Well, as mentioned in the previous post that was restarted (thanks - I think - for reminding me of this type of project!), I have started looking at my old electro-static deuterium accelerator again (Third pic.) The unit is complete but needs an electric field to accelerate and guide the deuterium ions. The extra large Van da Graaff I built I have never bothered to test since at the time, I happened to obtain a fusor grade transformer that could do fusion (alas, a unit I will have to return in the near future ...sigh. Just so grateful they lent it to me!)
Well, here is a pic of the new voltage multiplier (First, second, forth & fifth pics.) The long vertical "rod" (see fifth pic) is two high voltage resistors - one 5G-ohm and a single 1 M-ohm. The first two pics show more details of the stack - since the two cap types have different thicknesses I had to use spacers to help keep the units fairly inline - not the easiest solution but works. This entire system will be under oil (still finishing that new tank.) The resistor stack will serve two purposes: 1) as a bleed resistor to discharge the system when I turn off power 2) As a voltage divider so I can measure the actual voltage (if it does go over 100 kV.) That vertical stack will also have a plastic pipe cover as well as be in oil.
In an air test using a NST (7.5 kV, a few ma at best) driver I did get the output of the VM to read 50 kV (direct measure using my HV probe with a built in meter.) The variac was at 50% and since this was in air I hope when in oil it will do closer to 120kV (my red line for accelerator use.)
A few notes: I use two sets of door knob caps because these things, even surplus, are not cheap - I got a deal on each set so I use both types; each is rated for the same voltage - 20 kV; just an issue of different capacitance values. The diodes can handle 30 kV except for the first one - that is one of my chinese made 20 kV 2 amp models. I was short since I never thought I would have to have this many stages to get + 120 kV. The oil I use is synthetic motor oil - does a great job and is cheap. Removing and allowing the original stack to drip for two hours made the stack fairly dry and no issue to handle. Oil immersion isn't the hassle I thought it might be. Besides adding two more multiplier units (4 caps) I changed out the hardware from plastic to metal - decided the plastic was more trouble than I got from reduced corona issues.
The accelerator design is simple: my turbo and vacuum pump system can reach the low 10^-6 torr; the large SS globe contains a battery operated ionization unit (25 kV) to convert the deuterium gas into ions. A small deuterium tank (see image) feeds a needle valve and allows a very small amount of deuterium gas to bleed into the high voltage needle to be ionized and accelerated. I have electro-static lens along the tube length (in hind-site, way too long a tube and only two lens should have been made - live and learn.) The end target will be either a duetrated plastic or a Ti foil with deuterium. The target is water cooled. That target is electrically isolated so I can read the beam current directly. Also, the target has a reverse field coil near it to stop electron back flow (nasty source of x-rays from the unit.) I do have shielding but it isn't in place.
The VM will be used to provide a +120 kV charge on the globe (like charges repeal.) With the correct deuterium flow, this will produce a deuteron stream hitting the target. This can produce a decent neutron flux (like a fusor including 4pi Steradians directional flux) with a deuterated target or I can activate some types of materials - like boron ... . Assuming all works as planned. This project, compared to a fusor, was a significantly more difficult build - providing the high voltage using a VDG is NEVER worth ones time. Hopefully, I can get the VM to work at the full voltage; take note: VM's do not easily get to these high voltages - these are projects, too (my first attempt using slightly smaller caps never worked for reasons I think were driver/Freq. related but again, these are never simple projects despite what one reads by posters - lol; learning curves can be steep.)
Using a high frequency driver for a VM is, of course, the best method. My 60 Hz (really half wave) is an easy build but a poor way to get HV - door knobs are ideal for very high frequency driver circuits so (maybe not so much my one large diode), I really should have bit the bullet and constructed one; water under the gate now. Keep these issues in mind if anyone decides this is a worth while project to pursue - it isn't just get the high vacuum and the rest is easy (but that is another story.)
For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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- Dennis P Brown
- Posts: 1202
- Joined: Sun May 20, 2012 2:46 pm
- Real name: Dennis P Brown
- Location: Glen Arm, MD