bare copper plate and the other is fully covered with 1 mm thick ceramic insulator (permittivity >1000 for high capacitance). There is a 2 mm air gap between the plates.
1) Can such a capacitor be charged to 300 kV without dielectric failure.
2) Is there ways to experimentally measure the charge by discharging the the fully charged capacitor. Is such a method common practice. The accuracy needed is only about 5%.
3) Would a good experimental physicist be able to work with such a capacitor.
4) would there be significant charge leakage if left for 2 hr. A charge leekage of 5 % is acceptable.
Thanks.

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#1

Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 10:01 AM

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#10
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Re: 1 m x 1 m parallel plate capacitor at 300 kV. ... Pronunciation quibble.

09/19/2020 12:13 AM

Coulomb

Koo laam not ka Loom but otherwise nice videos.

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#2

Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 10:43 AM

This sounds more like an ionization chamber than a capacitor. It will certainly have a knowable capacitance but as radiation ionizes the gas between plates the very high bias voltage will separate the ions to produce a momentary current flow or count. At 300 kV this can easily be well into the useless continuous discharge region that is above the Geiger-Muller levels.

The use of air for an ionizing gas is also fraught with complications of impurities (pollen, dust, humidity), changing pressure levels, acoustic vibrations, etc.

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#3
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 11:00 AM

I can't understand why there is radiation causing the discharge. Radiation implies em radiation. What we have is a high voltage across the air gap; probably, the 300 kV may cause some momentary ionization of the air gap. But as the other plate is fully insulated, there may not be any continuous discharge. If there is any charge leekage, we may consider a slow continuous leekage of the bare plate to the surrounding.

If we take the example of a Van de Graft generator, the sphere may be charged to a million of Volt and yet may retain the charge for some time without spark/lightning discharge.

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#4
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 12:20 PM

..."Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating materials undergo breakdown when the electric field caused by an applied voltage exceeds the material's dielectric strength. "...

https://en.wikipedia.org/wiki/Electrical_breakdown

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#5
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 12:26 PM

Everywhere has some background ionizing radiation level. I introduced you to an ion chamber because your proposal resembles an ion chamber thus you should have a large volume of information one can research what can and might happen with this before attempting.

The pivotal difference between your configuration and an ion chamber is one plate has a ceramic coating with a high dielectric permittivity. This ceramic coating will make it less likely that ionization will cause a breakdown but without knowing the breakdown characteristics it is difficult to say with absolute certainty. I suspect this configuration introduces another problem you may not have considered.

Having an air gap in series with the high dielectric material will mean you actually have two capacitors in series with each other. Both capacitors will have identical areas, similar gaps (1:1 or 2:1 your gap description is ambiguous) between insulating boundary layers but over a 1000 to 1 ratio difference in the insulating permittivity. The air gap capacitor will be considerably smaller than the ceramic gap by this permittivity ratio. Remember, with capacitors in series the charge on each capacitor will be identical. Thus the air gap will have either a 300V or 150V gradient when 300 kV is applied.

Thus you do have an ion chamber with a series capacitor connected to the bias plate. This will reduce the chamber bias voltage from the applied voltage. Is this what you desired?

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#8
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 12:50 PM

You have given valuable comments.

I think my setup could be enclosed within a high vacuum chamber. In this case, there would not be ionization of air causing capacitor charge up failing to reach 300 kV. The problem is whether there is some insulating coatings of 1 mm that could withstand the high field strength and has relative permittivity > 1000.

I don't yet understand why it would be 2 capacitors in series when there are only 2 conducting plates. Of course the capacitance formula is not the usual parallel plate capacitor with uniform dielectric in between. When charging up, we expect the voltage between the conductors to reach 300 kV; this is what we aim for.

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#9
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 1:32 PM

Putting this inside a vacuum chamber may not make things less likely to arc in the gap for the breakdown voltage of a gas drops at modest vacuum levels before they improve. You are approaching another well studied device, the cold cathode vacuum gauge. The missing factor is a magnetic field to trap ion pairs instead having the E field separate the ion components.

