Beam on Target Project (BOT 2021)

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
John Futter
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Re: Beam on Target Project (BOT 2021)

Post by John Futter » Sat Jan 02, 2021 5:59 pm

Mark Well done.
You will find that you get a much higher ion current if you put a repeller electrode in the end of your glass tube 1500 - 2500 volts positive with respect to your cube potential.

during lockdown i made some microwave components for an experimental microwave furnace @ work. This is how a professional goes about getting microwaves to do what is required
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Re: Beam on Target Project (BOT 2021)

Post by ian_krase » Sat Jan 02, 2021 6:33 pm

This is fascinating work. Makes me ashamed for mostly sitting on my behind as far as the science goes these last several months.

I wish I was the equal of that machining for the ion gun -- do you have a cross section drawing of what you actually built?


I have only ever seen one other beam on target machine (Thomas Rapp's build) so this is very interesting. I've dreamed of one myself but doubt I will ever be working with the requisite voltage.


I'm curious -- why microwave rather than ICP?

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Mark Rowley
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley » Sat Jan 02, 2021 8:33 pm

Thanks John. Wow, that’s the “neutron star” of dense workspaces! The sign on the wall is also quite appropriate. But I sure do like the waveguide arrangement and am slowly collecting pieces to get something similar.

Hi Ian
I’m pretty old school on my project designs. At best I have tattered pencil and paper sketches with rough measurements scribbled about. Mostly everything is in my head as I machine the parts.

This is Carl Willis’ drawing but it roughly depicts the extractor geometry. The big difference between mine is that I use quartz insulators instead of the Ceramaseal insert and no CF fittings.
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Mark Rowley

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Re: Beam on Target Project (BOT 2021)

Post by ian_krase » Sat Jan 02, 2021 10:43 pm

Ahhh, it's built up out of tubing pieces and machined plugs? That could simplify things...

I'm still a bit dejected over the failure of my own Penning ion source attempt -- Andrew Setzman I am not.

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Mark Rowley
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley » Sun Jan 03, 2021 11:10 pm

“Plugs” Ian? I prefer the term “electrodes” lol!

Quick update
Extractor seems to work fine however the Pyrex tubing isn’t cooperating with the higher excitation power needed to keep the plasma alive below 10mTorr. Or, I could be that I haven’t learned to properly throttle the differential pumping arrangement yet. Either way, a decade ago both Carl and Doug made it clear Quartz is preferred over Pyrex due to arcing / heating issues. Tomorrow I’ll be ordering a couple sticks of 19mm quartz tubing. In the meantime it looks like a cheap diode met it’s maker on the HV psu. Easy fix but typical nonsense from the project gremlins.

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Richard Hull
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Re: Beam on Target Project (BOT 2021)

Post by Richard Hull » Sun Jan 03, 2021 11:21 pm

Frustration and repairs are the norm when breaking new ground. We are taking notes due to your reporting your successes and failures, which is a good thing for us all. The pioneers always wind up taking the arrows in their backs. Those who follow owe them a great deal for blazing the trail.

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

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Mark Rowley
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley » Mon Jan 04, 2021 4:11 am

Oddly enough the power supply was good. Ended up being a faulty piece of insulation within the HV probe feed line. Being an easy fix I decided to press forward with a deuterium run.

Interestingly, getting a beam with deuterium was quite easy. Not so with regular air. Neutrons became sparsely evident around -15kV and at -27kV it was hitting around 5kcpm. During this short run the beam was unstable as I was trying to keep the microwave exciter at a level where it wouldn’t interact with the Pyrex. Doing so caused a bunch of flickering and very troublesome electron backstreaming. I’ve yet to install the faraday cup as it blocks the viewport. I’d like to get a few pictures before putting it in place.

As the run progressed, I began ramping up the target potential to -30kV. Surprisingly the titanium disc popped out of its holder and fell by the wayside. Almost assuredly it was caused by heat expansion and a set screw arrangement with no indentation (that’ll be fixed).

Instrumentation is extremely crude but will eventually get better. Extraction was at -3.5kV and beam current was well below 1mA.

Once the quartz tubing is in place and some X-ray shielding is arranged I’ll begin some activation experiments.

Neutron producing deuteron beam
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Crowded BoT system with HDPE neutron moderator
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Most likely I’m going to replace the lab jack with a simple wooden shelf. Doing so will remove obstruction from the viewport and allow for much easier X-ray shielding.

Mark Rowley

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Richard Hull
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Re: Beam on Target Project (BOT 2021)

Post by Richard Hull » Mon Jan 04, 2021 7:06 pm

Great work, Mark. Once you get this rig really cooking it will be quite a coup, especially if it rivals or nearly equals the your fusor using velocity space volumetric fusion.

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

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Mark Rowley
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley » Mon Jan 04, 2021 9:21 pm

Thanks Richard. That would be nice. I’m hoping for a platform that’s a bit more stable than the fusor during sustained high output runs. I’ve got some ideas for an easy to machine water cooled target. If this current iteration pans out and cooling is ultimately required, I’ll probably go in that direction.

This morning I removed the dislodged Ti target. Very little ablation but then again yesterday’s run was only a few minutes and quite intermittent. It’ll be interesting to see how it holds up under sustained / higher powered runs.

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Richard Hull
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Re: Beam on Target Project (BOT 2021)

Post by Richard Hull » Tue Jan 05, 2021 7:00 am

Just curious. Is that a solid machined target or a standard 1 or 2-inch Al sample pan that is ridged? I assume it is solid of you own manufacture, but a pan is fairly flimsy and might move about if charged and not held down.

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

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