Electroscope experiment in vacuum

[Dan Haronian]

Hi
I am running the electroscope experiment in vacuum. This is shown in (a) in the uploaded figure . Once the aluminum leafs are charged the chamber is pump down (b). As soon the pressure decrease low enough (around relative pressure of 30 inHg) the leafs collapse to each other abruptly.
Here is a short video of the collapsing leafs:
).
There is still a gap between the two leafs but it's much less than in atmospheric pressure. This effect takes place every time I run the experiment.

In another experiment, the chamber is pumped down before the leafs are charged. The gap between the leafs is much smaller than that in atmospheric pressure.

Can anyone explain why?

 

[DrZoidberg]

That can probably be explained with Paschen's law

 

[Dan Haronian]

Thank you drzoidberg,
I thought of Paschen's law as well but I am not sure because of the abrupt collapse. It looks more like some kind of breakdown. In addition there is only one charged electrode. V in Paschen's law refers to the breakdown voltage between two electrodes. Still there is an electric field between each leaf and the surroundings. So maybe some variation of Paschen's law. Paschen's law Is gas dependent. I am going to try different gasses to see how this affects the leaf collapsing.

 

[tech99]

I know that with a discharge tube, the discharge starts fairly abruptly as the pressure goes down.

 

[Dan Haronian]

The setup here is different than in a tube which Paschen's law refers to. V is the breakdown voltage between the two electrodes and d in p*d is the distance between the two electrodes. Yet, if we divide the two axis by d we get E as a function of p. E is the electric field at which breakdown takes place which is a function of the pressure. Referring to one leaf, E is proportional to 1/x^2. So the breakdown field is such that the MFP of electrons is such that they gain enough energy to ionize gas molecules. A breakdown in this case will be creation of many such ions (chain reaction) that flow to the leaf and recombine with electron on the leaf. E will diminish as the recombination progress and the overall effect will slow down. I assume that the abrupt effect where the two leafs collapse to each other (but not completely as in the video) is the chain reaction. The slow collapse of the leafs towards each other, that follows, is due to slower process of ion generation and recombination with electrons on the leaf.
Does this make sense?

I will run this experiment with He gas where the ionization energy is higher. My goal is to have the leaf stay charged for long time. Any ideas are welcomed.

 

[Tom.G]

My goal is to have the leaf stay charged for long time. Any ideas are welcomed.

Some possible approaches:

• Pull a good vacuum before charging
• Fill the container with Argon
• Fill the container with Sulfur HexaFluoride (HF₆)
• Super cleanliness of insulator around the electrical contact to the outside world
• Hydrophobic coating on that insulator
• Of course no sharp edges or points on external connection, a sphere is best
• Enclose the whole thing in another vacuum chamber or in a dry Nitrogen atmosphere
• If the whole thing is not in a vacuum, then shield from ionizing radiation, including UV light

The above is just "stream-of-consciousness", and some or all may be no good at all!