Dennis P. Brown,
Thank you very much for your suggestions and insight. It has given me a lot of new ideas to think about that I hadn't originally planned to dive into, but you make very good points.
When I was typing out the last section, I knew I would probably have to specify more about the ion and titanium sublimation pumps. Thank you for bringing that up, I will go back and revise that last statement a bit. They are definitely very expensive pieces of equipment new. I don't even think I have come across titanium sublimation pumps used on eBay. However, in the past couple months of eBay browsing, I have come across a decent number of used ion pumps that appear to be in quite good shape physically. The nice thing is that a lot of pumps sized for 20-25 L/s are fitted with 2.75" Conflat flange hardware, which makes it very convenient to integrate into these smaller systems, and at ultra-high vacuum levels even 20 L/s is a huge pumping boost. These pumps that I have come across so far are in the $100-$250 range, which is quite reasonably priced for ultra-high vacuum pumps. Of course functionality is another matter, but generally ion pumps don't require much maintenance, especially if they are used properly. Old controllers are also not terribly expensive, and if someone can get a standard fusor working with proper instrumentation, then building an ion pump supply should not be too difficult or expensive as well.
I am quite interested in the titanium sublimation pump however because it would be very easy to construct a simple one with little effort and components, and the controller for such would be dirt simple (low voltage high current supply cycled on and off a few minutes a few times an hour). I found this guide that actually shows how to make a simple one based off of 2.75" conflat hardware that I am going to follow and modify myself:
https://www.rbdinstruments.com/blog/hom ... tion-pump/
All it would consist of is a nipple section, an insulated low voltage feedthrough with at least 2 feedthroughs, and some titanium wire. For my own system, I was able to obtain a 2.75" nipple section and an insulated low voltage feedthrough with 4 inputs for free, so all that is needed is the titanium wire and a simple controller. Interestingly, it is very easy to calculate the pumping speeds for both cryopumps and sorbtion pumps, but I have not seen much specifically on titanium sublimation pumping. I am working on a CAD model of the simple design, which I will post about, along with thermal modelling later. I would like to explore and implement a small one for my system to see how well it works.
In regards to the rest of the calculations, all of this work was originally and primarily done for myself for my own system. However, I figured that since I am doing all this work, others might be able to learn and benefit from the process as well, since a lot of people might not have access to a lot of the high vacuum engineering texts that I can obtain very easily, and I have found these texts have vastly more knowledge and insight than what I have found available online. I do try to tie this all back to fusor related operation as best I can, although the fusor is only one operating mode for my system, working at the highest pressures calculated.
It was motivated by two major driving points: 1.) since I will be spending a lot of time and money on this system, I had better make sure that it can do near everything that I want, with room for growth, and I wanted to estimate how reasonable my goals would be for this system, and 2.) how do I know that my system can support the process I aim to achieve? Can it actually support everything at high vacuum with reasonable gas flows? The answer to the second question is currently it can support all my goals except for one. I wanted to experiment with argon-fueled micro electric space propulsion engines at 10^-7 Torr. This was the most stringent limiting factor on my system, which helped push the optimization of speed for my system. Currently, as is it won't be able to support this project, but everything else for now can be run.
Another thing I was interested in, relating to the fusor side, is how much gas flow can my system support? Browsing through prior posts, it appears that the common sccm flow rate for deuterium for fusors operating in the 10^-2 Torr level use anywhere between a few sccm to a couple of tens of sccm of deuterium. How do I really know that my system can support this? I didn't want to spend all of this money and time to build a system to find out it couldn't do what I wanted. Based on all of the above math, I believe I have successfully shown that it is reasonable to support up to several tens of sccm of deuterium in the system at 10^-2 Torr. It also allows me to get a feel for how much gas flow I can support for deuterium beam systems in the high vacuum region, which is even more stringent and challenging. While the system can handle a lot of deuterium flow at low vacuum, it does not mean I will run the system at full flow - the manual gate valve will allow me to change conductance if needed, and I would put much less into the system anyway since it is very expensive. However, it is best to plan for extra room than not enough. Another unseen positive taken from these calculations is with the above calculations for max flow rate for air - if I have a leak that is preventing me from reaching some vacuum level, I at least roughly know how much load from air due to the leak is acting on the system, which could be very useful for troubleshooting and experimental run planning.
I think at this point, it apparent and demonstrated that anyone with a bit of motivation, determination, and some money can build a working fusor capable of fusion, without any rigorous design work or calculation. In comparison with other fusion capable devices, a fusor is incredibly simple, and it has been shown many times that a working system can be slapped together with moderate effort and scrounged components. I am in no way whatsoever downplaying the challenge and accomplishment of getting a running system both operational and proven, which still requires a lot of work and personal investment to achieve. However, there is a major difference between producing neutrons and maximizing neutron production efficiency, as many experienced fusor enthusiasts are well aware of by now. Especially with the new developments and push towards very small fusor systems, we may have reached the point where in order to make that next leap in improvement, more rigorous planning and engineering needs to be taken to maximize its potential, and to better understand the underlying principles of the device. For example, as you mentioned, water vapor is definitely a major bane for fusor operation, and certainly would effect efficiency. Yet a lot of systems presented generally do not appear to go through enough conditioning to fully drive off or reduce the water vapor loads. While running a plasma would certainly help drive off water vapor, and easily burn it out of the grid, for such short runs that the general fusor enthusiast operates at, there is not enough thermal energy acting on the entirety of the system surfaces or long enough pumpdown times at high enough vacuum to effectively drive out all of the water vapor of the whole system. Plasma cleaning definitely helps in bombarding the immediate surfaces, but the process still takes time. Even for systems that are baked to several hundreds of degrees C and pumped at high vacuum levels, it still takes many, many hours of continuous pumping and baking to effectively eliminate or reduce water vapor in the system. Once the system is admitted back to atmosphere, new layers of water vapor will immediately start adsorbing on the surface. Back-filling the chamber with nitrogen or other inert gas would help reduce re-conditioning times.
As you also mentioned, it would be very interesting to start exploring more experiments involving the relationships of water vapor and other contaminants in the system and fusion efficiency. Based on the work I have done so far for my system for example, it should be do-able to estimate for example the total number of water molecules present in the system, and work on minimizing this number. Maybe there is a certain point where decreased water vapor has no more effect on fusion yields. I do not know these answers. And recent developments in small fusor operation does show a higher efficiency for small devices over large devices, operating at higher pressures. Therefore it may be beneficial to see exactly how far a system can be pushed and how much deuterium it can handle at a given operating vacuum level.
There is a lot of info presented here so far, and a lot more that I will be posting and documenting. I apologize for the incredibly long posts, and applaud anyone who has the patience to trudge through them. Hopefully something useful can be gleaned from these efforts for all experience levels. I know when I first started I was eager to try and build this thing as fast as possible and start doing experiments immediately - however, I have come to thoroughly enjoy and savor the design process, slowing it down, and breaking it down to the most fundamental levels, then building on top of it. I do have almost all the parts I need now, and will start to actually assemble the system. I just have a one more post at this point for the main calculations, as well as some thermal modelling work. I hope this weekend to at least assemble the low vacuum roughing side and maybe qualify how well sealed it is and the ultimate vacuum of the roughing pump. I will post more about this as I get to it, as well as other developments such as the titanium sublimation pump build.