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I have recently built a flyback transformer driver with a 555 timer, and have been drawing electric arcs. But I noticed that the cathode electrode(the HV pin on the transformer) heats up a lot more than the anode(the HV cable on top of the transformer). I noticed this because if I use thin cables as electrodes, the cathode starts melting and getting red hot a lot faster than the other electrode. My question is why does this happen. Thanks

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In an electric arc, electrons are being stripped from their atoms in the air & fired with high energy towards the anode.

Some of the electrons are fired from the cathode directly, and others are stripped from air atoms, but eventually, if the plasma is sustaining, they end up striking the anode.

Meanwhile, the much larger & denser necleii of the electron-stripped atoms are not drawn through the arc with as much speed, any many end up dissipating into the surrounding gas, rather than impacting the cathode.

Due to this imbalance in the impacts of energetic particles, the vast majority of arc-caused heating to the electrodes occurs on the anode.

For more explanation, here is a detailed and informative article from lincolnelectric.com, explaining some of the various properties & arrangements of this tranferrance which are exploited in the verious metal arc-welding processes.

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  • \$\begingroup\$ I see... but in my case it is the cathode is the one being heated up more, maybe I have the cables on the primary set up wrong and creating a magnetic field on the other direction. But because flybacks have diodes in the secondary I though the output could only go one-way. \$\endgroup\$ – Bruno Feb 22 '16 at 5:46
  • \$\begingroup\$ @Bruno The thick high voltage cable coming from a CRT line output transformer (TV flyback) carries rectified positive HV output. An electrode connected to it is the anode. \$\endgroup\$ – jms Feb 22 '16 at 11:24
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    \$\begingroup\$ This is manifestly not correct. Why should the ions (not nuclei; an arc has an electron temperature of only a few eV) be subject to much greater losses than the electrons? A plasma is quasineutral, and how is current going to flow into negative space charge if the ions are mostly lost? Think about the local electric fields and hence the kinetic energies of the particles. The cathode is heated by ion impact; in fact the high temperature of the cathode (enough to sustain thermionic emission) is one of the most notable features of an arc. \$\endgroup\$ – Oleksandr R. Feb 22 '16 at 14:52
  • \$\begingroup\$ @OleksandrR. The ions are very slow compared to the electrons. The spark is somehow different phenomena then cathode thermoionic emission, where the cathode is externaly heated. In a spark the thermic ionization occurs in the surrounding gas, at first the free electrons are accelerated by means of electric filed, when those electrons hit the molecules they ionize and strip out new electrons - primary ionization. After the channel is formed, the current passes and heats up the channel, the secondary ionization happens - thermic. \$\endgroup\$ – Marko Buršič Feb 22 '16 at 15:19
  • \$\begingroup\$ @MarkoBuršič yes, and I agree with you, but the cathode fall is much larger than the anode fall, so the kinetic energies of the ions at the cathode are higher than those of the electrons at the anode, and their masses are larger, so that momentum transfer to the electrode is more efficient. Ion impact is an important heating process for the cathode in an arc, although I think your point about the current density at the electrode surfaces being even more important is very valid (+1 for that). \$\endgroup\$ – Oleksandr R. Feb 22 '16 at 17:48
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As noted by Marko Buršič in his answer, it depends very strongly on the current density at the electrodes as to which of them will be hotter. This in turn relates to their size and geometry, their material, the working gas, and to some extent the orientation of the discharge in space (because of convection). But if the cathode and anode are the same size and shape, made of the same material, and placed in the same relative orientation, then it is due mainly to ion impacts.

The electric field is not uniform within a discharge. Most of the potential is dropped close to the cathode, in the so-called sheath. This causes the ions to be accelerated into the cathode, while the electrons are not (to the same extent) accelerated into the anode:

Plot of plasma potential between electrodes
(source: egloos.com)

(This picture is for a glow discharge. The relationship is the same in an arc, although the falls are smaller in size and not as large in magnitude. This paper estimates them to be 14V and 4.5V for the cathode and anode respectively, for an air arc between Ag electrodes.)

Not only are the ions accelerated into the cathode, but electrons from the cathode are strongly accelerated into the discharge near to the cathode. This leads to a high power dissipation, and some interesting physics, in this region. The arc simply cannot exist without it! In any case, this volume of extremely hot gas also heats the cathode indirectly. There is no comparable structure next to the anode, and indeed, the heavy-particle temperatures can be seen (in the paper) to be much lower there.

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You should upload the photo of the arrangement. It depends on the shape of the electrodes, if they are plates, needles, combination of them. Possible phenomena that you observe is the corona discharge, where the current is flowing without being seen like a discharge, you should test in a dark to see the blue glow. Your electrode can be heated because of high current density at the tip of electrode, while the anode could have different geometry, so a pic can say as many words.

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I am going to go out on a limb and boil down Oleksandr R. 's answer to a super simple form even a child can understand, because my inner child, and my real child, wants to know why our flyback experiment is getting these same results.

Super tiny electron sparks are getting sucked into the anode over it's entire surface and drawn away back to the transformer.

Great big and hot ion coals are building up around the tip of the cathode, banging into it, and not getting drawn away but only pushed back into more incoming coals.

Hence the cathode tip gets white hot, melts, and fly's apart rapidly, while the anode barely gets warm and glows more evenly.

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