Beam-target neutron generator project

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Carl Willis
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Beam-target neutron generator project

Post by Carl Willis » Mon Jun 20, 2011 1:43 am

A few days ago I completed a small beam-target neutron generator and obtained my first neutrons from the system. It's a synthesis of a few modest innovations in design and construction that may lay some groundwork for others planning this kind of project. The top photo below shows the apparatus.

Right now, neutrons are detected at up to about 5000 cpm on my RS-P4-0404-253 He-3 tube moderated and held at about 13 cm from the target when the acceleration voltage is -100kV and beam current about 40 microamperes (but see below for discussion). This measurement enables only the coarsest estimate of source rate, probably putting it somewhere in the range of 30,000 - 50,000 / s. All of my dosimetric instruments are loaned out to other people.

I'll let a photo gallery do most of the talking with regard to construction. As a general overview, I wanted to accommodate an existing ion source (http://carlwillis.wordpress.com/2009/02 ... on-source/), avoid the complications of added lenses, and suppress electron currents from the target. SimIon v. 8 was used to plan a scheme in which the ion source extraction potential (in concert with the main acceleration potential) doubles as an immersion lens element to deliver the beam onto a target of a predefined size (the head of a titanium screw). SimIon was also used to plan the electron suppression concept, in which the target is shielded by another electrode a few hundred volts more negative. The potential difference could be produced by beam current discharging through a resistor of suitable value (in my case, 10 MOhm), or could be derived with a Zener diode chain (upgrade in the works). Some SimIon graphics are second and third below, illustrating the lens effect on ions (blue) and both good and poor suppression of secondary electrons (red) depending on the potential of the suppression shell versus target.

Next in the photo lineup are various shots taken during construction of the oil-sealed two-conductor HV feedthrough. The feedthorugh lip is soft-silver-soldered to the chamber. Oil-vacuum seals are viton o-rings captured in 5/16x24" brass acorn nuts. The 100-kV silicone cable seal is a standard brass Wilson compression fitting soldered into the aluminum cover plate.

Next up, two photos showing the titanium target and surrounding 304SS suppression electrode fitted into the interior of the HV feedthrough. The titanium screw is easily replaced; it threads into an aluminum support stalk that is not visible. Screw and suppressor are insulated from each other using Teflon sheet.

Next, a range of photos from operation. Photos showing visible beam were made at high pressure (1e-3 torr or so) and low voltage (about 40 kV). Different suppression-bias resistors were being tried, and in one of these photos the electron beam is poorly suppressed and may be seen exiting the target along the ion beam axis in a sharp line. A background yellow glow in this same photo results from the beam impinging on the downstream face of the ion source extraction electrode insulator. The last beam photo is made at high vacuum (1e-5 torr) and full 100 kV acceleration potential. During the 10-second exposure, a flashover occurred and its point of light illuminates an otherwise dark scene. X-ray noise on the photo is also apparent.

Last up is a shot of the neutron detector at about 4000 cpm when the shot was taken.

More to come on this project. Known issues:

-Poor ion source performance, tied to issues with new L-match network in use on the RF excitation coil. Right now, the ion source can only manage a meager ~40 microamps of extraction. Hopefully I can get it back in the game. If not, I will be pushing forward with another ion source design that has been on my plate a while.

-A funky field-emission-related conditioning effect, in which the electron current surges, terminating in a vacuum arc and restoring previous low levels of electron current. This effect heals over time. I suspect it results from irregularities on the suppression electrode outer surfaces that eventually will erode away.

