Arc in a vaccum?

Evil Bunny

Can you create an arc in a vaccum?

sophiecentaur

More volts are needed than in air. They make 'vacuum capacitors' for use in high power transmitters and they operate at pretty high voltage. But they will arc across, given high enough volts.
The electric field around the plates would be high enough to ionise any significant number of air molecules and to start a discharge. The field at the metal surface needs to be higher to release electrons and to start an arc. A pointed electrode can easily produce enough field around it to start an arc in a vacuum with only a few kV applied to a nearby plate.

nsaspook

Sure, you can create field emission electron flow on the surface of the electrode in total vacuum.

http://en.wikipedia.org/wiki/Field_electron_emission

davenn

Interesting question :)
and to you sophicentaur, do we really need any ionisation at all ?
if the electric field was hi enough wouldnt the electrons just jump the gap anyway ?

I'm just pondering ( mind wandering haha) a vacuum tube of any sort where there is a flow of electrons between any 2 electrodes eg. filament and plate
now its not an arc, but it is still a current flow. So ummm the arc is just a visual manifestation of the electron flow cuz of the ionisation of whatever gas is present ? air, neon etc......
somewhere in there is a fundamental question as of the true definition of an arc.. what is it. like the flame of a fire....
we all know some flames are easily visible some are near invisible depending on the chemical elements present
so as to electrons jumping across a gap their presence can be seen or not depending on the presence of chemical elements

Dave

NUCENG

What we typically call an arc is the visible current path caused by ionization and heating of air. The vacuum tube is a perfect example of flow of current between conductors in a vacuum. There is no visible arc because there is so little gas to ionize. BTW do they still teach vacuum tubes?

KavinMassey

Can someone tell me an electric arc is visible in a vacuum or not?





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yungman

This is a good question. You really don't get a perfect vacuum. I worked with HV in high vacuum, we do have arc, but mostly are from creepage on the surface of insulator. But I am not sure you arc through vacuum. This is very different from emission of electrons, arcing is an avalanche of electrons, not just an emission. The instantaneous current is in hundreds of amps.

Also, when you have an arc through creepage, some sort of plasma or something get started and you get arc all over at different spots, so I don't think you can be sure it really is arcing through vacuum. Bottom line is easy to talk than to actually perform an experiment. Think about how you put the two electrodes into a vacuum chamber....through a flange with insulator. The creepage will start on the surface of the insulator and ionize something that I don't know how to even explain and get the arc going. Once you get some plasma going, you might see arc across the electrodes but that is not really arc through vacuum.

My experience is when one arc going, everything arc!!! you have to be careful in theorizing this.

yungman

Arc always visible, we catch arc by turning off all lights and search for arcs.

sophiecentaur

I think this is heading for another 'definitions' phase. Not too fruitful, I think.

A current can pass through a CRT, which just consists of electrons from the gun. As there's nothing for them to collide with, then the beam will be invisible. Would it actually matter whether we called this a (special class of) arc or an electron beam? In a low pressure gas discharge tube, the same thing is happening - mostly there's a stream of electrons but this time they are colliding with and ionising gas atoms. Would you call that an arc? So how low a vacuum would you need before it was no longer an arc? There's no end to this.

nsaspook

If the voltage is increased to the point the field emission electrons striking other atoms causes secondary electron emission you then form a electron cloud that can cause an avalanche of electrons (arc). On our machines (Semiconductor Ion Implanter beam created electron clouds) this radiation is usually in the X-ray range with visible light from the ionization of materials from the electrodes and walls.

http://en.wikipedia.org/wiki/Secondary_emission
http://en.wikipedia.org/wiki/Electron-Cloud_Effect

sophiecentaur

So that's an answer. If there are no gas atoms in the way then no avalanche can occur and so, by definition, you can't have an 'arc'. It's just a beam of charges on the move.

yungman

Yeh, I don't know the physics side of it in detail, I just been around various mass spectrometer, semi-conductor analyzing equipments for many years. Just learn from listening to the discussion between scientists mechanical engineers that design the chamber, lens, egun etc.

Maybe it's like what you said about the electron cloud. As I said, once the arc starts, you get arc all over in the surrounding. So it is hard to say who start it first. Just a lot of theorizing and not much solid evidence.

yungman

I think what he meant is the electron beam striking any surface and cause secondary electrons to be emitted and start the avalanche. When electrons emitted from an electrode, it spray all over.

I better stop talking as I don't know much on this, just kept hearing this all the time, all the secondary electron emission, plasma and all. It is not just vacuum.

cmb

If you mean 'a vacuum' (as in low, medium, hard, scientific, etc..) then the answer is that not only can you get an arc, but it is easier to get an arc, down to around 10 Torr, below which it gets more difficult.

Interestingly, it is more difficult to get an arc from electrodes closer together, than further apart. This is because electron cascading doesn't work well over very short distances.

The other thing you need for an arc is an ionisation source. Normally, once a gap is at sufficient potential it will spontaneously undergo cascade breakdown because it is exposed to cosmic and background radiation, which will ionise some molecules in the gap, triggering the cascade.

This piece of information is relevant because although it is pretty difficult to create a totally radiation-free volume, the inverse can be used to more easily initiate an arc by exposing the volume to intense radiation. But in any case, you need to exceed the Paschen breakdown and have some ionisation source.

cmb

Of course! An arc is an arc. If you don't see it, it isn't an arc! What you see is the recombination of freed -ve electrons dropping back into +ve charged atoms.

nsaspook

As you say, it's a matter of 'definitions'. You can have a avalanche caused just by the positive feedback of secondary electron generation in a pure vacuum. This in turn causes thermionic electron emissions from localized Joule heating of the electrodes causing even more secondary electron generation. Usually at this point we have intense ionization forming a plasma. I've seen this happen at 1e-7 torr vacuum when the electron suppression circuits fail.

Evil Bunny

So... the consensus is that you can still have a visible arc in a vaccum.

Does the arc look any different than it would in our normal atmosphere? Same orange-ish color and everything?