cyclotron p-B11 fusion

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Chris Mullins
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cyclotron p-B11 fusion

Post by Chris Mullins » Sun Apr 29, 2018 5:37 pm

My son originally suggested a cyclotron as a final project before he left for college around a year ago, and as mentioned here viewtopic.php?f=46&t=11481&start=20#p79495, it's now working. The "stretch" goal, assuming we did get it going, was to use the cyclotron to produce and detect p-B11 fusion. The cyclotron design specs allow it to go a little past 160 keV to hit the first fusion resonance. We've got to 67 keV now with the magnet at half strength, and should have the magnet at full power in another few weeks (winding 2nd coil now). Further (much further) down the road, a boost to 10 kW would take it past the 650 keV resonance point. This is perhaps an effort along the lines of Jason Wells' with his VDG a while back, but with a cyclotron instead.

Although there's probably 6-8 months of refinement to the cyclotron itself before it's ready for any serious work (mostly improving repeatability and stability, beam quality, instrumentation, and cooling), I'm starting to think ahead about how to go about achieving and definitively detecting B11 fusion.

My initial thought is to use a very thin B11 target (B11 evaporated or sputtered onto some thin substrate), and a PIPS detector shielded with suitable foils to prevent false scattering detection. Acquiring (or making) these targets is yet another challenge .... This would be modeled after the setup in Low-Energy Cross Sections for 11B(p,3a)*, by H.W. Becker, C. Rofls, and H.P. Trautvetter (https://link.springer.com/article/10.1007%2FBF01284459), where low energy cyclotrons were used to map out these resonances. Essentially this stretch goal is to reproduce a few of the results in that paper.

The easiest way to vary the beam proton energy is to vary the target position in the chamber - the theoretical max energy depends on the radius from the center. I could vary the target (and PIPS detector?) position, and use the sharp spike in the 150 keV resonance to help demonstrate that fusion was occuring, along with the absolute PIPS measurements themselves. If the PIPS output is both correct, and changes sharpely as expected as the beam energy changes, that should be fairly definitive shouldn't it? Although the cross section of the 150 keV resonance is lower than the 650 one, it's much sharper, and that should make it easier in some ways to confirm. Acquiring the detectors and instrumentation, and getting repeatable, calibrated PIPS readings is yet another challenge ....

I'm not sure how practical it will be to physically move the target and PIPS detector on the fly in the chamber though, especially for my relatively poor machining skills. An alternative method for changing the proton energy is to vary the magnetic field. Unfortunately, that also changes the cyclotron RF resonance frequency, which requires retuning the matching coil. At the moment, I have to physically swap out coils to do that, and it's not very repeatable. Ultimately I plan to use a rolling inductor to allow for continuous rough RF matching (I already have an autotuner in place for fine tuning). Ideally I could vary the beam energy by both mechanisms, and see the same PIPS spike in output - that would be even more definitive I think.

Any other ideas on methods to achieve and confirm the detection of proton-B11 fusion in a cyclotron? This is a pretty tough, results-oriented crowd, so if it passes muster here it's probably working!
Last edited by Chris Mullins on Mon Apr 30, 2018 11:10 am, edited 1 time in total.

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Richard Hull
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Re: cyclotron p-B11 fusion

Post by Richard Hull » Sun Apr 29, 2018 6:52 pm

Detecting the alphas to a critical, competent audience will be the real bear in the room.

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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