-Carl
Attachments
setup.JPG
beam_target_generator.JPG
HVPS.JPG
S_Good_suppression_1.png
S_Poor_suppression_1.png
feedthru.JPG
oil_reservoir.JPG
Wilson_seal.JPG
chamber.JPG
target.JPG
benchtop.JPG
ionsource.JPG
rf_hardware.JPG
swr_meter.JPG
discharge_poor_supp.JPG
discharge_good_supp.JPG
discharge_100kv_hivac.JPG
4kcpm.JPG
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

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Carl Willis
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Re: Beam-target neutron generator project

Post by Carl Willis » Mon Jun 20, 2011 7:09 am

Hi James,

My beam current right now is somewhere under 40 microamps and above about 20 microamps at 100 kV. This low level is difficult to measure on the PS metering because corona and voltage-divider current are a significant background, and as I mentioned, there is periodic vacuum arcing that is always preceded by an exponential rise in current (all of which seems to be electrons). The best way to measure the current is to reduce the target voltage to about 30 kV, which still allows the extracted beam to arrive at the target but doesn't result in as much background from the variable sources of low-level leakage that show up on the meter. When the extraction supply is turned on, beam current immediately adds to the leakage current on the acceleration supply meter. When the extraction supply is turned off, the beam current component disappears and only leakage remains. My ion source has accomplished more than 300 microamps in the past, so I'm hopeful that a few corrections to the RF generator will get me back in that range at least--which is much easier to measure.

My friend Tim Koeth made me a deuterated titanium target that I intend to try in this generator sometime soon. I'm not too worried about heat removal from the target. The target has a short copper and aluminum thermal path to the oil on the other side of the feedthrough.

For RF generators to use with ion sources, you might be on the look-out for diathermy machines (typical units made in 1930-1950 use a single 810 tube in a shunt-fed Hartley configuration, good for at least 100W at ~30 MHz), or "tube sealers" (if you peek behind the scenes at a blood drive, you'll likely see one of these in use; 50W at ~30 MHz is typical from the solid-state generator). These possibilities can be picked up on eBay for a lot less than lab-grade or ham-radio gear delivering equivalent wattage.

-Carl
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Richard Hull
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Re: Beam-target neutron generator project

Post by Richard Hull » Mon Jun 20, 2011 3:00 pm

Carl,

Fabulous project! I have long wished to do a beam-target setup with a palladium target as I have a couple of 1 oz ingots I could fashion as needed.

I was hoping to just make a fusor like environment with a gas pressure of 10 microns or so with no Ion gun, as such. I was looking at a bulk and wasteful ionizer of a heavy tungsten needle grounded to the positive chamber end with an isolated tunsten grid mesh very close by, powered by a positive 0-500 volt ~1amp variable supply. At the other end of a short 6'" glass stub with a 2.75 conflat, I would have Pd target on copper rod with and the entire conflat hooked to the negative 30-50kv lead. Sort of a bull-head arrangement just to see how operational loading of the PD might occur and neutron numbers might climb over time and operation.

At some point, I might imagine the Pd loading due to bombardment to equal the unloading or deuterium emission from the target due to same bombardment.

Long before I try the above, I hope to place a simple Pd bead or ball on a tungsten shaft, replacing my normal fusor IV grid. This might do far better than the proposed linear device above. We have already admitted to low or near worthless recirculation within the fusor itself and such a bead would not interfere with neutral-fast collision fusion.

Just thoughts.

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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Doug Coulter
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Re: Beam-target neutron generator project

Post by Doug Coulter » Mon Jun 20, 2011 4:45 pm

Good show Carl, you've beaten me to it this time! I got tied up in discussion about what and how to manipulate the ions on the way in so we weren't trying to get reactions in violation of CPT conservation or Pauli's exclusion principle in doing this, but that's more or less finished now so I have a hardware design that can support all that. (cheap compared to a fusor, BTW)

I am also in the process of building up a beam on target or beam on beam device, which I'll post a link to here once I finish building it and have some results - it won't be long till I have all the bolts tight on it. I'll be making my own Ti over silver targets in another vacuum system to keep the Ti layer thin, and thermal conductivity high. I believe you only want the Ti (or Zirconium, but Ti is better) layer about 1-2 microns thick.

The Phillips book/chapter I've linked here many times has a design for this and some pretty good talk about target design for beam on target, and I intend to start by duping their design and results, which are on the order of a million neuts/second per microamp (with T).

