Small Scale Multi-Purpose High Vacuum Chamber

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Michael Bretti
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Small Scale Multi-Purpose High Vacuum Chamber

Post by Michael Bretti » Wed Nov 29, 2017 3:12 am

Several months ago I shared a post regarding some very large pieces of vacuum equipment I have had the opportunity to obtain. In addition to that very long-term project, I have been working on the design of a separate small system that I will be using to test a wide variety of systems, which I would like to integrate eventually into my larger chamber. These include beam on target systems, ion guns, plasma sources, electron guns, and a variety of thrusters. All of these parts will be integrated into 2.75" conflat blanks. Unlike my large chamber, this small chamber will be eventually used for neutron production, primarily beam on target, but could certainly run a traditional fusor, with ion-gun injection as well. Since the auxiliary systems would be the same for both the large chamber and the small chamber, including water cooling for the diffusion pump, rough pumping, pneumatics for the valves, low and high voltage power supplies, instrumentation, and controls, this small test stand will also serve as a way to set up and test these systems before I integrate them into a larger chamber.

This system is based off of 2.75" conflat equipment, with the main chamber made from a 5-way 2.75" conflat cross. Since I wanted this system to accommodate much more than just a fusor, I have designed it in a way to maximize inputs while keeping the design streamlined and expandable with plenty of capabilities to host the variety of systems I would like to test. Below are the various components I have collected for the design so far:

20171122_153033.jpg

These parts include: a 5-way 2.75" conflat cross, a 4-way 2.75" conflat cross, a 2.75" manual isolation butterfly valve, an inline 2.75" conflat pneumatic gate valve, a manual KF25 90 degree valve, and a KF25 to conflat 2.75" pneumatic 90 degree valve. These serve as the main components of the system. Other than the 5-way cross (which was still obtained for a fraction of the cost that it normally goes for), all of the parts where obtained from eBay for reasonably less than $100 each, and all of them appear to be in either new or exceptional condition.

Below is a CAD model of the completed high vacuum system including the diffusion pump and the adapter plate I will machine out of 1" aluminum plate stock. After going over various design iterations, I settled on this topology to maximize vacuum throughout to the main chamber, while allowing for isolation between the various vacuum stages, and allowing for numerous instrumentation and system inputs. The system will include ports for a thermocouple gauge, a pirani or ion gauge, a purge gas inlet, isolated roughing pump inlet, and an inlet for the diffusion pump with both inline gate valve isolation as well as throttling via isolation butterfly valve (if I where to run the system with deuterium for neutron production) and roughing side isolation. In the main 5 way cross chamber, I have the front facing port dedicated for a viewport, which leaves me three inputs for my systems. Various combinations can include: fusor with up to two ion gun injectors, beam on target system with target/faraday cup and additional port for movable target or inline beam instrumentation such as beam profiling instrumentation, small magnetron sputter source with target, electron gun with faraday cup and beam profile diagnostics, ion thruster with thrust plate, and other combinations of devices and diagnostics.

2.75 Conflat Based High Vacuum Test Stand.jpg

I still need to obtain the rest of the KF25 hardware, but all of the big ticket items are at least out of the way at the moment, and I am not in a rush to get this running. I will be constructing a mobile stand from 8020 that will house all of the auxiliary systems such as cooling, power, control, and the roughing pump. I am also interested in exploring the possibility of creating an automated control system for either semi- or full- automated pump down control, which would monitor vacuum as well as auxiliary systems, provide automated pump-down control, as well as emergency shutdown, in addition to normal automated shutdown of the system. Since I know what the inputs and outputs of the system needs to be, this should be relatively straightforward to set up utilizing an Arduino as a controller, which could be easily built and run under simulated testing before high vacuum is established. Though I would like to get the big chamber running for large scale experiments, this test stand will serve as my main high vacuum system for all of my future high vacuum projects.

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Andrew Robinson
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Re: Small Scale Multi-Purpose High Vacuum Chamber

Post by Andrew Robinson » Fri Dec 01, 2017 6:41 am

Kudos for using CAD. Wish more people on here would go this route initially. Would save them much time, money, and headache...
I can wire anything directly into anything! I'm the professor!

Michael Bretti
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Re: Small Scale Multi-Purpose High Vacuum Chamber

Post by Michael Bretti » Fri Dec 01, 2017 1:27 pm

Thanks! I feel that CAD, especially for something like this, is an amazingly powerful tool. It definitely has helped me better plan out my system designs, and I plan on CADing everything for my vacuum projects before I build anything. It has allowed me to both save money and optimize my design for my intended goals so far. The thing I also really like about this approach is that you can actually build your system ahead of time - since this hobby usually requires a lot of time and patience, waiting and saving for the right parts for those like myself trying to save as much money as possible, it is one way to start working on these systems while waiting for parts/money without spending anything. This goes for simulation in general too, such as electrostatics, magnetic fields, and thermal, though this may be a little tougher to get going on than CAD. Not to mention that almost all of the CAD models for most standard vacuum components are already done and freely available on sites like Kurt J. Lesker and Ideal Vacuum. I use Fusion360, which is definitely one of the most powerful piece of free engineering software I have come across yet, and strongly encourage anyone to use it as well for there endeavors. Fusion360 also has some very impressive thermal modelling capabilities, which I will also be using to model heat transfer from some of my systems such as Faraday cups, targets, and fusor grids, through the feedthroughs and to the chamber itself directly in my CAD models.

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Re: Small Scale Multi-Purpose High Vacuum Chamber

Post by Andrew Robinson » Wed Dec 20, 2017 6:02 am

I can wire anything directly into anything! I'm the professor!

