Hi, there is no simple solution for you, I think. Usually the layer formed on the surface can be quite thick, let us say up to several micrometers in our conditions and it persists for more than one month of the operation. Deposition, of course, depends strongly on the surface temperature. Sometimes, additional gases like nitrogen are used to decrease deposition. But I have only a little experience with this on the tokamak, try Paco Tabares from CIEMAT, Spain: [email protected]

The electrodes are made of SS? Or copper?

How do you plan on getting the decaborane into a gas phase?

Micron-sized particles will occur unless the pressure is very low (few Pa) and/or the gap distance between electrodes is small (a few mm). Typical deposition rates are of nm/s depending on conditions. I don't know the focus set-up very well, but maybe you could try a protection flow of an inert buffer gas (argon) near the electrodes?

Windows, for example, can be put at the end of tubes to reduce deposition on them.

Otherwise, clean off the boron using an etching plasma (maybe some fluoride-containing gas); or demount the electrodes and clean in acid.

We will be getting gaseous decaborane by heating the whole chamber to about 120 C and feeding vapor in from a heated container of the powder. We expect to run with a density of about 10^19 ions/cc. The electrodes will be beryllium. The good thing is that in the case of the experiment, the gas will cool down in about 10 ms or so. Can we calculate what fraction of the boron will deposit and what fraction will condense into suspended particles? If they are micron sized, we can presumably blow them out of the chamber.

I assume that under your experimental conditions decaborane will decompose to boron and hydrogen. Applying Le Chatelier's principle I would try to add some hydrogen to your plasma therefore pushing the equilibrium back to decaborane (or another borane which is certainly more volatile than boron itself). I would not recommend the addition of nitrogen because boron nitride is not volatile at all.

Additional gases like nitrogen are used to decrease deposition.

You can remove boron from the electrodes by running plasma focus device with argon gas. as a result, the electrons sputtered the anode and boron removed.

I have a reference for the deposition rate of boron with arc plasma device (PISCES-B linear plasma device)

I should emphasize that we are running a much denser gas than is mentioned here--10^18-10^19 ions/cc. Is there any data on deposition rates from such dense plasma? If we can calculate the exact size of the solid particle formed, then we could I assume, figure out from that how thick a layer would be deposited on any surface.

Eric - how fast are you pulsing the plasma? If it is really fast, the formation of B radicals will be fairly small. In fact, if you can answer... what are you trying to do with the plasma? That will help you determine how to clean the system. So for example, some of the sputtering techniques suggested above would not be good if you are trying to deposit a very thin layer of B on a surface. The dust formation will kill the process.

Hi

We have been using several boron compounds for the inner coating of tokamaks and stellarators in Madrid, including decaborane, the only one not containing carbon. We use simple Glow discharge techniques, with the anode at several hundred volts and the chamber to be coated as the cathode and grounded. We feed the decaborane from simple ovens at 100ºC into a He glow discharge, in order to enhance the ionization and fragmentation of the molecule, at total pressure of 1Pa aprox. We have never had problems with boron deposits on the anode, basically because all the ( positive) ionized fragments of the parent molecule got to the cathode as expected. However I must say that our plasmas are by far much less dense than those you are planning ( by 8 to 9 orders of magnitude). If you boost the plasma density, the mean free path of the molecule will decrease enormously, so that everything will deposit very close to the gas inlet, i.e., no coating at all into the vessel. If you are not flowing any gas, just simply filling the chamber and pulsing the plasma, then you will deposits everything from the gas phase on the walls. The growing rate of the film will be simply giving by the rate at which you can refill the vessel with decaborane vapor and the geometrical surface to volume ratio of your device. As somebody suggested above, there is only one kind of plasma able to remove boron, at least at a significant thickness, which are halogen containing plasmas (chlorine will do). If the films are very thin, then it seems that chemical sputtering by H plasmas will work. The sticking of the fragments acting as film precursor is very much depending on energy for ionized ones. I dont know what energy do yo expect ( voltage difference across the cathode sheath) but if not high enough, then you may expect polymerization and low quality deposits...

You may be interested in our articles "First boronization of the TJ-II stellarator" Vacuum 67 (2002) 393–397 and "Boronization of the TJ-I tokamak by Trimethylboron an Decaborane" J. Nucl. Mater. 220-222 (1995) 688

I think you would have more problems about boron deposition at the isolation between electrodes. You could use Li6-P fusion because lithium evaporates at a lot less temperature. Of course you would had less barns

There are following ways to remove boron:

If it is possible to fill in the fusor with water:

1. Using weak acid, diluted in water or water+water peroxide

2. By electrolysis, be careful that working function of boron is less than the electrode ones

Without filling with water:

1. By using aluminium, polished steel or other high reflectance electrodes and apliying a laser. Laser does not damage aluminium but removes low reflectance materials

2. By using a high power electromagnetic pulser (ask to [email protected]), it is possible to remove using one pulse the entire boron surface. It is possible use during fusor operation

Good luck Lerner!