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Proton Accelerator

Posted: Thu Jan 15, 2015 3:11 am
by Jason C Wells
Upon achieving high vacuum reliably with my diff pump I've turned toward getting a nicely collimated beam for shooting protons at my boron target. What you here see is the Van De Graaff generator with the accelerator column penetrating the shell. The test is being performed at about .5 torr and 350 kV. The gas in the tube is ionized air. The high pressure was chosen to make the shape of the beam visible. I was looking for poor beam behavior before I go on to lower pressure with hydrogen gas. The "target" for lack of a better word was just a grounded copper wire located in the center of the accelerator tube. (not visible in this picture.)

No increase in background radiation is observed with this test. No nuclear phenomenon were expected with this test.

This is what it looks like with the lights on.
The VDG and column with the lights on.
This is what it looks like with the lights off. I don't have anything better than my smart phone to take pictures. This picture isn't quite as exciting as the purple glow given off in my darkened garage. The beam was very bright, straight, with no streamers running off to the side of the tube wall. I was very excited to see this result. Previous attempts resulted in all manner of bad behavior from the beam line.
The same photo with the lights off, showing the beam.
Another curious phenomenon occured. When I grounded the VDG terminal with my magic wand, the beam turned orangish and appeared to be tighter and more focused. I think the planar geometry of the end cap of the accelerator column is a factor. I plan to try a couple different configurations of the gas nozzle/end cap to see if I can get a tighter beam yet.

I have a neat video of this test run. The server won't let me post it because it's too big. I doubt I could make it small enough and still provide any useful visuals. If you really want to see it, I can send it to you off the boards.

Air was introduced via a vinyl tube running inside the Van De Graaff drive column. Some people cautioned me about loosing terminal voltage but the leakage doesn't seem to prevent a nice beam. I did take the advice to ground my gas supply tube very near to the VDG. Well... actually... I had to learn the hard way. I got bit (not too bad) by voltage on my brass needle valve even as far away from the VDG as 36 inches of tube length.

The plywood box serves as an environmental chamber. I have a dehumidifier routed to the box.

Re: Proton Accelerator

Posted: Thu Jan 15, 2015 8:20 am
by Richard Hull
Interesting effort. Question. Do you really beliieve you had 350kv applied from your VDG? I hope not, as it is highly unlikely you had even 20kv. A study of currents in the beam and VDG loading would be in order. Basically, if you have a glow your voltage is low.....Very low.

Richard Hull

Re: Proton Accelerator

Posted: Fri Jan 16, 2015 2:17 am
by Jason C Wells
Adding a beam current meter is on the todo list. The more correct statement about VDG voltage is that I can get a ten inch spark on a good day with zero load on the VDG terminal. I certainly do not have zero load with my the accelerator column in place, the gas tube leading to the VDG, and even occasional glowing along the belt which probably needs to be cleaned again. The terminal is nominally capable of 600kV.

I did ground the VDG terminal with my grounding wand during the test. The spark was on the order of an inch which supports the voltage number you gave. To wit: No. I do not believe I had 350kV.

Glow is a function of pressure as I understand things. My pressure here is pretty high. I guess I'll have to do my homework on the voltage aspect of gas discharge phenomenon. That's why I keep coming back. I learn something every time. I hadn't been worried about voltage as long as I had interesting phenomena to look at and manipulate. Getting a better idea of VDG performance under load is probably a good thing to do next.

The express purpose of this test was to observe good collimation. Of that I am somewhat satisfied. A thinner beam would be nice.


Re: Proton Accelerator

Posted: Fri Jan 16, 2015 9:07 am
by John Futter
well done so far
you will find as the pressure goes down the focus gets better
the ions naturally want to repel each other so the higher the pressure the more beam divergence
the higher the accel voltage the less this has time to take place but you have a limit on beam power so you will have to throttle your gas flow to allow the voltage to increase this effects both the effects I have mentioned above

keep working at it

Re: Proton Accelerator

Posted: Tue Jun 09, 2015 11:08 am
by Dennis P Brown
Adding Einzel lens would provide a far superior ion focusing system. By the way, very nice first step. However, do measure the current since that, and not whether you have 350 kV or 20 kV will really determine the VDG potential as a power source for driving a linear accelerator for gun use. Hooking up an analog meter from the dome to ground can be very useful in optimizing current output of a VDG. You can then study issues like what the rope or plywood box does to your current output (likely a large draw.)