My real point is most every configuration of applying voltages onto different materials and combinations of materials has already been done. The Physics is baffling and not intuitive but well understood.

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#11
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/19/2020 12:13 AM

I have to say I am not an expert. My attempt is to propose a certain experimental setup. We must be able to charge the capacitor with the 2 mm gap to about 300 kV. The experiment would be a success if the capacitor could retain its charge for say 2 hrs; a 5 % charge leekage after the 2 hours is acceptable.
1) The capacitor is to be charged only after the desired vacuum is achieved.
2) I don't understand yet about the cold cathode vacuum gauge. I now understand there may be a cathode ray problem. In a vacuum with the bare copper plate as the cathode, there may be cathode ray discharge. But our setup can have the ceramic coated plate as the cathode. Would there still be any cathode ray discharge?

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#16
In reply to #8

Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/19/2020 8:32 PM

Here are a few calculations: Calculate the force between charged plates (assuming you could actually charge to 300 kV)...

Force = εAV2/2d2 (Force between 2 charged plates)

http://www-eng.lbl.gov/~shuman/XENON/REFERENCES&OTHER_MISC/electric_forces.pdf

where:

ε0=8.854 x 10-12 F/m : Permittivity of free space

A=1 m2 : Area of plates

V=3x105 volts : Voltage on Capacitor

d=.003 m : separation of plates

Force = ε0AV2/2d2 = 60256.4 newtons = 13443.5 lb = 6.7 tons

This is for free space (vacuum or air). Put a dielectric in there and that multiplies the force by the relative permittivity (relative permittivity ε ~1000)

Force=εε0AV2/2d2 = ~ 6700 tons (with dielectric)

Capacitance: C= εε0A/d = 2.95 x 10-6 F

Stored energy: E = 1/2 CV2 = 188.8 KJ

You should stand back, when the separators give way, it could get interesting...

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#17
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/19/2020 11:21 PM

OK. Amateurs don't always view things in so many angles. I doubt we could work with these tons of force!

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#6

Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 12:40 PM

"Most insulating ceramics, including alumina, have dielectric breakdown threshold values in the order of 10–25 kV mm− 1."

https://www.sciencedirect.com/topics/materials-science/electrical-breakdown#

Dry air has a breakdown voltage about 3 kV/mm

https://en.wikipedia.org/wiki/Electrical_breakdown

So, obviously you cannot charge this capacitor to 300 kV as electrical breakdown would occur at about 30 kV (2mm of air and 1mm ceramic).

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#7
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Re: 1 m x 1 m parallel plate capacitor at 300 kV.

09/18/2020 12:41 PM
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#12

Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV

09/19/2020 12:43 AM

The greatest problem I see with the proposed device is guaranteeing the integrity of the ceramic insulator such that no porousness occurs to cause a breakdown flashover.

Mica sheet may act as a sufficient insulator but I fear at the high voltages employed it will fail at a 1mm thickness for the air in the other 1mm will become ionised and act as an extension of the bare plate.

If this is set up in a Vacuum then you will certainly have one hell of an xray generator,

Good luck with your frying.

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#14
In reply to #12

Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV. Make it Round

09/19/2020 2:18 AM

Dial 459-2222 (Mr. Gattis pizza joke)

You need an insulator with an extremely high dielectric strength. The table at the link includes diamond at 2000MV/m. You will have trouble finding a perfect diamond the size you need. The table also includes fused silica at 470-680 MV/m but you will have to have a very good process for low(or zero) defects in your fused silica. Mica is only 118 MV/m but tends to come in sheets if you can find a perfect one big enough. Mica is often used in capacitor manufacture but they tend to be smaller ones than yours.

Now I am concerned about the edges of your capacitor. Sharp edges are a big problem because of corona. Sharp edges create extremely high local fields at the points which is very bad. You may have to use pizza pan shapes with rounded edges curving away from the opposite plate.

