Beam on Target Project (BOT 2021)

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

Post by Mark Rowley »

Being such an odd and potentially hazardous little device, the safety issues (if any) will be the first thing I’ll report on.

Progress wise, I finished the coupler this evening and am now moving onto the target / electron suppression assembly.

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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Spent about 4hrs last night trying to get the MW exciter to work. Nothing but arcs, hissing, and a very hot maggie. I know the Q is astronomically high so finding the sweet spot between all three adjustments might be the issue. The problem with this is reflected power. It’s so intense that one can’t just switch on the system and tweak the controls for a few minutes until the plasma ignites. Doing so would spell certain death to the magnetron.

I did get brief plasma ignition one time following an occurrence of arcing. Lasted for about 2 seconds until arcing occurred again.

The two places it’s arcing:
1) at the point where the hardline connects to the magnetron antenna.

2) between the tips of hardline and the tuning stub (if they’re close enough).

I fixed #1 by abandoning the machined copper conical antenna cap and instead directly attaching the 1/8” brass hardline directly to the mag antenna.

#2 only arcs when the tuning stub gets within 3mm of the hardline tip. And that’s only with a non-polished stub. The polished stub is a much smaller diameter and does not arc, however, it’s reduced size may is about 30% less than what the design calls for.

So today I’ll try a few more things. Larger polished tuning stub. I’ll separate the filament and HV between two xfmrs so I can throttle the hv. And lastly I’ll try to pre-ignite plasma so the mag can have an easier time latching on to a load.

I was able to land a W284 waveguide on eBay so if this utterly fails I can resort to that. But I won’t until I throw the kitchen sink at this one.

Last two points...
Negligible microwave leakage. Nothing detected beyond a foot from the device. And what is detected at closer ranges isn’t in the danger zone.

Pyrex or quartz discharge tubes have no effect on the above conditions. It’s 100% a reflected energy problem.

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Re: Beam on Target Project (BOT 2021)

Post by Joe Gayo »

Mark,

Have you considered hacking an existing microwave oven's waveguide setup?

I remember reading a BellJar article that discusses this and searched for it after reading your post - http://www.belljar.net/plasma.htm

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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

I'm familiar with the belljar article. After parting out a multitude of ovens over the years I've yet to see one with a waveguide as shown in Fig 2 and 4. In reality they only have a very small launcher ( a couple inches) which is incorporated within the oven cavity. Fig 3 could possibly work but I have yet to consider it. And lastly, due to being too bulky and kludgeworthy, Fig 5 is just plain unreasonable for this application.

It's highly likely the W284 will be the answer:

viewtopic.php?f=12&t=5047#p32355

https://www.youtube.com/watch?v=Zbsnf7XaFKs

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

Post by Richard Hull »

Carl's three slug tuner sounds like a great idea. One slug might be too critical of adjustment. But what the heck do I know.

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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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Success!
Typical, just as I was close to shifting gears towards the W284, it worked.

A couple things resulted in it working:

1) As Carl pointed out, running two transformers (one for the filament and one for the HV) is apparently the standard. Being able to throttle back the 600w magnetron was key to striking the plasma. Switching it on at full output caused debilitating feed line issues resulting in no coupling to the plasma. Lesson learned... a variac is the way to go.

2) The original tuning stub was machined at 4mm diameter. This was apparently too small so by increasing it to 6mm it widened the tuning range to a point where it would properly interact.

Specific to safety, the first 50% of the power range results in no detectable microwave leakage. However, going higher it begins to become problematic. Using brass screen totally fixed the issue. Once I get the screen shield cleaned up a bit I’ll post a pic.

One other quick point... the discharge tube is Pyrex. After 2 minutes of run time at about 60% power it only got warm. Eventually I have full intentions of using quartz, I just dont have any at the moment.


Plasma!
3362C766-3229-44AE-B525-F337DF628965.jpeg


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Re: Beam on Target Project (BOT 2021)

Post by Bob Reite »

Might be late to the discussion, it looks like you got it working, but I'd put in a circulator. That way, the magnetron would always see a good load, any mismatch would go to a reject load.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Beam on Target Project (BOT 2021)

Post by Rich Feldman »

Nice work, Mark!
As with motor-powered cutting tools and x-ray generators, the hazards of microwave power can be mitigated with knowledge, care, and instrumentation.


Re: "Bell Jar" figures 2 and 4.
By peculiar coincidence, the last two MWO's I broke down for recycling had sections of conventional rectangular waveguide between magnetron probe and cooker box.
(Conventional dimensions, but made from stamped sheet metal.) I knew that was common in 1980's units.
These are much more recent, I think both configured for "above the range" installation.
wave2.jpg
wave3.jpg
Oh, and there was one which had no turntable; waveguide ran under the cooker box & fed in through a dielectric floor plate.
All models are wrong; some models are useful. -- George Box
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Mark Rowley
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Thanks Rich. I’ll keep looking for one of those ovens. I cringe at the thought of drilling holes in my newly obtained WR284.

