Role of the Coulomb Barrier
The Coulomb barrier is
the reason that nuclei up to carbon or oxygen can fuse directly to heavier elements, but
fusion of heavier elements like silicon goes through the equilibrium process described in the left panel.
For larger atomic number, the nuclear charge increases and so does the
Coulomb barrier to fusion of the charged particles.
Thus, for silicon fusion
the particles must move very rapidly
(high temperature) to overcome the barrier.
But when the temperature gets very high there are many photons
in the plasma with enough energy to break apart the silicon nuclei. Thus, the very conditions
necessary to fuse silicon mean that it is likely to be broken apart by photons.