Bottom line is coat two pizza pans with fused silica. Coat each with slightly less than one half of the total thickness desired so any individual flaw will be met with an intact opposing surface. Coat all the way around at the edges and use rolled edge pizza pans. Mount two pans bottom to bottom with a thin layer of teflon dieletric film as a squashable gasket layer between the coated pans. The assembly probably will squirm enough to crack the fused silica due to the mechanical forces once charged and fail catastrophically so be ready for that as you test. Good luck and test from a considerable distance with lots of polycarbonate layers between you and your capacitor to deflect any blasted molten pizza pan metal. Discharge carefully after testing with no arcs close to the pans.

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#13

Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV

09/19/2020 1:40 AM

About 4 decades ago I made capacitor with aluminum plates with window glass dielectric. The glass was approx 4mm thick. The aluminum plates were about 12" square, and there were a total of 6 plates. The glass was several inches larger than the plates. I ran it at about 10kv at 450khz. It was a resonant tank circuit.

Amateur radio handbooks have some pretty good formulas for building capacitors and inductors.

The experiment was a bust - at 450KHz at those voltages the circuit radiated huge amounts of energy at the intermediate frequency - it played havoc with radios in the building --- oops.

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#15

Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV

09/19/2020 10:24 AM

From the feedback I received, I think this capacitor setup of mine is not feasible. I have learned something from the discussion.

Many thanks.

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#18

Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV

09/20/2020 7:53 AM

Just in case any form of this goes ahead, make sure the insulating sheet is much bigger than the plate it is attached to. (Otherwise the arc will just go round the edge.)

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#20
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Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV. Tesla Experimenter Tricks

09/21/2020 11:04 AM

Hacks and Tips from the World of Tesla Coil Development

Well. Randall got close to a set of tips one can find if searching the web for homemade hi voltage capacitors for Tesla coils so he gets a GA. The other half of that online tip for high voltage homemade caps is a bit less obvious and I have held it back because it costs a bit more, has safety issues, is messy and some skeptics may guffaw but... Even though the dielectric strength of mineral oil is 10-15 and one has to be careful that any sneak path through mineral oil is long, mineral oil immersion provides a very important feature to a homemade hi voltage cap.

Mineral oil immersion makes the cap more self healing. That is, if there is an air bubble somewhere (with air in the table rated 3) and an arc through that air forms, the arc heats that air rapidly, and the oil disperses the exploding bubble. If the cap is immersed in oil, that oil not only acts to remove the bubble, it also removes heat from the location, replaces the air with the higher dielectric strength liquid (at 10-15) and prevents oxidation of the surrounding solids since it does not carry any new oxygen to the previous location of the bubble. Distilled water can operate similarly but unfortunately it is such a good solvent that it tends to pick up material vaporized by the arc. This dissolved material increases the conductivity of the local water and continues heating of that water until you get steam production, electric dissociation of the hydrogen and oxygen and continued oxidation of the surrounding solids. Unless the high thermal mass of the water overwhelms the heat from the arc, the arcing continues catastrophically. So, water self heals some but not as effectively as oil because the water still provides continuous oxygen to the defect and the steam keeps cooling water off of the arc site.

Looking at the table Benzene(at 163) could conceivably work even better but it is highly flammable and toxic to humans through the skin so I strongly recommend against Benzene immersion of experimental caps by anyone not fully versed in handling Benzene in explosion prone environments. I can make similar comments about other exotic fluids both in and out of the linked table that one might consider for their high dielectric strength/arc quenching possibilities.

Mineral oil immersion even has fire and excess pressure issues that an experimenter must keep in mind but it has been used a great deal and is well understood. It even took the entire electric power industry a long while to realize(with some government prodding) that they should ban mineral oils high in PCBs so these issues can be rather subtle.

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#19

Re: 1 m x 1 m Parallel Plate Capacitor at 300 kV

09/21/2020 2:50 AM

There was a similar question in an A-level physics exam recently...

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