Fusor Forums

Most of the discussion appears to center around electrical current, but what if I were building a Farnsworth reactor with exposed electrodes, using only the difference in voltage (potential) to drive ions around? From your experience what is the minimum difference between positive and negative electrodes with which D-D fusion reactions would reliably and detectably appear?

I like what Frank just said about this FAQ's purpose being to help newcomers size up power supplies, and his emphasis on keeping the ensuing thread simple and practical.

Frank's FAQ on this topic and Richard's FAQ say some different things about desirable voltages and currents, as Jerrod pointed out. Frank's emphasis was more on minimum practical power requirements (~0.1 mA / 20 kV) assuming an excellent neutron detector. Richard's point of view was more towards the "ideal" power supply (for painless progress and richly-satisfying neutron yields, look for ~20 mA and up to 80 kV.) A key understanding that should inform Jerrod and anyone else confused about this apparent discrepancy is that, in theory, there is NO voltage / current threshold for fusion; these FAQs are talking about PRACTICAL capabilities of the power supply, with implicit assumptions about what's involved in detecting neutrons. FAQs on this site are almost always written by single individuals and promoted a posteriori to the canon of FAQhood wisdom by administrators based on perceived utility. Readers can expect stylistic, point-of-view, and emphasis variations as well as the occasional factual contradiction. It's just like the four gospels of the New Testament (with the exception being that we go back and edit for clarity and accuracy).

-Carl

Dan DT wrote:
> To expand on what Chris Bradly said, I have the impression that you (Rich) are thinking about typical current in a wire. The electrons are the only mobile charge carrier, and thus exclusively carries the current (ignoring holes and other complications). But in a plasma (ionized gas) the electrons and ions are both mobile, so either can carry current . The ions move in the opposite direction compared to the electrons ...>
> Dan Tibbets

With apologies to Frank for academic nit-picking on a FAQ thread:

That's exactly my point, Dan. Suppose the inner-grid feed current is -1 mA. That means 1 mA worth of electrons (6.2E+15 per second) are drifting along the wire from HV PS toward grid.
Now if we consider a closed surface that encloses the inner grid, and don't count the feed wire current, there is 1 mA worth of net charge crossing that surface in the inward direction. We can't easily determine the partition between X mA worth of electrons and negative ions crossing that shell outward, and (1-X) mA of positive ions crossing the shell inward -- they both contribute.

I think Chris wants to count some allegedly much larger flux of ions that are "recirculating", with equal numbers crossing inward and outward in a given time interval. So what? I could draw a spherical surface in water of neutral pH and figure the astronomical absolute current of both H3O+ and OH- ions "recirculating" across that shell due to thermal motion. In slightly acid water (pH 6), 99% of the mobile ions are positive, and the thermal "recirculation" current density would be even higher -- with no practical consequence except to satisfy academic curiosity.

Yes, they both contribute. The net current is the sum of the two. The point about recirculation is that if an ion recirculates 10 times, then it is only contributing 1/10th of the net current between the anode and cathode (which means the electrons are contributing 9/10ths of the current- they enter and leave the reaction space 10 times as fast). Remember that the opposing charges have to almost match each other or the voltage goes through the roof. It gets more convoluted as you consider neutral ionization by energetic electrons, Z, etc, but that is my understanding of the basics.

A weak attempt to return to the topic. The required current to do and detect fusion is highly dependent on the recirculation current/ net throughput current. Thus all of the schemes to greatly change this ratio, which is very difficult, if not impossible, to do - especially when both species are considered. This ties directly into any attempt to improve Q.

Dan Tibbets

I just posted an FAQ on this issue and hope to avoid this totally different discussion topic in this FAQ discussion. check out......

viewtopic.php?f=14&t=6957#p42640

Richard Hull

I'd just point out that yes, there is a theoretical ion current and energy to get to detectable fusion with a given detector - BUT!

Without an ion gun setup or other additions, you can't just run an arbitrary volt/ma set of numbers and have a fusor keep itself lit very well, if at all. At some given pressure and geometry, putting on a certain voltage will get you a certain current related to all the re-ionization losses, Paschen's law and other issues, and if you try to lower the current by lowering either the voltage or gas pressure, you're going to find that it "goes out" and goes to zero current - you can't achieve the full continuum here in a "simple" fusor.

In other words, the gas pressure required to get ANY current to flow at 20kv might just be enough to draw significantly more current than desired once it lights off - and doing anything you can at the point that cuts current down to sub-ma just makes it go out. Anyone who has run a fusor through the parameter space knows this well.

All of what you touch upon were covered in the very old FAQ in the operation forum.

viewtopic.php?f=24&t=3111&hilit=FAQ#p12621

Here, I give tips on actual, real time, fusor operation, provided all needed appliances are in place.

The delicate spider monkey routine and artice of fiddling with the voltage, gas flow, vacuum and current interactions are laid out in fusor operation. The simple fusor is operated over a very narrow and often ragged zone between runaway arc discharge and simple glow mode. Holding it in this sweet spot while doing fusion is the art of operation.

Once mastered it is second nature, but in learning mode, grids are often melted and power supplies or their fuses or diodes are blown.

Richard Hull

Wow, well thanks for all of the discussions in response to my question. That certainly cleared it up for me. Essentially the main post in this thread is describing the theoretical requirements needed to reach levels of actual fusion... but the rates may be so low that really sensitive equipment and statistical models may be required to properly judge if fusion is actually taking place. Whereas extremely higher ma ratings may be required for my readily testable fusion (aka easier neutron detection with cheaper equipment).
However, as has been stated here, you can get quite deep in the rabbit hole, so to speak, with coming up with some figures and taking other variables into account. But, for me, the basic info is all I need, for now at least.