Need help with an electrode setup

[laser_dude]

Hi guys,
I'm stuck on a tricky problem. Thought there might be some smart people here who could help me. I have two metal plate electrodes at 50V, 20cm separation. I want to get a current flowing across them (1mA would be great, but microamps would possibly do). Air is an insulator, but if you create ions/plasma between the electrodes it conducts.

Can someone help me think of a setup that will work? I've attached a visual of the problem with an attempted solution using a separate spark generator.

cheers for any help

[klugesmith]

Sounds like the classic setup for _measuring_ amount of air ionization.
Ion recombination might be an issue with plates so far apart.
Can you put a series of plates much closer together in the same air volume?
Search Internet for DIY ion chambers.

You could irradiate the air space using a radioisotope or an x-ray generator.
Not hard to figure the necessary exposure rate, in R/h or Gy/h,
but it would take a seriously scary level to get 1 mA. (well maybe not, if plate area is measured in square km).

Can you adapt your experiment to work with sub-microampere currents?
I can attest that 10 nA is enough to get a visible glow from a LED (with dark adapted eyes).

[davekni]

What are your constraints?  Can 50V be changed to 50kV?  That would make it workable.

If you really need 50V across 20cm, heating the air to 5000K might get enough current.

[Uspring]

Ion drift velocity is only about 20 cm/s at the field strength you're considering. In the setup you have drawn, negative ions will be pulled to the left away from the spark and positive to the right. As a ballpark: Since ion travelling times are around 1 sec and current is 1 mA = 1 mC/s, there must be a charge of 1 mC travelling between the plates. That is a huge amount for a charge with no opposite charge being close to it to cancel it. What happens is actually, that the negative and positive charge clouds wont separate since they are mutually much more attracted, than by the plates.

That problem can be avoided by klugesmiths suggestion by ionisation with radiation. Then there are no separate clouds of positive and negative ions i.e. they are at the same place, so that they won't create a big field. Due to the slow drift speed, ion densities would have to be large, around 10^12 / cm^3. Since ions tend to recombine fast i.e. in about 10us at this density, you'd have to create 10^17 ions per second and cm^3 to keep the current going. Looks to me like a radiation flux in a reactor core.

These schemes are quite far away from being workable. Can you share some details about your intents?

[Twospoons]

A dielectric barrier discharge would let you fill the space with ions, though I'm not sure if the ion density would be sufficient.  You would need an AC supply of about 25-30kV at >30kHz to supply the discharge. Ozone generator supplies would be suitable for this.

What's your application? Its quite difficult to suggest solutions if we can't see the bigger picture.

[laser_dude]

Thanks for the replies, yeah kinda what I was expecting to hear, my idea might not work but I'll think a bit more on it.

Basically the idea was to design a more efficient no moving parts solid state fan. If you ionise air and apply an electric field, you can push the ions and get a net force in one direction. Lookup "ionocraft" or "lifter" on youtube to see people making devices that seem to magically float in the air without propellers. You can make a working fan with a piece of wire held near and parallel to a piece of foil with 25 kV between them.
   A large 20cm electrode gap means much better momentum transfer per Joule because the ions smack into more air, surprisingly the ratio goes to infinite in theory if you keep making the electrode distance bigger, but as klugesmith pointed out recombination brings the ions to a stop.

There's two design issues I want to overcome:
1. Producing a high ionisation rate of the air high enough for 1mA
2. Somehow get around the recombination issue so that the gap can be large

Cheers for the replies

[klugesmith]

What features of proposed fan make it "solid state" ?   

[Uspring]

There's two design issues I want to overcome:
1. Producing a high ionisation rate of the air high enough for 1mA
2. Somehow get around the recombination issue so that the gap can be large

In order to propel air, you should only have one sort of ions, either positive or negative ones in the air space you want to move. Positive and negative ions move in opposite directions in a field, so their effect of dragging air along with them will cancel. In a cloud of either positive or negative ions recombination does not take place.

Creation of ions in a typical lifter usually happens by corona. (Don't google this, it may point you in the wrong direction). The current there is limited by space charges, which pile up near the point of the electrode. The high field of say a positive tip will generate positive ions in front of it, which will lower the field there and limit the current.  So the main obstacle to a high corona current is the slow movement of ions away from the tip. Higher voltages help there, but as you said, efficiency drops with larger fields.

Another issue is the amount of ions, that are kept in transit between the plates. If there are too many of them, they will repel each other and not go, where you want them. That puts a limit on ion density and with the rather slow speed of ion drift, a limit on the current.

[laser_dude]

I'd like to check my understanding of the lifter theory (a lifter has one small electrode, one large electrode and a high voltage. ie corona between asymmetric electrodes):
[see attachment]
A positive small electrode and a larger negative electrode cause dielectric breakdown/ionisation at the positive terminal (higher electric field there). Negative ions don't drift away from the positive electrode and positive ions can. For a DC voltage we get a continuous current that creates a net positive cloud between the electrodes. Low concentration negative ions already present in the air from background radiation/thermal effects, particularly near the negative electrode are attracted to the cloud and small amounts of recombination occurs, but not enough to remove the positive cloud. The higher the current, the higher the charge of this cloud. This cloud and the negative ions stuck at the positive electrode reduce the net electric field between the electrodes, resulting in a limit to how much current can be applied for a given voltage and electrode separation distance.
So a really efficient fan, that requires a large electrode distance for maximum ion-air collisions, would require a very very low current for a low voltage. Hence high efficiency fans = impractically low powered fans. So say this another way, the charged cloud forces a trade-off between high power and high efficiency. One way to combat the charge cloud reducing the net electric field is to make the electrode plates km wide, spreading out the charge distribution, which is kind of silly in practice.

In order to propel air, you should only have one sort of ions, either positive or negative ones in the air space you want to move.

This is I think the generally accepted explanation for how they work, but there's some interesting exceptions to the rule. It's been reported many times that when you reverse the electrode polarity in lifters, the force is still in the same direction, but is slightly larger when the electrode is positive. This is because of an asymmetry in positive and negative ions that are formed. Maybe you are making O2- and N2+ ions and cations, which have different weights, one pushes air molecules a little harder than the other. So in theory you don't need an asymmetrical setup, you could get a weak fan, maybe 1-2 orders of magnitude weaker, by just applying a uniform electric field to air and ionising however you like, radiation, light etc.
Even if you did this in a chamber of pure nitrogen gas, you'd get N2- and N2+ which have a slight difference in mass due to the extra/missing electron, so this would again cause a slight net pushing force in one direction when applying an electric field to the air, but further orders of magnitude weaker. So I would suggest that it is actually possible to create this effect with a neutral distribution of ions, you just have to keep remaking the ions due to recombination.

I guess I'll share a bit more about the failed goal. I wanted to use the fan technology to make a lithium ion power drone with no moving parts. Some people have already done this, where the drone can lift it's own battery, but it's dodgy. The structure is so light weight it's flimsy and it only flies for 1 minute with maybe 10 grams of payload. I wanted to improve the specs to be better than current rotor based drones but it's not happening. I guess the only conceivable way left to make the thing fly is to use a high energy density battery that outputs like 5MW.... I'm not getting one of those batteries.