Hello.
I'm now considering to generate plasma which electron density ranges from 10^13 to 10^14 cm^(-3). The choice of this density range is from the fact that this plasma would be actually test bed for fusion-plasma diagnostics techniques. Plasma length is also important in this case and would be several centimeters (maybe 1 to 5 cm should be okay). We have high voltage box (Glassman) thus I think HV gas discharge can make plasma satisfying our requirements. Problem is that I don't know how to design plasma source such as what kind of gas I have to use or how to control ionization degree in discharge, etc. Could you give me some good advise for this purpose?
A plasma torch may produces plasma with the density you require.Argon is often used as plasma forming gas. The plasma formed is a kind of jet, and its length is dependent of the background pressure and the gas flow rate. The ionization rate is related to the input power and gas pressure in the torch. More information about plasma torches can be referred in articles by J F Coudert, V Rat, J Heberlein, etc.
Oh. Thanks. It looks microwave plasma is best option for our requirement. I have some question for DC discharged plasma: Why does DC plasma have limit for generating relatively wide volume and moderate density? Some people said when density becomes moderate like 10^14 cm^(-3) plasma immediately become arc and arc plasma is narrow line. Why should Arc be narrow?
As you refer to arc, you may mean the part of a plasma carrying intense electric current. The electromagnetic compression, called "pinch effect", may make the arc column narrow. If you define the diameter of the arc by its brightness, maybe the diffusion to periphery is an important reason.
Usually, cathode spot occurs with arc near the electrode, and it is very small. Almost all the current is flow through the cathode spot, and hence, its size may restrict the diameter of the arc column.
If small diameter of plasma is acceptable, you can consider use of hollow cathode discharge. The best paper I know on this topic is White, A. D. (1959), New hollow cathode glow discharge. J. Appl. Phys. 30, 711–719.
Oh good. Thank people. Maybe I need to extent of my study to hollow cathode plasma. And I finally understand that the arc is typically narrow plasma line. Can I have one more question? Why is hot spot on the cathode generated? Could you tell me generic mechanism of its formation?
Do you have a way of determining electron density? Reproducing a plasma from another study with a specific electron density is difficult. It is probably easier to build a plasma source with available parts, vary the gas composition, pressure, and change gas flow rate to change the degree of ionization. You can also change the discharge characteristics by changing the frequency using DC.
It is hard to explain why the arc spot forms in cathode.
Some books mention that the constrictions of the arc spot are associated with condtions where the electrons are emitted and with the characteristics of the arc. Some explain the constrictions of the arc spot from the view of stablility. It is said that a moderate increase in temperature or field on a small area even requires less energy than a small increase on a large area (refer to Andre Anders, Cathodic Arcs, Springer, 2008). Hence, this favors the forming of small area arc spot.
A Helicon plasma will exactly produce your desired density. you can see a review paper on this type of plasma source in the following link:
http://iopscience.iop.org/article/10.1088/0963-0252/24/1/014001/meta
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