Final Design and Molecular Flow Simulations of the Integrated High Vacuum Test Stand

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Michael Bretti
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Joined: Tue Aug 01, 2017 4:58 pm
Real name: Michael Bretti

Final Design and Molecular Flow Simulations of the Integrated High Vacuum Test Stand

Post by Michael Bretti » Tue Dec 11, 2018 4:29 am

New update regarding the recent work on my vacuum systems. If you have been following the progress up until this point, you will know that I have covered the design of my system ad-nauseam, with CAD, thermal simulations, vacuum engineering calculations, and walking through the entire process. The design for the Integrated High Vacuum Test Stand has been completed for a while now, but I have been waiting until after I finalize the pumping parameters with molecular flow simulations. Here is the link to the system project page:

http://appliedionsystems.com/portfolio/ ... est-stand/

The final CAD design of the test stand going forward includes the closed-loop cooling system, the roughing system, and the high vacuum diffusion pump assembly, mounted within the 80/20 test stand:

Integrated High Vacuum Test Stand Assembly.jpg

The full specifications and details are on the project page, so I won't review them here. However, I have recently completed the final portion of the vacuum engineering for my system, which includes molecular flow simulations of the pumping stack using Molflow+. With these results, along with all of the other hand calculations and thermal simulations, I have been able to fully design and qualify every aspect of the entire assembly before assembly, and validate that the design will work for my experiments.

The project page for the Molflow simulations can be found in the link below. The page details the model preparation process, results, discussion, and includes all of the simulation files for the system:

http://appliedionsystems.com/portfolio/ ... -assembly/

The simulations look at the pumping assembly blanked-off, for three different cases of vacuum preparation: unbaked and pumped for <1hr, unbaked and pumped for >24hrs, and baked and pumped for >24hrs. This is to compare system response and ultimate vacuum levels for the system just assembled, and ideal case vacuum for a well prepared and pumped system. The full details are on the project page above, so I will just briefly summarize some of the results here.

For the first results, average molecular trajectories can be seen for the system. Based on a pumping speed of 600 L/s for air from the diffusion pump, the resulting effective speed at the chamber inlet blank-off was found to be between 392 and 408 l/s, which shows that the effective pumping speed is retained between 65% and 68% of the speed of the diffusion pump, verifying optimized baffle design for maximum throughout for a baffle with an indirect line of sight, as well as optimal pipeline geometry for maximizing pumping speed at the inlet of attached chambers on the second adapter plate.

Integrated High Vacuum Pump Assembly - Molecular Trajectories.jpg

Reducing the number of particle trajectories viewed, we can better see the results:

Integrated High Vacuum Pump Assembly - Molecular Trajectories Reduced.jpg

As can be seen from the results, a large fraction of interactions occurs between the upper and lower baffle fins, which is expected due to the indirect pumping path set up by the baffle geometry. This exemplifies the mechanism of baffles in dealing with particle backstreaming. As a general note, a baffle still needs to be cooled to minimize backstreaming. As observed, particles will continue to bounce around on wall surfaces until they stick, backstream, or are pumped out. The oil vapor needs to condense when it comes in contact with the baffle surface so that it does not migrate upwards. However, the indirect line of site and large surface area of the fins helps increase the probability of interaction dramatically, while maintaining high throughput.

The main power of Molflow+ however is in determining ultimate pressure of the system. I will just provide examples for one of the three simulations mentioned - all three are compared on the simulation results page. For the first case, of an unbaked system pumped for an hour, the pressure profile distribution can be seen as follows:

Integrated High Vacuum Pump Assembly - Unbaked, Pumped 1hr - Normalized Pressure Final.jpg
Integrated High Vacuum Pump Assembly - Normalized Pressure Scale Final.jpg

By creating a transparent measurement facet through the system, we can generate a pressure profile plot. For the same simulation, the results are as follows:

Integrated High Vacuum Pump Assembly - Unbaked, Pumped 1hr - Pressure Facet Final.jpg
Integrated High Vacuum Pump Assembly - Normalized Pressure Facet Scale Final.jpg
Integrated High Vacuum Pump Assembly Final - Unbaked, Pumped 1hr - Pressure Profile Plot.jpg

For this case, pressure at the blank-off is around 2.6 x 10^-6 mbar. With further baking and long-term pumping, this value, as seen in the other two simulation cases, drops to the mid 10^-8 mbar region. However, in the simulation results, you will find that the difference for a long-term pumped system that has been unbaked vs. baked is very small. This is due to the ultimate pressure by the permeation through the viton o-rings used on the baffle and adapter plates. With a very short, wide, and direct pumping pipeline, pumping speed is maximized, allowing for low pressure despite outgassing loads from the viton.

This simulation was used to purely qualify the above pumping assembly, and was used as the basis for the simulations for my actual chambers. I have 3 main systems I am working on, and will create separate posts for the final design details and Molflow simulations for each of these. One of the reasons I have put so much engineering design effort into this stand is to be able to accommodate all of my experiments going forward, and have made the system highly modular so that each of the three chambers can be rapidly mounted as needed.

