Differences between a 4.5 and a 2.75 CF cross

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Nathan Hattrup
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Differences between a 4.5 and a 2.75 CF cross

Post by Nathan Hattrup »

I'm planning on building a fusor out a 6 way CF cross but am still deciding on the size. In other posts and in Images du Jour I have seen references and images of 2.75 and 6 in. CF crosses. However, in another New User Chat Area post, Richard Hull recommended a 4.5 in. CF cross. I was wondering what the practical differences between the sizes are.

I know that the 2.75 in. is cheaper, however I have heard of it having a voltage cap at around 38kv. I was never planning on supplying more than 40kv, but I am concerned about arcing. I think Jim Kovalchick abandoned his 2.75 CF cross because of this. Although I also imagine because of its smaller volume the vacuuming process takes less time.

My current plan is to decide on the cross size after I acquire my vacuum pumps as this was recommended in some of the FAQs. I have a slight disposition to a 4.5 CF cross because while it is a bit more expensive, it seems to be safer and less limiting for future growth.

Any and all comments and advice would be much appreciated!
Nathan Hattrup
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Mark Rowley
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Re: Differences between a 4.5 and a 2.75 CF cross

Post by Mark Rowley »

Nathan,
Dig deeper into the archives. You'll see that many other folks who ran the 2.75" system didn't have arcing issues. It all depends on how you design it. I will say, however, that a larger system will provide a more stable operation.

An important consideration when going big is the power supply. If you start getting beyond 4" (definitely 5 or 6), you'll need a VERY beefy (and costly) supply capable of 10-20mA throughout it's voltage range.

Sufficient power supplies for the larger fusor builds are the #1 limiting factor. If you compensate by getting a small psu, then you'll be limited to the same issue you were trying to avoid by going with a big chamber.

Mark Rowley
Last edited by Mark Rowley on Thu Apr 15, 2021 2:58 pm, edited 1 time in total.
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Liam David
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Re: Differences between a 4.5 and a 2.75 CF cross

Post by Liam David »

I second what Mark has said.

My main chamber is a 2.75" cross and I've pushed the voltage as high as ~55-60kV (and 2e6n/s) without arcing issues (viewtopic.php?f=18&t=13542). Smaller chambers are more sensitive to misalignment and sharp corners can cause issues, but with some attention to detail and cleanliness it can be done. As standardized conflat fittings go, 4.5" might be a good balance between "easy stability" and power supply requirements, but a 2.75" cross will certainly do.
Nathan Hattrup
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Re: Differences between a 4.5 and a 2.75 CF cross

Post by Nathan Hattrup »

Ok thanks for the help. I think I'll first try and see what kind of power supply I can get my hands on before making a decision.

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Richard Hull
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Re: Differences between a 4.5 and a 2.75 CF cross

Post by Richard Hull »

All of this goes to ....

1. What do you want in a fusor? Are you really serious about fusion and want to do it to a specific final research usage or long term experimentation into the future? 95% of folks are not serious, not planning anything and will, if successful, be gone in under 6 months after doing fusion. Two years ago, the figure would have been 99.5% would be gone the day after they do fusion and are ensconced in the Neutron club. This.. IS .. the number one question in all efforts to do fusion. If answered honestly it will affect all the following number of issues.

2. Assuming you are serious and have a post-fusion goal. You will need to ask yourself what amount of fusion do you need to reach your goal. This is usually expressed in Total Isotropic Emission of Radiation, TIER, in neutrons per second. This will impact your design decisions down the road. You will be spending money like crazy and you want to make wise decisions that will not come up short of the mark. Remember, you have a goal.

3. In designing your fusor, especially if it is your first, You should take a long time steeped in researching the successes and failures done here and that means a lot of reading, perhaps months of it. No matter your goal, you should overdesign and over think every aspect of the project. Even with this, as a newbie, you will fail with silly decisions made to save money or time all along the way. The fusor vessel or reactor is the easiest of all to make and do well. The nearly impossible items are a good power supply suitable to actually reach your goal within your fusor. Also difficult is the electronics detection gear needed to prove you are really doing fusion. The idea of a sphere, a cylinder, or a cross for the chamber is trivial for all types and all sizes have been shown to do fusion. (When worked by the right and learned hands)

4. Once the fusor system is complete there is the steep learning curve attendant in learning how to run the fusor to advantage. That means nursing it to its full potential of its original design parameters. The fusor is not a precise fusion engine. Many have a major issue with obtaining and operating an adequate, successful vacuum system. The fusor is cantankerous and resistant to doing fusion in the hands of first timers.

The above is a simple overview for the newbie who dares to think himself serious about doing fusion. If you build small your, skill set will be tested with little wiggle room available to success. If you take the middle of the road, you will be safe against total failure unless you do not have the verve to succeed or pack the gear needed to make it to fusion. Read all the FAQs and look at the many images of working fusors and especially the support gear.

Good luck.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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