Voltage breakdown - positive vs. negative

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Post Reply
Todd Massure
Posts: 443
Joined: Thu Mar 11, 2004 12:38 am
Real name: Todd Massure

Voltage breakdown - positive vs. negative

Post by Todd Massure »

Hi everyone,

A general high voltage theory kind of question I've had for a while:

Since electron emission (thermionic or cold cathode emission) comes from a negatively charged electrode, not a positively charged one, are there some instances where a very high positive charge on an electrode is less likely to result in breakdown than if it were the same voltage applied to a negative electrode in the same geometry setting? My question is generally in regard to steady DC voltages in vacuum situations, not in atmospheric conditions.

A good example of what I'm talking about would be the inner grid of a typical fusor with a grounded outer grid/shell. It seems that the inner grid would be more likely to arc when at a high negative potential (as in the typical IEC fusor) rather than a high positive potential since a positively charged inner grid wouldn't produce any outward thermionic or cold cathode emission. Also, a positive inner grid seems less likely to be able to induce a high enough negative charge on the outer grid to result in substantial enough electron emission to breakdown/arc due to distance and the larger area ratio of the outer grid. This is the basic principal of how a vacuum diode works--the electrons only flow from the cathode. I've also done some simulations on FEMM where there is a high positive potential on an electrode, but the surrounding materials don't reach a high enough negative potential to produce electron emission. This being said, I wanted to ask the question here to see if anyone has input about whether in the real world high positive and negative voltages can really be treated differently from a design, engineering and safety standpoint.

One other point--I don't want to give anyone reading this a false sense of security when using positive high voltage. It IS very dangerous and unpredictable for sure, and will still jump out (and can kill you) so be extremely careful! As many here will tell you, there are a lot of variables with high voltage that may not be accounted for, and calculations/Paschen's law, simulations etc. never tell the full story of what's going to happen in the real world.

Thanks,

Todd Massure
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Voltage breakdown - positive vs. negative

Post by Richard Hull »

Positive high voltage regarding its danger is no more dangerous than negative HV as both at their point of origin (electronic power supplies) have their "cold side" firmly grounded. As such, should you approach say 100kv on a leaky or corna point. You will by nature be charged to the opposite polarity and a target of opportunity to have that voltage leap out beyond its normal air arc range via a corona ion bridge. DC is especially bad in this regard. No matter your resistance to ground you will be moved towards ground based on you capacitance charging time related to even a high resistance path to ground that you think is not there. Once an arc forms your resistance goes down via the saline blood path and out of your shoes to the nearest better ground point. If you pull real current....well....that's it for you.

Wish I could help regarding the vacuum. At at some point regardless of shapes as voltage goes up, even in a vacuum you will see electrons ripped from electrodes

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Frank Sanns
Site Admin
Posts: 2119
Joined: Fri Jun 14, 2002 2:26 pm
Real name: Frank Sanns

Re: Voltage breakdown - positive vs. negative

Post by Frank Sanns »

For equivalent geometry, the E field is the same at both ends no matter the polarity.

The E field can be dispersed or concentrated depending upon geometry though. A needle point will concentrate the field while a smooth hemisphere will spread it out. This is why we avoid sharp corners and small radiuses in our HV feeds. Nothing really changes in a moderate vacuum like in a fusor.

What is important is that ionization does not start at a particular polarity. It starts when gas molecules close to it are ionized and produce charge carriers. The sharper geometry, the more ionization (splitting into electrons and positive ions) occurs. When there are sufficient charge carriers formed, a conductive pathway forms and an arc ensues. Somewhere in the Images section was a post that I did some photography of the arc initiation in the outer and inner grid. It seemed pretty symmetrical but I have those sharp pointed inner and outer grids in my setup and not just a smooth outer chamber wall.

Vacuum diodes are not the same case. They have a heated cathode to be sure there is a large supply of electrons being supplied. Electron flow then is pushed in one direction. Alternatively, different work function materials could be used in the anode and cathode. This would force current to only flow in one direction. This is only good for 2 or 3 volts at most though because work functions are relatively small numbers.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
Todd Massure
Posts: 443
Joined: Thu Mar 11, 2004 12:38 am
Real name: Todd Massure

Re: Voltage breakdown - positive vs. negative

Post by Todd Massure »

Richard and Frank,

Thanks for your replies. You both bring up the various factors involved. I think the scenario that I'm describing may occur if the conditions were right, but it would require a high enough level of vacuum that the ionization factor is negligible and geometry, voltage/electric field strength and work function are the only real factors. I'm still noodling on this one, and may want to do an experiment to check it out at some point, but it's probably not something that would be useful in the realm of anything related to plasma since (per Frank's comments) there would be sufficient charge carriers to negate the effect.

Todd Massure
Post Reply

Return to “High Voltage - Fusor Input Power (& FAQs)”