Hello.
The mechanism of the gas discharge is quite well-known when the gas is filled between two plane electrodes. In this case electron emission from cathode have significant role to get into self-sustain regime such as glow discharge and even arc.
But what about hollow electrodes? In our case, we're using hollow type electrodes between which the gas is filled about several tens of torr and DC-discharged with High voltage up to 30 KV. The gas filling length is about 3 cm. With holes of both electrodes, plasma can ejects to outside and laser also can be sent to plasma to have some interaction. In this case, it looks that electron emissions from electrodes doesn't have much job but still very high plasma density is observed as several 10^18 cm^(-3). How to explain this case? what is the mechanism of this? Is there no room to apply theory for plane electrodes to this? If so, how can we call this as arc plasma?
what you describe is known as a hollow cathode discharge. it is essentially a glow-like discharge with increased cathode surface area; charge multiplication is caused by release of secondary electrons from the cylindrical cathode surface due to ion impact and UV photons from the central plasma. thanks to the hollow cathode geometry, most photons and ions from the plasma reach the cathode and are not "lost in space", whence this kind of discharge can be much more intense than ordinary glow discharges in the same range of parameters.
there is plenty of literature describing the physics of hollow cathode discharges, and also many technical applications like switches, gas discharge lasers, electron and ion sources, UV light sources, plasma sources & thrusters, etc.
Thanks Hartmann
I'm not sure our plasma is the one what you imagined. In our case, hollow electrodes are separated with sapphire tube in which the gas is filled. The tube radius and length are ~300 um and 30 mm respectively. One of the electrodes is cathode while the other is anode thus this setup is geometrically similar to two plane discharge but difference is that plane electrodes are replaced with hollow electrodes for laser injection to and ejection from the plasma. As major discharge appeared within sapphire tube, I'm not sure secondary emission is major role here. As plasma temperature is measured as just several eV, secondary emission coefficient for sapphire could be so low, I think.
Better for you to visit plasmafocus.net website and study the relevant literature or thesis, also u can study similar discharge handbook .
The best paper I know on this topic is White, A. D. (1959), New hollow cathode glow discharge. J. Appl. Phys. 30, 711–719.
White described the Hollow Cathode Discharge (HCD) and its I–V characteristics, in detail, including experiments with a real micro HCD with a spherical cavity of the cathode having a diameter of 0.75mm. According to White, the major features of the HCD that distinguish it from the normal (‘‘flat’’ cathode) glow discharge are as follows: (1) HCD extends and flattens the volt–ampere characteristics for the same cathode area and (2) the cathode current density at HCD can be quite high.
Thanks Gutsol. I definitely have to read the paper. Unfortunately Our university can't access to the paper you mentioned directly since it is pretty old paper. If possible, could you give me the paper via [email protected]? It look very helpful for my job!
Look at our paper "Analytical model of the hollow cathode effect" available in my researchgate web site. We have written a review "Glow discharges with electrostatic confinement of fast electrons" that will appear in J. Phys D in a couple weeks (I hope)..
I have always been interested that below the Paschen's law minimum, the discharge will actually occur between the furthest points and not the closest points. At low pressures I have used this to maintain an arc between two hollow cathodes.
Since you have a sapphire capillary between your electrodes as well as low pressure, the properties of your plasma will strongly be influenced by diffusion and wall recombination processes in your capillary. Thus you will have to take into account both the hollow cathode effects mentioned by Werner Hartmann, Alexander Gutsol, and Vladimir Kolobov, and a capillary discharge model (e.g. Schottky model, see http://dx.doi.org/10.1051/epjap/2011110157). Details of the model to be applied will depend on the current, which you didn't mention at all.
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