On our last Atomic physics lecture, we experimented with gas discharge tubes, and the dark-light zones. During the experiment, bright, white flashes of light appeared on the anode of the tube. The number of them increased with the anode-cathode voltage, and, if I remember correctly, didn't seem to be related to the pressure inside the tube. I asked the professor and the demonstrator about them, but at the end we could not come up with a statisfying explanation.

The phenomena is observable on the following video I took after the lecture:
Flashes on the anode of a gas discharge tube (YouTube)

A frame from the video (though the video gives a much better insight):

Flashes on the anode

The flash rate seems to be consistently 3-4 flash per frame @ 29.7fps or approx. 104/s.

Setup: The tube was an approximately 1m long, 5cm diameter cylinder filled with air, connected to a vacuum pump and an adjustable valve to dynamically sustain an adjustable degree of vacuum. The anode and the cathode are both approx. 1cm wide Aluminium cylinders with a small hole in the middle, with their sides exposed to the enviroment outside of the tube. An adjustable 1-6kV voltage supply is connected to them at the exposed sides of the cylinder with a banana plug.

The question:

What is the explanation of these flashes?

Arisen possible explanations:

  • They are caused by the dust in the air, turned into plasma (scource: The professor)

  • They are sparks on the anode's surface (scource: The demonstrator)

  • They are caused by the electrons when leaving the surface (my idea)

  • It is some kind of a scintillation effect (my idea)

  • $\begingroup$ What's the material of the anode or screen? In my experience, Maybe the material of screen has a scintillation effect. $\endgroup$ – Feynman Mar 22 '17 at 2:44
  • $\begingroup$ @Feynman as I said, both the anode and the cathode are made from aluminium. There is nothing else I know about them. The device is a self-built one, so the source is probably an ordinary aluminium block a technician lathered. So oxidation and others could occur. $\endgroup$ – Neinstein Mar 22 '17 at 22:31

If you are pretty sure the flashes are on the anode and not distributed within the air/gas or in the air near to the anode it would appear to be a surface phenomena. To check this repeat experiment and look at anode from different angles, perhaps with 2 or 3 persons viewing same flashes. If convinced they are from anode surface then remove anode and examine it under magnifying glass or microscope to see if there is anything unusual about surface. Also try cleaning anode or covering it with fresh aluminium foil (may have to do this v. carefully to avoid sharp edge effects).

My personal guess would be it is a surface phenomena caused by impurities on the anode itself. Most airborne impurities are more likely to become positively charge and attracted to the cathode.

  • $\begingroup$ Thanks. The flashes happen on the surface without a doubt. (They are much more visible in real life than on the video, and they are in the plane of the anode. The system is impossible to dis- and reassemble with reasonable effort (as the glass is glued to the aluminium...), and is fully closed except the small tube connected to the vacuum pump. Impurities could be the reason as the device is a "home-made" one, and most probably built from a random CNCed aluminium block... maybe the oxidation on the surface is the key? $\endgroup$ – Neinstein Mar 22 '17 at 22:24
  • $\begingroup$ Cleaning the anode may be tricky then ! Perhaps ithe whole assembly can be fitted into an ultrasonic cleaning bath filled with appropriate cleaning fluid, probably something pretty volatile like industrial alcohol. You would probably have to run the vacuum pump for several hours to get the last few molecules of the cleaning fluid out of the tube. $\endgroup$ – BetterBuildings Mar 23 '17 at 19:40

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