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Joshua Turbyfill
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Greetings!

Post by Joshua Turbyfill » Fri Jun 23, 2017 2:06 am

I'm a high school student and as a summer project I'm constructing a high-powered fusor. I'm well in to the project, with about half of the system built. At the moment I'm setting up a He-3 detection system. I have to say the surplus of information that you have on this forum is invaluable, and has been a great resource for me. Its nice to meet you all.

Best Regards,
Joshua Turbyfill

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Dennis P Brown
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Re: Greetings!

Post by Dennis P Brown » Fri Jun 23, 2017 9:29 am

Welcome.

You might want to post on a He-3 detector system (your design) and read some FAQ's on detecting fusion in a fusor; (a He-3 detector will not work on a fusor, ) oops; LOL - misread the post! A He-3 detector is ideal. Sorry about the mis-read.

Posting on your current build and what you are going to build would be a good start.

Building a real neutron detector would be extremely useful since it can be used with a fusor and is required for proving fusion in a fusor.
Last edited by Dennis P Brown on Fri Jun 23, 2017 9:56 am, edited 3 times in total.

John Futter
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Re: Greetings!

Post by John Futter » Fri Jun 23, 2017 9:36 am

Dennis
You wrote
"You might want to post on a He-3 detector system (your design) and read some FAQ's on detecting fusion in a fusor; a He-3 detector will not work on a fusor, frankly (rate of fusion is many orders of magnitude too low for a He-3 signal"


Not quite right. A He3 detector will work beautifully on a fusor with a moderator
It is the most sensitive detector available to hobbyists albeit very expensive compared with other options such as BF3 and corona tubes ( russian).

And it doesn't crap out after about a year like bubble detectors - and yes the metrology has to be right to elliminate gammas and HV noise from clouding the results with false positives

Joshua Turbyfill
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Re: Greetings!

Post by Joshua Turbyfill » Fri Jun 23, 2017 2:34 pm

For the detection system I have a Russian СИ-19Н He-3 detector tube with a BNC splitter. Quoting the seller on eBay, "the splitter contains a 500K resistor for HV bias and 100pF capacitor for HV bias filtering". Its powered by a Fluke 423A 0-3kV power supply and connected to a PicoScope 2204A PC oscilloscope which in turn is connected to my laptop. Custom software, written by the same guy on eBay, utilized the PicoScope's SDK and outputs neutron counts, neutron spectrum, and a few other things that are apparently rather useful. As for a neutron moderator I'm planning on using paraffin wax, although I'm not sure how much so if you have any suggestions I'd greatly appreciate them.

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Dennis P Brown
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Re: Greetings!

Post by Dennis P Brown » Fri Jun 23, 2017 2:52 pm

As for the amount of paraffin, not too critical if one has at least 4 cm or more (exact amount not critical. One is just thermalizing the 3 MeV (or so) neutrons.) Its a good idea to surround the detector since neutrons scatter and tend to "randomly walk" from all directions when a modulator is present. Also, shielding for x-rays is a good; these tend to be line of sight. If you chamber has fairly thick steel walls (a few millimeters) and the power supply energy is 30 kV or so, or less, that should be sufficient shielding. For mine, I did use a 2.5 mm lead shield to be on the safe side to prevent issues but again, not critical.

Joshua Turbyfill
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Re: Greetings!

Post by Joshua Turbyfill » Fri Jun 23, 2017 4:47 pm

Thats great, thanks for the info. Now the chamber is pretty thick, but the power supply is way over 30kV.
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MDC 8" chamber during first stages of assembly
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120kV X-ray transformer as it arrived
The transformer is rated at 120kV, but apparently X-ray transformers output several times their rated output when not under load. I was going to operate the fusor in my workshop but at that input voltage I'm assuming I'll need to be a fair ways behind some sort of shielding, like lead as you suggested, or maybe even cadmium and concrete.

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Dennis P Brown
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Re: Greetings!

Post by Dennis P Brown » Fri Jun 23, 2017 5:49 pm

You will need serious radiation shielding if you operate in the highest ranges of that transformer. Also, do not leave any area that could expose someone to a dangerous level of x-rays without proper shielding. While current does matter (will control total flux, not x-ray energy), generally, if you operate above the 25 kV range one starts to get into issues where a steel shell will start to become transparent to the x-rays.

