RE: starship-design: Staged Fusion Power

[Timothy van der Linden <Shealiak@XS4ALL.nl>]

Hello again Lee,

>Table I:  Fusion Reactions Among Various Light Elements
>D+D   -> T (1.01 MeV) + p (3.02 MeV) (50%)
>      -> He3 (0.82 MeV) + n (2.45 MeV) (50%)  <- most abundant fuel
>      -> He4 + about 20 MeV of gamma rays (about 0.0001%; depends
>                                           somewhat on temperature.)
>      (most other low-probability branches are omitted below)
>D+T   -> He4 (3.5 MeV) + n (14.1 MeV)  <-easiest to achieve
>D+He3 -> He4 (3.6 MeV) + p (14.7 MeV)  <-easiest aneutronic reaction
>                                     "aneutronic" is explained below.
>T+T   -> He4 + 2n + 11.3 MeVHe3+T -> He4 + p + n + 12.1 MeV (51%)
>      -> He4 (4.8) + D (9.5) (43%)
>      -> He4 (0.5) + n (1.9) + p (11.9) (6%)  <- via He5 decay
>
>p+Li6 -> He4 (1.7) + He3 (2.3)      <- another aneutronic reaction
>p+Li7 -> 2 He4 + 17.3 MeV (20%)
>      -> Be7 + n -1.6 MeV (80%)     <- endothermic, not good.
>D+Li6 -> 2He4 + 22.4 MeV            <- also aneutronic, but you
>                                              get D-D reactions too.
>p+B11 -> 3 He4 + 8.7 MeV <- harder to do, but more energy than p+Li6
>n+Li6 -> He4 (2.1) + T (2.7)        <- this can convert n's to T's
>n+Li7 -> He4 + T + n - some energy
>
>As you can see from the table D+He3 is a good candidate for the second stage
>if you use D+D as the first stage and then somehow manage to fuse the He3
>produced before it can escape. Of course you also have to worry about
>neutrons from the D+D reaction. 

The first stage D+D->He3+n+3.27MeV does not really add a lot of energy
relative to 18.3MeV and as you mention it has a neutron as reaction product
which may disturb the second stage so much that the second stage reaction
isn't as efficient anymore. (Maybe even more than 3.27MeV less efficient.)

I wonder why our tables don't show p+D->He3+5.5MeV it looks like an
excellent candidate to be followed by a second stage (as do all single
end-product reactions). If this is a valid reaction, it may be followed by
my D+He3->He4+p+18.4MeV that would give a yield of 4.78MeV per nucleon.
(Fusing upto Fe54 starting with H2 and H3 will give a yield of 7.43MeV per
nucleon.)
But again I'm not sure if a second stage is as "easy" as we hope, it still
is very far away from the current fusion technology.

>Too bad we can't use the gamma rays produced in the D+D reaction though...

What would you use them for?

>As you said, your notation is a little different but it is understandable.
>Where did you get the 18.4 MeV though?

D+He3 -> He4 (3.6 MeV) + p (14.7 MeV)
 3.6+14.7=18.3MeV  My 18.4MeV is actually 18.35MeV, so the difference is
likely due to rounding errors.

>Notice that most of the reactions in this table produce He4 as a by-product.
>My first choice would be a straight single stage D+Li6 or p+B11 using
>antimatter catalysis or muon catalysis to start the reaction.

p+B11 would be better since it doesn't have D+D reactions as D+Li6 has. And
a bonus is that both p and B11 are abundant in nature, while D and Li6 are
harder to find. The only disadvantage is that it only yields 8.7MeV while
its reaction  materials consist of already 12 nuclei in total. (0.725MeV
per nucleon)
I wonder if a second stage reaction could significantly increase this.

Regards, Timothy