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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Pinky's Brain wrote ...
Proud Mary wrote ...
In a Coolidge tube there is (ideally) no gas at all. Electrons boil off the heated cathode in a sort of dance, and before they can catch their breath they fall under the attractive power of the anode, and accelerate towards it. Were there to be any significant amount of gas present - as in a clapped-out tube that has gone 'soft' for example - the mean free path of the electrons is so short that they make almost no headway before bumping into something, all hope of reaching the anode brought to nothing.
With a large enough field you can get runaway electrons even at atmospheric pressure (ie. they keep speeding up, even with the occasional collision). These are probably the source of x-rays in atmospheric discharges.
I dare say you are right, Pinky, but it's not something I know much about.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Ash Small wrote ...
Most plasmas, however, exist in partial vacuums.
I should have thought that the most common example of a plasma discharge is to be found in electric arc welding at atmospheric pressure.
No matter what the gas pressure, from the abysmally low to the oppressively high, plasma can be made to form wherever there is at least some gas to ionize.
If we consider two electrodes, an anode and a cathode in a 'hard' vacuum (10E−9 Torr), there are but two known ways in which a current may flow.
1: electron field emission as we find in the cold cathode X-ray source.
2: vacuum breakdown as we find in flash X-ray sources.
Despite my hangover this morning, I'll attempt to try and clarify a point.
The last thing you want in a capacitatively coupled plasma is arcs.
Arcs can form if the pressure is to high, although there are other factors involved (excessive voltage, points from where corona breakout occurs)
(There are also two types of capacitatively coupled plasmas, AC and DC (three if you count pulsed DC seperately, and four if you count AC with a DC component, either pulsed or continuous)
But, to keep things simple, let's just consider DC.
The charged plates ionise the gas in between, producing a plasma. the positive particles then migrate to the cathode, the negative particles (usually electrons) migrate to the anode. This results in a current flow, similar to the current flow in a thermionic valve (vacuum tube), excapt the current in a thermionic valve only flows one way (electrons).
An arc involves a breakdown path, as you've pointed out.
I'm trying to highlight the difference between an arc and a plasma, and the similarities between a plasma and thermionic emission, but I'm not sure if I'm doing a very good job. More coffee, I think.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
Ash Small wrote ...
Most plasmas, however, exist in partial vacuums.
I should have thought that the most common example of a plasma discharge is to be found in electric arc welding at atmospheric pressure.
No matter what the gas pressure, from the abysmally low to the oppressively high, plasma can be made to form wherever there is at least some gas to ionize.
If we consider two electrodes, an anode and a cathode in a 'hard' vacuum (10E−9 Torr), there are but two known ways in which a current may flow.
1: electron field emission as we find in the cold cathode X-ray source.
2: vacuum breakdown as we find in flash X-ray sources.
Despite my hangover this morning, I'll attempt to try and clarify a point.
The last thing you want in a capacitatively coupled plasma is arcs.
Arcs can form if the pressure is to high, although there are other factors involved (excessive voltage, points from where corona breakout occurs)
(There are also two types of capacitatively coupled plasmas, AC and DC (three if you count pulsed DC seperately, and four if you count AC with a DC component, either pulsed or continuous)
But, to keep things simple, let's just consider DC.
The charged plates ionise the gas in between, producing a plasma. the positive particles then migrate to the cathode, the negative particles (usually electrons) migrate to the anode. This results in a current flow, similar to the current flow in a thermionic valve (vacuum tube), excapt the current in a thermionic valve only flows one way (electrons).
An arc involves a breakdown path, as you've pointed out.
I'm trying to highlight the difference between an arc and a plasma, and the similarities between a plasma and thermionic emission, but I'm not sure if I'm doing a very good job. More coffee, I think.
I really can't see where you're trying to go with all this, Ash, but I can recommend a remedy for hangover:
Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
It seems to me that ash is trying to say that a plasma is a sustained arc, and an arc is simply a discharge. Thats what I got out of it. Also, if you're looking for another example of ion flow representing current flow, the galvanic cell works nicely.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
. I really can't see where you're trying to go with all this, Ash, but I can recommend a remedy for hangover:
.
magnet18 wrote ...
It seems to me that ash is trying to say that a plasma is a sustained arc, and an arc is simply a discharge. Thats what I got out of it. Also, if you're looking for another example of ion flow representing current flow, the galvanic cell works nicely.
I was trying to clear up an apparent (or peceived) misconception that I thought Magnet may have had, that being the difference between a 'plasma' and an 'arc discharge'.
(The thread is titled 'plasma X-ray machine' yet the content regards 'arcs'.)
Maybe I shouldn't have bothered.
BTW, Thanks for the advice regarding hangover remedies.
EDIT: @Magnet, I'm not sure a plasma can be defined as a 'sustained arc'. (I think my choice of words in my first post was incorrect) (Although a 'sustained arc' eg from a Mazilli flyback or plasma speaker, is usually described as consisting of plasma)
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Plasma x-ray machine?
