In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Where D-D peaks out in our fusors
Date: Aug 19, 2:35 pm
Poster: Richard Hull
On Aug 19, 2:35 pm, Richard Hull wrote:
I will discuss what looks like the limit of our little fusors with D-D fuel loading.
First, many of us are amateur experimenters. There is a limit to what we can afford. We will all bump against our economic ceiling well before we reach the peak reaction rates of D-D fusion.
What then is a practical and realizble limit? It still depends on a gang of things, but I would imagine very few of us will really go much beyond 50-60kv input voltage. This is based on corona problems becoming severe, power supply expense, chamber size and insulator limitations in the real world. Expense folds over rapidily above these voltages.
With this said, we can look at the D-D reaction cross section for the range of voltages we are looking at.
The D-D cross section is pretty bad. (stinks, actually) With optimum fusion cross sections normally given in 'barns' (10e-24 sq.cm) for most reactions, the D-D cross section never hits 0.2 barn even near 2mev!!!
I supply a small table (extracted and extrapolated from two sources) below:
fusor volts Energy(collisional) cross section
10kv 20kev .0003 barn
20kv 40kev .003 barn
25kv 50kev .005 barn
30kv 60kev .0075 barn
37.5kv 75kev .01 barn
40kv 80kev .014 barn
50kv 100kev .018 barn
100kv 200kev .03 barn
One can see that the curve is great and has a lot of slope up to about 100kev collisional or 50kv applied. From here, it is almost a linear stepping to 2 mev where the reaction abates. The important note is the 10X improvement from 10kv applied to 20kv applied.
Yes, we are doing fusion at 10kv applied, just very little of it. one might consider this the absolute threshold for fusion. The neutrons should be immeasurable at this point, however.
At 15kv we have more than doubled the rate of reaction. In my old fusor II, I actually succeed pulling a measured 3x10e3 n/s out of the mud the one time I ran it for fusion at ~14kv. At 20kv applied we are just starting to cook. 37.5kv applied, places us about 35 times above the threshold value! At the above stated "amateur maximum" voltage of 50kv, we are doing over 50 times the threshold level, but not over 5 times what we were doing at 20kv. We have sort of "run out of nuclear steam".
D-T mixtures of 50:50 will push over 100 times the reaction at the same voltage as will D-D. The comparison reaches a maximum at 300X between 20 and 25 kv applied or 40-50kev energies.
These figures compare well with real world data, as Bob Hirsch was getting between 10e6 and 10e7 neutrons/sec with ~20kv applied using D-T. Scott Little and myself are seeing about 3X10e4 neutrons per second using D-D with the same ~20kv applied.
Both Fusor III and the Hirsch/Meeks fusor have the same volume. So we have similar volumes and used similar pressures, too.
Armed with the above data, a practical limit for fusion neutrons from a D-D fueled, 50kv applied, 6" fusor would be on the order of 2.5x10e5 neutron/sec. or about 5X10e5 fusions/second. (The sum of both of the reactions, including the aneutronic one.)
The above figures are not pie in the sky values but reduced from real data and not theoretical or purely calculated values.