Michael Bretti
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Re: Small Scale Multi-Purpose High Vacuum Chamber

Post by Michael Bretti » Sun Jan 07, 2018 11:31 pm

Over the past month or so, in addition to designing and optimizing my system, I have been diving intensively into the high vacuum engineering calculations for my system. After many hours of scouring numerous technical articles and several textbooks on the subject, I have started applying more rigorous design calculations to better estimate the operation of my system under numerous operating modes. From my initial calculations, I was quite surprised by some of the numbers I was getting, and it has forced me to re-evaluate my design. I have since been working on streamlining and optimizing the system due to these initial results, and have come up with the next iteration of my design for this chamber.

I have replaced the inline valve and butterfly valve combo with a manual gate valve that I found for free a few weeks back. This helps improve my overall pipeline conductance and effective speed, as well as gives me the functionality of isolation and throttling control in one valve. I have also eliminated all of the KF25 hardware on the high vacuum side, simplifying the design with all 2.75" CF hardware. This should help reduce the amount of area for gas permeation through less viton o-ring area, which is especially useful since I would like to ion-pump this to higher vacuum levels eventually. On the bottom four way tee I will be implementing two different vacuum gauges, most likely a nude ion gauge as well as a Infinicon BPG400 dual pirani/ion gauge unit (depending on locking in the prices for some units I have found.) The top 5 way cross, which is the main chamber and beamline, will stay the same, with two ports on the left and right side for systems and instrumentation, as well as a viewport for the front. The top port however will eventually be used for an ion pump, so that I can seal off the main beam line and keep it continuously pumped under high vacuum and for extended bake-outs without the need to constantly run my diffusion pump and roughing pump.

On the roughing side, I also greatly streamlined the design. I figure that since the main chamber and pipeline are literally the same size, it would be a waste of valves to implement a separate roughing line for such an effectively small overall volume. I have eliminated the extra roughing port, and will just pump the entire system directly through the diff pump inlet. I am keeping the roughing line as minimal and close to the diff pump inlet as possible as well, with a KF25 90 degree replaceable foreline trap directly connected to the roughing pump, which will attach to the diff pump via very short bellows section and through a KF25 tee into a manual isolation valve to the diff pump inlet. The tee will include a single thermocouple gauge so I can both check the roughing line vacuum isolated from the diff pump input as well as read the pressure at the inlet. The recommended CFM for my diff pump is 4CFM, however I am going with an 8CFM 2 stage backing pump for an extra bit of safety factor.

In regards to the calculations, so far I have completed the calculations for characterizing the pipeline conductance as well as effective pumping speed for a large range of operating parameters for the two systems for comparison. These are but the very beginning variables for figuring out the rest of the design numbers (such as pumpdown speed, gas sorbtion, gas loading, etc.) I am ultimately working towards figuring out the maximum gas input load for each of the operating conditions. Right now I have calculated the pipeline conductance and effective speed for both the molecular flow and transitional flow regimes, for air, water vapor, argon, and deuterium. This will effectively allow me to calculate the rest of the system parameters for just about all of my operating modes:

1.) Initial pumpdown from atmosphere (transitional flow with air after roughing)
2.) Water Vapor loading at low vacuum (transitional flow with water vapor)
3.) Standard fusor operation without deuterium (transitional flow with air as the initial benchmark)
4.) Standard fusor operation with deuterium (transitional flow with deuterium)
5.) Pumpdown to high vacuum (molecular flow with air as the benchmark, followed by molecular flow with water vapor as a more realistic scenario)
6.) Beam on Target Systems for neutrons (molecular flow with deuterium)
7.) Ion Beam systems with argon injection for non-neutron systems (molecular flow with argon)
8.) Optional system purging with argon (molecular and transitional flows with argon)

where molecular flow is in the range of vacuum lower than 10^-3 Torr (down to 10^-8 Torr for my current system ultimate goal), and transitional flow in the region of 10^-1 to to^-3 Torr, where I calculated the conductances and speeds at the value of 10^-2 torr. The molecular flow regime dictates experimental operation for beam on target systems, thrusters, electron beams, and other ion beam systems, where the transitional flow regime dictates operation of a standard fusor. Once I finalize my numbers, I will start posting these for reference. I should also be able to verify or disprove my calculations once I have a way of delivering gas and measuring the flow into the system. I can also provide PDFs with my calculations for those who are interested in the more rigorous analysis of high vacuum systems. Note that for the standard fusor builder this is grossly overkill, however, I am personally looking to fully understand the engineering behind high vacuum systems, and apply this knowledge to better understanding, designing, and predicting the operation of my system. While the numbers are still only estimates, they provide a very good indication of what to expect. Since I am also going to be using this for much more than just a fusor, I also want to know how to characterize it across a wide range of experimental setups.

Here is the new design iteration for this setup (now my third and hopefully final one). I still have many more hours of calculations to go, but this has been a great way to work on my system even though I don't have the parts yet. However, I have started putting orders in for the remaining parts, and will update the build status of the system as I go.

2.75in Conflat Multipurpose High Vacuum System V2 - Render 2.jpg

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Richard Hull
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Re: Small Scale Multi-Purpose High Vacuum Chamber

Post by Richard Hull » Mon Jan 08, 2018 3:03 pm

You are wise to spend a lot of time on the vacuum system. Once assembled it sounds like you will have a great system. All the best in getting it sealed and in good working order.

Richard Hull
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Retired now...Doing only what I want and not what I should...every day is a saturday.

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