Since you don't need to go much above 60 kV since the very high range buys little extra performance, consider limiting the x-former to that max (you can, with experience, raise that when you are ready; remember, good fusion levels occur in the 30/35 kV range and even the 20 - 25 kV range with good detector systems.) Until you can handle high voltages, consider experimenting at far lower voltages but in no case, exceed the rated voltage of your cables!

Current (power) for fusion is important, too and transformer's provide less power the higher in voltage one goes. Also, voltage insulation becomes difficult above 25 kV and above 60 kV exposed points can act in dangerous (read unpredictable) ways. Few ever try voltages in that top range (over 70 kV) and those that do are very experienced with high voltage.

You need to really understand high voltage systems and related safety - issues such as proper (fail safe) chamber grounding, properly insulated cables (rated for the max voltage/power), and feed-thru's (very power dependent) are all critical issues that get very difficult the higher in voltage one goes. Corona issues are significant above 30/40 kV and especially severe above 60 kV. The list goes on. If that X-ray x-former is rated to 120 kV that is a supply that requires extreme caution in use - one mistake is death and could kill others trying to help the person. Posting High Voltage signs (warnings with the max rated voltage) are important; as is having proper radiation signs. You are responsible for not exposing innocent parties so how you shield around the chamber will depend on various parameters - keep that in mind, too.

An aside: one can buy any advanced equipment but experience is earned; read the FAQ's and read web based information and ask questions here before exciting a x-former and taking needless risks - fusors are deadly on many levels and for many reasons.

Looks like you are well on your way; so, keep at it and ask questions after reading up on the desired topic.

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Re: Greetings!

Post by Richard Hull » Sat Jun 24, 2017 6:30 am

Obviously, you will have to variac that transformer to hold it back to the beginner's range of voltages, ~20kv, and work up with operational experience. I can't tell the size from your image. I hope it can deliver a continuous current of about 15-20ma at high voltage. Once you have a fully functional 3He system check your background count. Let us know what you get in the way of a background in counts per minute.

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.

Joshua Turbyfill
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Re: Greetings!

Post by Joshua Turbyfill » Tue Jun 27, 2017 1:17 am

Thank you for the information. As per your suggestions I will start low and work up to 60kV. If very little performance is gained above that then I don't find it worth the radiation and HV issues that will arise. I plan on remotely operating it from 20m away, so I doubt that shielding should be an issue considering the inverse square law (correct me if I'm wrong). I'll also make sure to give you the background readings once I get everything setup.

The power supply setup includes a variac as well as a rectifier diode and a metal oxide varistor. It was explained to me that the diode in front of the transformer will make it output negative pulsed DC, and that the varistor will protect the input somehow. Will this regulate the current to keep it from sucking into too much current? Also, the transformer is rated at 120kV @ 15mA, so it should fit within the FAQ's suggestions of a 60kV @ 30mA power supply.

I'm a bit confused on how to setup the transformer. There are two wires with screw connections on them, which I assume to be AC inputs. Theres a naked wire on one end, which is apparently for a mA meter. Then on one side there are two pins and one pad. If I'm inputting DC current because of the rectifier, which input wire do I use? As for the output, the seller said to use just one of the pins as the HV output.
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The seller also said something which I don't entirely understand: "You will want a three grid farnsworth fusor system to use [the transformer] as it is. The outer one breaks down and ionized the gas with +60kV HV on the outer electrode, the center is the common earth and ground point, and the third is -60kV which accelerates nuclei to the center. This method works great since the hard work of ionization and electron stripping is being done by the two outer electrodes while the inner one does the actual confeinement and fusion." If the fusor outputs -60kV, where would I get the +60kV from? In the FAQ's it says that the positive lead of the power supply should be grounded.

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Re: Greetings!

Post by MatthewL » Tue Jun 27, 2017 4:10 am

Hi Joshua,

You are correct that the two wires with the bolts on them in the first photo are your input wires, and the bare wire you say is for your mA meter is the center tap of the secondary winding and should be grounded along with you outer grid. The two pins are your secondary (AC) outputs. I would recommend you use an ohmmeter and familiarize yourself with what wires are connected to what. Do you plan on inputing a direct current or were you talking about your output? I know that there are posts in the FAQs that give greater detail as to an X-ray transformer power supply setup and the wiring within the transformer.
As for the three grid fusor I haven't heard anything about that elsewhere, but if you construct your power supply correctly it will work fine for the classic two grid design.

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