dangerous plasma generation?
dangerous plasma generation?
(OP)
Hypothetical (before I do it for real)>>>
Setup: Take a glass bulb 4 inches in diameter with a glass tube in one side to attach a vacuum pump. Pump a (roughing quality) vacuum through the glass tube inside a 1kw operating microwave oven cavity. As the air is removed and a certain vacuum level is reached, the microwaves may couple to the rarified gas and turn it into a plasma.
Question: If the gas is converted to plasma, can the heat/pressure possibly damage the bulb or will it behave like those novelty plasma globes?
Consider: the bulb is vitreous quartz and thick enough to take a hard vacuum at a high temperature
Second question: below a certain pressure, the gas can't couple well, but what if there is a hot spot inside the bulb? (no spinning vane and turntable disabled in oven)
Setup: Take a glass bulb 4 inches in diameter with a glass tube in one side to attach a vacuum pump. Pump a (roughing quality) vacuum through the glass tube inside a 1kw operating microwave oven cavity. As the air is removed and a certain vacuum level is reached, the microwaves may couple to the rarified gas and turn it into a plasma.
Question: If the gas is converted to plasma, can the heat/pressure possibly damage the bulb or will it behave like those novelty plasma globes?
Consider: the bulb is vitreous quartz and thick enough to take a hard vacuum at a high temperature
Second question: below a certain pressure, the gas can't couple well, but what if there is a hot spot inside the bulb? (no spinning vane and turntable disabled in oven)





RE: dangerous plasma generation?
Yes conduction occurs through evacuated tubes using thermionic emission, but I don't see a similar mechanism for the microwave coupling. The dielectric constant of the rarified air will be slightly less than normal air but normal air is already something like 1.0007 (guessed)
RE: dangerous plasma generation?
RE: dangerous plasma generation?
But does a gas where a few of the outer electrons are stripped really meet the full physics defination of a plasma? I don't know.
RE: dangerous plasma generation?
thanks for the reminders about the non-contact glowing tubes. I remember those experiments now you mentioned them. They do glow in preference to the surrounding air so I guess if you stick one of those tubes in a microwave it would also glow (and then some!)
RE: dangerous plasma generation?
Anyway, considering those plasma balls are sealed, I suppose a conversion of the evacuated sphere gas to plasma shouldn't pose a threat. It's just that I have seen a vacuum chamber of metal pumped pumped in with 3 kw uwave. When the vacuum (pressure) reached a certain low level, a plasma formed and started to melt parts inside the vacuum chamber. Then, as the vacuum improved, pressure further reduced, no plasma. I read about this subsequently but have no idea if the plasma might harm a glass ball if it formed inside. The metal vacuum chamber was quite strong and cold.
RE: dangerous plasma generation?
I believe if your vessel is considered a dielectric similar to the antenna, that arcing may start on the inner surface of that vessel wall and start burning it until it breaks down the vessel material, just a thought.
kch
RE: dangerous plasma generation?
So, there's heat but no pressure. I know the quartz won't get that hot without the plasma. Odd that the plasma stops at the neck.
RE: dangerous plasma generation?
If the E field is constant across the vessel and the voltage is E field strength in Volts per unit length, then only a certain length may generate enough voltage to generate the plasma. Decreasing the pressure may send the plasma into the neck further.
If you move the vessel closer to a side wall, you may see the plasma disappear closer to the wall. The voltage at the door is shorted and the E field goes to zero in the up/down direction.
kch
RE: dangerous plasma generation?
The electron temperatures are supposed to be in the thousands of kelvins, while the molecular temperatures might not even be above 1000 kelvins, which would mean that the gas pressure would not be a problem, either for the vessel or the pump, since the gas will be cooled by the time it actually reaches the pump itself.
TTFN
RE: dangerous plasma generation?
lighterup
RE: dangerous plasma generation?
Also, immersing the vessel in rf semi- transparent Castor Oil will cool it. I've tried heating Castor Oil in a microwave oven for 3-4 minutes with no rise in heat detectable. There may be differences in Castor oil of course from one vendor to another.
Castor oil dielectric is probably in the 2.5-3.5 range which of course makes a reflection, but you can always taper the input shape.
kch
RE: dangerous plasma generation?
RE: dangerous plasma generation?
On further thought it may be that there is a certain low amount of spontaneous ionisation occurring all the time as some electrons occasionally reach "escape velocity". These will not have much to combine with in a rarefied atmosphere so their mean time to collision and hence their mean lifetime is probably longer. Now these miserably few ionised particles will be strongly affected by the intense electric and magnetic fields and will move like crazy as a result. Being super-excited they will necessarily bash into nearby un-ionised particles with tremendous energy and cause a cascade of ionisation. Now you get a self-sustaining plasma.
So I feel happy with the theory versus experimental results again