Bob, a circulator sounds interesting. Could one be built without much machining?

Finished off the evening with a deuterium test. I was curious if there was any major difference with coupling which there didn’t seem to be.

Here’s a pic of the current mess with brass screening.
4CB8CEF2-C97F-4DD8-8B8B-4A5A5F93AAA6.jpeg


Brilliant Deuterium Plasma
BC15F552-141C-4A30-87B1-0B6E56DF806A.jpeg

All these shots were at 50% power input. No detectable microwave leakage.


And lastly, here’s a video of the system powering up with both air and deuterium.

https://youtu.be/-m--DU1LsYI


From this point forward it’s housecleaning before attempting beam extraction. Too much clutter and mess in the shop before I can safely move forward.

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

Post by Richard Hull »

Great save Mark! WE are looking forward to reports on the BOT fusion results.

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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Re: Beam on Target Project (BOT 2021)

Post by John Futter »

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
thumbnail_IMG_20200730_145045.jpg
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Re: Beam on Target Project (BOT 2021)

Post by ian_krase »

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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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

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.
8949087C-8826-413E-BC54-18301F367C01.jpeg
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Re: Beam on Target Project (BOT 2021)

Post by ian_krase »

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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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

“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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Re: Beam on Target Project (BOT 2021)

Post by Richard Hull »

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Beam on Target Project (BOT 2021)

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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
BA0BC60C-A7A4-4A98-B34D-6EF8A6E87577.jpeg
BA0BC60C-A7A4-4A98-B34D-6EF8A6E87577.jpeg (25.29 KiB) Viewed 27699 times

Crowded BoT system with HDPE neutron moderator
82562872-6212-424D-8591-0C0B01B74F01.jpeg

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.

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Re: Beam on Target Project (BOT 2021)

Post by Richard Hull »

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

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.

AE000D34-E706-4759-BC8C-AB89CF44EEC4.jpeg

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Re: Beam on Target Project (BOT 2021)

Post by Richard Hull »

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Solid machined aluminum target holder. Not flimsy at all. Here’s a good shot of it with the Ti target seated in place. Due to the angle of the pic, the set screw is not easily visible (on the right side).

9817BC73-56CC-4A09-A92D-C1F0F6735A10.jpeg

The Ti target is about the size and thickness of a quarter.
The set screw wasn’t centered and it appears to have seated just above the lower edge of the target. So when it heated up, the tension caused it to snap out.

Details on the target:
Titanium (Ti) 24.26mm Metal Disc... https://www.amazon.com/dp/B07V3YL9X8?re ... b_ap_share

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Re: Beam on Target Project (BOT 2021)

Post by Jon Rosenstiel »

Mark,

Very nice indeed. Is your Ti target pure Ti or the usual 6Al-4V alloy?

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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Thanks Jon
The seller claims 99.7% purity and no mention of any alloy content. His ratings are good so that's about all I can put trust in.

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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Just some eye candy shots today while waiting on the quartz tubing.

This was a very low power run while I was learning how to maximize the differential pumping arrangement. As you can hear in the video I was getting a small smattering of neutrons (louder beeps).

https://youtu.be/sWgvnNpuL7k

Through the viewport
48084695-977B-4EEC-91FB-687594CB9F98.jpeg


The camera was on the inside wall of some lead shielding. Note the X-ray interference.
EA6B6264-C020-4197-A231-9BAB8619CD91.jpeg

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Re: Beam on Target Project (BOT 2021)

Post by Mark Rowley »

Finished the ECR yoke and adjusted it to ~890 gauss at the center using a Hall effect magnetometer. It’s fairly stable within the 19mm diameter of the quartz tube and begins to change a bit as you get closer to the yoke.

This one part made a substantial difference at maintaining a plasma to 1mTorr and below. Even by opening the Varian HS-2 diff pump to full throttle I couldn’t extinguish it. This effect in concert with the crude capillary type differential pumping should make the ion source quite stable during sustained long term operation. Heating is almost non existent with the quartz and microwave leakage is zero when measuring past 6” from the exciter coupling.

Here’s the deuterium plasma with the TC gauge bottomed out.
D3FCAD19-97F5-4F1E-928A-2004D1B2996C.jpeg


Better shot of the ECR yoke with deuterium plasma at 50mTorr. This was taken before the yoke was properly dialed in.
42F75883-9453-495D-973A-CD0C849F768A.jpeg

Still lots to do. The magnetron supply is still in “oven” pulse mode but throttled back with a ballast resistor and a 0.2uF cap. The entire power supply will be re-arranged in a week or so to provide continuous DC. Just waiting for parts. Electron suppression is also on the immediate to-do list.

Gotta say, so far this project has been (and will continue to be) a phenomenal learning experience. I highly encourage anyone to proceed in this direction after the fusor. And what’s especially cool is the potential of applying this tech back to the fusor after ion source tech is learned. Lots of fun to be had.

Mark Rowley
Last edited by Mark Rowley on Sun Jan 17, 2021 4:29 am, edited 1 time in total.
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