Hi all, Yesterday i was trying microvawe popcorn.
I putted at max power and, after 2 minutes suddently there was a reddish colour in the microvawe, i pulled the plug and discovered a bulge in the inner case.

Does anyone have an idea about what could be happened?
I don't think this was plasma, because It persisted for some seconds After turning off.

In the photo, you can clearly see the bulge in the upper right corner.

Where is the "bulge" you are talking about?
You mean the dome above the mica covered window on the side?
That bulge is supposed to be there, it is around the antenna stud of the magnetron. From there goes a waveguide (formed from the other side of the right hand inner wall of oven, with sheet steel cover welded on it) towards the mica covered window, where the microwaves radiate out into the oven box itself.

Now if that dome became red, most likely there was an arcover between the magnetron antenna.
The cause could have been either power surge in the mains, the antenna cap getting lose and/or some contamination (dust or even an insect crawling there when the oven was off), or a combination of above. Normally the 2.45GHz AC voltage on the antenna tip isin the 10's kV range, so once a discharge gets ignited there, the arc stays. And wgat caused the metal to go red was the huge heat from the arc there.
Unfortunatelly most likely the antenna cap is ruined, it is made of a rather thin sheet metal, so very readily melts with any arc. And once damaged, it does not have exclusicely rounded surface anymore, so it will become even way more prone to any arcing.
If you are lucky, it is just the antenna cup, which could be replaced, and not the RF output lead of the magnetron itself.
But do not mess with the microwave internals unless you really know what you are doing. There is 2.5..3kV, capable to deliver about an amp, so practically instantly deadly. Plus on top of that there us a capacitor, which may have remained charged to those 3..5kV (in case its bleeder resistor had failed before), so really safety rules for working with high voltage electricity apply there.
After replacing the cap, remove the mica cover, inspect it for cleanliness and any arc damage and completely clean up the waveguide cavity. Also clean the soot from the magnetron insulator just beliw tge antenna, so it won't trigger any flashover there. If the mica is damaged, better to replace it too.
 Any sharpish speck on the wall and you will get an flashover again.

Burned antenna cap replacement video
Example of the antenna caps, but you should make sure you order the correct size for your magnetron, there are 4 sizes I have seen so far (different diameter and length)
Material to cut the waveguide aperture window cover from - be careful to not disintegrate it to million pieces...

Unless it's a very high end machine, then you're better off just getting a new machine, 

As Medved says unless you know exactly what you're doing microwaves can be lethal.!!!

Hi all, think you for the info, i didn't know about that antenna cap.

I didn't noticed that dome until It happened, so this cheap machine Is doomed.

I know about internal capacitor and voltage and, unless the lamp, i'm not going to save anything.

Do you think could’ve overheated the magnetron? I heard microwaving popcorn is hard on the magnetron.

No. Magnetron is quite hefty block of thermally very conductive copper, so it has very uniform temperature. And that is monitored by a cutout switch, so it is never allowed to really overheat it, even when the cause is elevated power dissipation. Plus the supply system (HX transformer with series capacitor on the output) is designed so it delivers the maximum power when the food absorbs all of the power without reflections (so like a big chunk of juice meat or so), so any form of reflection in the oven leads to lower actual power.

What is very vulnerable are the high voltage conditions. Not because the magnetron itself, but for the risk of such a flashover. But when all components are in a good shape and clean, it won't happen either.

What is dangerous in the terms of high risk of flashover is when mainly the mica waveguide window cover gets dirty. It is the place with the strongest field in the oven, so any junk residue stuck on it gets quickly turned into red hot glowing carbon. And mainly at the top, close to the magnetron antenna, it tend to trigger the flashover, mainly when the oven content leads to the higher voltage condition. When the flashover happens, usually it means hole byrned into the antenna cap and the mica melted on the top.

So to prevent this, make sure the mica cover stays clean. Always make sure no mess can reach the mica cover, use the plastic lids to cover anything you are heating there and regularly inspect that mice for anything that seem to turn brown/black.

After a flashover, usually there is no other damage than the mica cover and the antenna cap.

That mica cover could be easily replaced from the inside, without opening anything where the lethal voltages could be. You can wash it, but then let it completely dry out before installing back, the water soaked into the material may form that arcover initiation spot also. Often when the dirt is already "fried in", it is better to replace that cover and not riskthe flashover.

The antenna cap, which you maybe can see or even touch when the mica cover is off, is safe, as (except for the 2.5GHz power, but that is there only when the oven is heating) it is quite hard grounded.
But to replace the antenna cap, you need to remove the magnetron from the oven first and that, meside being way more laborious, means opening the compartment where the lethal electrical power is.
So better prevent the flashover in the first place, by maintaining that cover clean or replacing it when suspicious even a bit.

Probably the popcorn itself being overheated and ignited, hence the red glow. This can sometimes occur. But 2 min at full power shouldn't causes this.

Maybe microvawe was detective: i used it rarely, was clean and popcorn were in bag.

You may inspect that mica, if is burned there or not. If it is clean, it is likely the bag sparking (and the light from that just being reflected by the mica), so there is nothing wrong with the microwave at all.
If there are some scorch marks on the mica, mainly on the top side close to the dome, it may be \needed to remove it and at least turn around, so the scorched edge is at the bottom (there the field is by far not that strong).
And once it is off, you may peak into the cavity onto the antenna (e.g. using a phone or so), if there was flashover, the eventually damaged spot will be most likely facing down, so it will be visible.
If no damage, you can just order some mica sheets to replace the cover once it arrives and the microwave will be in normal working order again.
In any case in the meantime you may still operate the microwave, the damaged spot on the mica cover just has to be on the bottom side (away from the bulb in the sheet metal, where is not that strong field). The oven may be a bit more sensitive to flashovers (so better to not use it for popcorn), but you may still heat up normal food with it, till the new mica arrives or till it starts flashing over again.

Too late, yesterday it went to local recycling center to be recycled.
I think this was something near the magnetron (as Medved suggested before), because the event was this:
1) Stopped heating popcorn.
2) Started glowing red.
3) I shut it down.
4) After 1-2 seconds, it stopped glowing red.

There was hot metal smell and not burning smell inside the microvawe oven.

Dor123, it wasn't popcorn bag, because it wasn't burned at all.