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I've checked specs of Russian "Dynamic Albedo of Neutrons" module on Mars MSL rover - it can do 10^8 neutron pulses at 10 pulses per second. It works on D-T reaction in tiny linear accelerator, and power consumption is quite low (14W).

If you check specs for all these fusors (Farnsworth, Polywell, e.t.c) - they are happy to have 10^6 neutrons (best ones barely scratch 2*10^8) per second while having huge power consumption and short construction lifetime (<1hour). Does that mean that fusors are just a waste of paper from the science prospective, and real scientists routinely make neutron sources based on nuclear fusion with much better parameters/efficiency?

Update: Fusor: http://en.wikipedia.org/wiki/Fusor

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DAN: http://mars.jpl.nasa.gov/msl/mission/instruments/radiationdetectors/dan/ More info on Russian: http://www.federalspace.ru/main.php?id=58

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Link to some quick descriptions would be useful. I looking into "neutron generators" the other day and found a number of COTS tools with specs like those you list the the on-board instrument, but I don't know off the top of my head how the fusers run. –  dmckee Aug 23 '12 at 14:25
@dmckee Hopefully these links makes it clearer... –  BarsMonster Aug 24 '12 at 8:06
I am looking at their Astrobiology paper. Do you mean 10^7 neutrons @ 10 Hz = 10^8 neutrons/second? –  mng Aug 25 '12 at 16:38
@mng On the federalspace.ru page they were saying that each pulse is 10^8 neutrons. So 10^9 per second. –  BarsMonster Aug 25 '12 at 19:08
Hmm, well, the paper has different numbers. –  mng Aug 25 '12 at 19:29

1 Answer 1

up vote 3 down vote accepted

I would not say that fusors are a waste, but so far they have turned out to not be very efficient. There are several factors working against them, compared to linear accelerator-based devices. Basically what it boils down to is the neutron production cross section.

In a fusor, the ions are in a plasma, and the mean free path is huge. So the effective cross section is small (in other words, ions have a relatively small probability of hitting each other and fusing), and you are spending a lot of energy accelerating ions (and electrons) that fuse very rarely. Perhaps efficiency can be improved, but the results speak for themselves -- the best efficiency I could find was 10^4 neutrons/W [2]. Despite a lot of work over the years, we do not have a good qualitative understanding of what goes on inside a fusor.

Linear neutron generators have a huge advantage because they use solid tritiated metal hydride targets [1]. The ratio of hydride atoms to metal atoms is typically ~2, which presents a very large effective cross section compared to a plasma. In addition, the generated beam(s) of deuterons can be optimized for the target geometry. Furthermore, we have a better understanding of what goes on: it amounts to a fixed-target setup, which has a long history in nuclear physics.

There is the side issue of D-D vs D-T. Since the fusor is fed by gases and typically purges to atmosphere, hobbyists (and most scientists) operating fusors are restricted to the use of deuterium due to safety concerns. For a given energy in the typical range, the microscopic cross section for D-D is lower by an order of magnitude or more. But when the efficiency deficit is 10^4, 10^5, there is still a big gap even if D-T was used in both cases. fusion cross sections

[1] See chapter 7 of PhD thesis by J.M. Verbeke, UC Berkeley, 2000

[2] Miley group, UIUC, 2004

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Do you know if cross section reduction due to colliding ions instead of neutral atoms is also the issue for fusors? –  BarsMonster Aug 25 '12 at 21:19
Certainly there should be an effect, but I'm not sure how large it is. People have been talking about injecting neutral beams into IEC devices but so far I've been unable to find results. –  mng Aug 25 '12 at 22:29

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