"Cool", what a word to describe blowing up the Sun. :)
About fusing He-3 and N-15 in the core: haven't you just reinvented the
"helium flash", a phase in the normal evolution of stars? My
recollection
is that according to the accepted models of solar evolution there is a
very brief phase when all the He-3 that accumulates in the core during
the H-burning phase ignites, and the fusion rate goes as temperature to
some obscene power, that's why they call it a "flash". I'm a bit
suspicious of your estimate of the time required for the sphere to reach
the core, because the effect of the helium flash is supposed to
propagate
to the surface for thousands of years (i.e. nothing spectacular can be
seen on the surface). I understand that you're hoping to keep the bomb
in solid state all the way, but so many orders of magnitude still make
me
nervous... Also, even if it does reach the core and ignites, the output
is still gaseous and has to travel upwards for ages. (Unless you can
collect neutrinos, of course. :)
Regards,
--dv