dangerous plasma generation?

[ProfK]

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)

 

[logbook]

If the microwaves don't couple to air at normal pressure I'm not sure why they should suddenly start coupling to the rarified air.

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)

 

[biff44]

Sounds like a recipe for ball lightning. You DO NOT want to touch it!

 

[Comcokid]

There are plenty of examples of ionized gasses in everyday live where the glass container is not damages - neon bulbs, fluorscent tubes, and similar low pressure ionized gas devices. Even air at very low pressures in a strong electric field will glow. You can light a fluorscent tube in the presecence of a strong electric field without direct electrical connection - fluorscent tubes held by hand in the field of atelsa coil at science museums are one example. Many of these examples are not even hot to the touch.

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.

 

[logbook]

comcokid

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!)

 

[ProfK]

Yes, Comcokid, ionized gas is called a plasma. Some consider plasma a fourth state of matter.
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.

 

[Higgler]

I can picture a similar problem we had with high power rf cable into an antenna. Arcing occurs at the dielectric interface.
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

 

[ProfK]

Well guys; I tried it. Nuked a vacuum pumped quartz flask. Yes, a very nice purple plasma filled the flask. The plasma stopped at the neck where the smaller tube was fitted and where the vacuum was pullled from. After 30 seconds, the flask was hot to the touch and clearly heated by the plasma. Conversion to plasma did not seem to alter the vacuum/pressure load on the pump. After i get better gages, I'll know better.
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.

 

[Higgler]

Cool project. A photo would be nice.

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

 

[IRstuff]

While there might not be any other issues, you should be careful about the possibility of UV emissions from the plasma, since the plasma contains high energy electrons.

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

 

[lighterup]

Anyone interested in this art can follow the progress by doing a search on

http://www.uspto.gov

for patent number 6,617,806. This in turn leads to a company called Fusion Lighting in Maryland that has been researching uwave excited plasma lighting devices for several years. Actually, Nicola Tesla first discovered this phenom, and I believe holds an early patent on same. Check out the patents issued to this company. It is interesting. In the mid-nineties, this company had a microwave electrodeless lamp system installed at the National Air and Space Museum in DC as a demonstration of the technology. Whether it is still in operation or not, I do not know. Also if you read through the prior art, you will quickly see that a major impediment to developing this technology is cooling the containment vessel, as you found in your experiment. ;-)
lighterup

 

[Higgler]

For cooling, you could add metal heatsinks to the top and bottom of the vessel which would be cross pole to the E field (assumes E field is vertically polarized for most ovens).
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

 

[biff44]

I sure hope Fusion Lighting doesn't go anywhere, because if they do you can kiss your Sirus or XM radio reception goodbye! How about a 2.1 and 2.3 GHz receiver trying to find a geosynchronus satellite with a strong 2.45 GHz interferror on every street light.

 

[logbook]

My first posting suggested that the field wouldn't couple to the rarefied gas. It clearly does, but why?

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