FAQ - fusion...the battle in our fusors, X-section vs. voltage

If you wonder how/why fusion works, or how/why the Fusor works, look here first.
Post Reply
User avatar
Richard Hull
Moderator
Posts: 13186
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

FAQ - fusion...the battle in our fusors, X-section vs. voltage

Post by Richard Hull » Thu Jun 24, 2021 6:51 pm

We need to look at all fusion possibilities in our fusors. Some of this has been covered to specific degrees before. Based on the Lawson criteria we may completely ignore containment! we are not going to do any power fusion as we work in a "velocity space" fusion scenario where loss is the order of the day related to confinement. In short we are not looking to contain our fusion... We just want to do it....to make it happen. This is easy, as we all know.

What is left of the Lawson criteria for us

Temperature

This is a direct function of voltage in our case. More voltage means more temperature for the colliding nuclei.

fuel type/cross section

The selection of our fuel will determine the fusor's fusion collisional cross section. Easy to fuse nuclei at any given voltage will mean more fusion.

Density (faux confinement in velocity space)

How many neutral fuel atoms are in the reactor per unit volume. Increased ionization opportunity with some possibility of more ion/neutral fusion. This is a peculiarity of fusor based fusion over all other fusion type scenarios.

Amount of nuclei/ions are in the velocity space

As we increase current at any density of neutrals at any given voltage, in any given fuel, we will create more high speed nuclei capable of fusion.

Now, as to fuel

The attached cross sectional chart below speaks volumes. The best fuel is deuterium-Tritium without question. As amateurs, we are not allowed to purchase, obtain or use tritium. D-T fusion is out!!

As amateurs, the voltages that we can reasonably and inexpensively be able to obtain and control in a smallish fusion device might max out at 150 kilovolts, Most will never be able to reach 60 kilovolts. If you can get 150kilovolts then two fuels pop up with the best being deuterium-helium3 at that voltage. While you can buy helium3, it is outrageously expensive in the many thousands of dollars for a very small amount.

We note that deuterium-deuterium, D-D, is an all around best choice due to costs. D-D also satisfies all the Lawson criteria above relative to required voltages and currents that the amateur can get ready access to. Thus all amateur fusion will forever be limited to D-D fusion efforts. Another positive is we only need purchase and be concerned with a single fuel gas. No mixing required and the gas is available to the amateur relatively inexpensively via tanked gas at high pressure or via electrolysis in a PEM reversible fuel cell at STP or slightly above.

Continuing to deal with the Lawson criteria above having knocked down the fuel and density issues, we come to the most important and most limiting issue in amateur fusion......Temperature, which is voltage.... Here comes the rub. D-D fusion while possible, is voltage limited. We see the curve in the chart below has a nice sloped rise. we must also note the ever decreasing rate of cross sectional rise or return on voltage increase. It is seen that another 50kv increase after 150 kv applied just doesn't win much more fusion for the exceedingly more dangerous and complex apparatus needed to go higher in voltage. In the end, the most gain in real neutron numbers via cross section for D-D fusion is seen between 35 and 120kv. After that you are pushing a stalled bus up a mountain road. virtually nothing is gained from 150kv to 1megavolt applied!

Serious and killer info. on D-D limits

There are but 3 order of magnitude increases in cross section for D-D fusion!
1. 6kv-13kv...hardly noticeable on a neutron counter.
2. 13kv-28kv...Very noticeable and where most in-it-to-win-it DIY folks make the neutron club and then disappear forever.
3. 28kv to 150kv....Where the real action is in D-D fusion. The activation zone. The amateur research and discovery zone. The zone of max D-D fusion at the amateur level.

Above 150kev is the end of all gained D-D fusion results. The dead zone of wasted energy per unit fusion result.

The last chance for increased fusion over 150kv applied

The final item is the number of nuclei present to do fusion. This is Current! - More current means more ions/nuclei ready to do fusion. Thus, there is even a limit here in that the internal components of the fusor suffer under power limitations. At any given voltage, more current not only means more nuclei, it means more energy is injected into the fusor which must be absorbed by the more delicate internals within the reactor vessel.

The simple fusor we build will forever be limited. This is not to say that novel construction alterations might not be advantageous, for they will be. It just means that the limits that are in place related to the physics of fusion will always be there, even in a more complex amateur device attempting to push the envelop.

Richard Hull
Attachments
crossSections.jpg
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.

User avatar
Joe Gayo
Posts: 308
Joined: Mon Jan 07, 2019 2:34 am
Real name: Joe Gayo
Location: Los Angeles, CA

Re: FAQ - fusion...the battle in our fusors, X-section vs. voltage

Post by Joe Gayo » Fri Jun 25, 2021 1:40 am

The predominant species of ion in a fusor is D2+ meaning that with 150kV applied, as you know, has the equivalent energy of a 75KeV D+ ion (with 2x the ion current). I would argue that an applied voltage of 300kV is the real ceiling.

User avatar
Frank Sanns
Site Admin
Posts: 1814
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: FAQ - fusion...the battle in our fusors, X-section vs. voltage

Post by Frank Sanns » Fri Jun 25, 2021 4:05 am

It is a complicated situation with IECF. Beam on wall fusion, beam on grid fusion, and beam on neutrals fusion ratios will depend upon the configuration and operating conditions.

Then there is also beam on beam fusion where reference frames tilt the curves a bit.

Of course this has to be applied to all of the moieties flying around including but not limited to D, D2, D+, D++, D2+.

Then there is the matter of median vs mean energies where optimum neutron production may not necessarily correspond to optimum Q. Tuning for the optimum results in a given configuration is necessary. While the core physics principles will prevail, picking just one voltage and current as absolute, misses some of the factors above.

Also missing is the maximum current plateau for a given voltage that I have seen with data on with just about every iteration fusor.

Did I say it was complicated?

User avatar
Richard Hull
Moderator
Posts: 13186
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: FAQ - fusion...the battle in our fusors, X-section vs. voltage

Post by Richard Hull » Fri Jun 25, 2021 5:32 am

Yes, without rigid controls which the simple fusor doesn't have nor will it ever have, makes it a hodge-podge of many right things and many wrong things. Fortunately, like the early battle of real matter over anti-matter, the right things in the simple fusor conspire to win the day due to the right combination of construction and operational experience. Throwing a whole bunch of power into the device to get fusion of highly variable results that allow for smaller and smaller run periods based on higher and higher fusion numbers has been the norm. The real surprise came a number of years ago with large area wall loading adding a bit of totally random, uncontrolled BOT in the velocity space mix.

Thus far, little extreme modification of the simple fusor reactor has been seen. All the new big fusion numbers have come from the application of higher voltages. This, as I note in the FAQ, is to be expected.

Pretty up the simple fusor to a "fusor-like" device, i.e. pre-loaded D targets to enhance BOT, and you can do better via blunting a few bad traits found in the simple fusor. Make it into something that doesn't resemble the simple fusor, i.e. adding ion sources, guns, etc., and one might do better still. Purity and elegance in the fusor was never the amateur goal. Fusion was the goal, be it 10e5 or 10e10 TIER. There was always that race to 10e6 TIER and a good number of us have made it. Two, maybe three true amateurs have either done 10e7 or almost made it. The ultimate limit of physics in D-D fusion will always haunt the amateur fusor. Where that limit lay, is yet to be seen.

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.

Post Reply

Return to “FAQs: Fusion and Fusor Theory”