According to Phillips, you need to keep peak target temperature below 250c or you blow the H isotopes out of it.

Yet again, at the end of this chapter is a decent borehole tube design and construction, with real numbers. http://www.coultersmithing.com/data/Dat ... n/Pch8.pdf

If they could do this well in the late '50's, I think we all have a shot at it.
Here's the startup of my build: http://www.coultersmithing.com/forums/v ... f=24&t=387
Why guess when you can know? Measure!

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Re: Beam-target neutron generator project

Post by Larry Upjohn » Mon Jun 20, 2011 5:29 pm

Carl, You wrote:
Next in the photo lineup are various shots taken during construction of the oil-sealed two-conductor HV feedthrough. The feedthorugh lip is soft-silver-soldered to the chamber. Oil-vacuum seals are viton o-rings captured in 5/16x24" brass acorn nuts. The 100-kV silicone cable seal is a standard brass Wilson compression fitting soldered into the aluminum cover plate.....

What type of solder did you use on the brass to aluminum joint. I have a similar brass tube to aluminum joint to bond and your recommendations would be greatly appreciated. Great project and your inspiration urges me back to the bench to get on with my next chamber project. Thanks again for your help, Larry Upjohn
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Re: Beam-target neutron generator project

Post by George Schmermund » Mon Jun 20, 2011 6:07 pm

Nice effort, Carl. It's good to see progress in the beam on target arena. I'll use it as an inspiration for a couple of things I have going.
Anything obvious in high vacuum is probably wrong.

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Carl Willis
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Re: Beam-target neutron generator project

Post by Carl Willis » Mon Jun 20, 2011 8:51 pm

Larry, I use this stuff on aluminum: http://www.muggyweld.com/super1.html

It's expensive, but very easy to use--unlike a lot of other solders that claim to work on aluminum. On the likely assumption that this is a zinc alloy (official composition is proprietary), I would be cautious about using it in a vacuum application. The joint illustrated here is just oil-air. Good luck!

Richard, I made some palladium-plated strips of copper to be cut up and used as targets in this generator. It'll be a while before I try anything else in it (Pd or deuterated Ti), as I want to learn about its basic behavior with the ordinary titanium target at the moment.

-Carl
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Re: Beam-target neutron generator project

Post by Richard Hull » Mon Jun 20, 2011 9:11 pm

Carl,

Thanks for your noting you have the Pd stuff in the background awaiting further familiarization with what is going on in this new arena you are in.

By the way.....You think you gotta' nuff ferrite donuts on that signal line in the pics???
I assume it is a signal line, but I stuff 'em on the HV lines as well, but just one at each location. You musta' found a box of 'em at a hamfest or sumpthin'.

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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Carl Willis
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Re: Beam-target neutron generator project

Post by Carl Willis » Mon Jun 20, 2011 9:24 pm

Hi Richard,

I'm sure it's overkill. These cores are some kind of un-numbered low-mu VHF material that I've never used, and yeah, they came in a box of other junk from a hamfest. My philosophy was to put them all on there since I'd rather have too many than too few. The purpose is to choke off currents flowing on the exterior of the hardline, which radiate and mess up electronics like the turbopump controller and vacuum gauges.

I have a good mind to just sack the 200-MHz scheme and go with an oven magnetron. I'd rather do pulse modulation to control average power like ovens do, rather than linear amplification of CW signals as I do now. At higher vacuum, a high-power signal is needed to ignite the discharge, although average power can remain low.

-Carl
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Re: Beam-target neutron generator project

Post by Richard Hull » Tue Jun 21, 2011 3:10 pm

Tom Ligon used a microwave oven maggy ionizer on one of Bussard's original systems when EMC squared was located in Northern Virginia back in the late 90's. I got to see those early iterations back then when I would visit Tom.

You are right, the maggy is a bit easier to impliment, but you probably had the other HF stuff on hand to play with when you started your effort and used what was at hand.

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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