The first is my Small Scale Multipurpose High Vacuum System V5 design. This is based off of 2.75" conflat hardware for a compact and modular test platform, where I will be exploring miniaturized intense beams. I have posted extensively about this system in my previous walkthrough posts.

The second system is my Micro Propulsion Testing Chamber. This is based off of two 6" conflat tees. I have not shared details of this build yet here, but will be providing more details now that the design and pumping simulations are completed. All of the parts for this chamber have been acquired, and just needs to be assembled.

The third system is the biggest, and most exciting system I am working on, and is my big, long-term project. I have dropped hints of it here and there (though more information on social media), and will be sharing some small details about it for the first time here. This project is named EXEDA, and will be, as far as I am aware, the highest peak power and energy particle beam system ever built by an amateur effort. This system is completely designed and built from scratch, and self funded with no external help or affiliation to any university or research lab. The system will be in the power class of systems used at national research labs and industry. More importantly, it will be accomplished using surplus parts, home-built equipment, and on a budget at the levels anticipated for a quality fusor system. There are a wide range of experiments that are planned for EXEDA that until now have never been done at the hobbyist/maker level. In addition, EXEDA will be used to drive a modified topology of the system, dubbed EXEDA-MEVI, that will focus exclusively on very high peak energy beams.

The goal of these systems is not fusion research, but rather to explore the physics behind high intensity particle beams (as well as propulsion for small satellites in the case of the propulsion chamber). However, depending on the topology, neutrons can be generated, both as intended output, or as a biproduct. Neutron production will only be one possible and very small subset of experiments that can be accomplished with the system, where the primary focus will be on high-power physics. In addition, as a broader and further reaching goal of the EXEDA project, I hope to open the doors to true high power and high energy accelerators at the maker level that has currently not been achieved or explored before. Note that I am not proposing any new technology - through my efforts and research into this field, I have come across very old and niche technologies that can be built at the hobbyist level, and open up this realm of physics to the community. While I will not be releasing the specifics of the project publicly yet, the project will be fully open-source with all engineering details available, like all of my other projects. Currently I have most of the parts required, and just need to start prepping everything for assembly. I expect to have vacuum in February, and hope to start producing the first low-power beams for testing soon after if everything goes to plan. I am also currently working on a detailed historic literature and technological review of previous systems, experiments, and precedents that will be used in the design of EXEDA and EXEDA-MEVI. I expect to make the official project announcement with preliminary details after the design of the first iteration has been complete, and the pumping system is running, hopefully in early 2019.

Chris Mullins
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Real name: Chris Mullins
Location: Shenandoah Valley, VA
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Re: Final Design and Molecular Flow Simulations of the Integrated High Vacuum Test Stand

Post by Chris Mullins » Sun Dec 16, 2018 7:44 pm

Thanks for posting the Molflow files and detailed discussion on your website! I'm planning to model my vacuum system, to help decide among several options to improve my chamber vacuum. I looked into Molflow a while back, and it seemed very capable but with a significant learning curve.

Michael Bretti
Posts: 177
Joined: Tue Aug 01, 2017 4:58 pm
Real name: Michael Bretti

Re: Final Design and Molecular Flow Simulations of the Integrated High Vacuum Test Stand

Post by Michael Bretti » Mon Dec 17, 2018 12:26 am

Chris,

No problem, glad that the info is of help to you! I would say that for anyone looking to do serious vacuum engineering and analysis on their system, Molflow+ is really a must, and an incredibly powerful tool. This was really the final verification stage for my systems before I go on to building them - I went through and calculated everything by hand first to get a ballpark estimate of what would be reasonable to expect, as well as for obtaining the necessary outgassing parameters for my simulation inputs. It definitely has a learning curve to it, although this is much less than a lot of the other free physics simulation programs that don't have any user interface. The biggest challenge, like any simulation software, is learning to properly set up the simulation inputs and configuring the outputs. To load a Molflow simulation file (for example, like the ones I have posted on my website), you just open the entire .zip file in Molflow.

Molflow goes hand-in-hand with CAD (since you need a model to input into the program), as well as a more in-depth understanding of vacuum engineering (such as calculating outgassing loads for various materials and surfaces, as well as where to apply them). Since it is a free simulation program with a full user interface and support tutorials, and an incredibly powerful one at that, it should be very readily available and accessible for makers and hobbyists to take advantage of. I always encourage others to plan out their system in CAD first, and adding this tool to one's repertoire makes for a very powerful combination, though I would also suggest anyone looking to use this to first familiarize themselves with more in depth aspects of vacuum engineering before rushing to try simulating their system.

I have two more simulations that I will be sharing in the coming weeks for two out of three of my chambers, so you will get to see plenty more examples of Molflow applied to full chamber simulations. I will also be revisiting Molflow for dynamic modeling of vacuum levels based on loading from both beam sources for my beam systems, as well as loading due to thruster output in my propulsion chamber.

If you need any tips or help using Molflow, feel free to reach out to me. Good luck with your system optimization!

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