SymenJ wrote:Maybe I was not too specific with my question but what I actually ment was DD fusion during p-B11 fusion.
That all depends on the choices of the designer. D-D fusion will proceed faster than P-B11 fusion all things being equal. This has been discussed in some variations before. The fusion of a mixture of fuel combinations is possible.
In an ignition machine D-D fusion could directly lead to further D-T and D-He3 fusion. With the addition of B11, the fusion produced protons could even participate in further fusion. This minimizes considerations about fusion cross sections, etc., but in a hot dense and long lived plasma a long list of various fusion reactions could proceed apace or sequentially until the plasma finally exhausted nearly all possible exothermic fusion reactions. This is what happens in large stars till Nickel62 is reached. The self heating, aided by gravitational heating then reaches a dead end. Other interesting things can then occur...
In man made reactors the gravitational confinement is not available and reaching fusion conditions with net excess energy release is much more constrained. Only the easiest fusion fuel combinations may be practical.
In Polywells, which are not ignition machines (self heating) there are further limits on the fusion soup. Mixtures of fuels may be incorporated, but often via selected interventions. Like produced tritium being used as a new fuel. The tritium ash has to be collected and reintroduced as new fuel. It is not a run away sequence of progressive fusion reactions as is possible in an ignition scheme (within the limits of the various fusion cross sections and equilibrium temperature).
More to the point...
Providing protons hydrogen nuclei ) fuel is simply done by injecting hydrogen from nature. This does contain some deuterium, and this deuterium will react with itself to produce the expected fusion products including neutrons. If you are trying to burn only P-B11 with it's very low neutron output you have to purify the hydrogen you use. The deuterium has to be removed. The neutron production by the P-B11 reaction is so low, that even rare deuterium contaminates can lead to relatively large neutron radiation compared to the ideal machine. It is the reverse of isotropically collecting deuterium from the natural mixture of hydrogen (protium?) and deuterium. Such isotopic purification is much easier with light elements compared to uranium, and with the small amounts needed it should be a trivial cost for operating the machines.
The D-He3 reaction is similar. Deuterium is required here, but the possibility of the deuterium reacting with itself (D-D reactions) is significant and will probably outpace the D-He3 reactions if the fuel is in equal proportions. This defeats the purpose of having fusion energy without most of the headaches of neutron radiation. There is a work around though. By diluting the deuterium compared to the He3, the possibilities of D-D reaction compared to the D-He3 rate can be reduced. The undesired reactions can be suppressed, but not completely eliminated. I have heard of recipes for having a mixture of 10 parts He3 to one part deuterium in order to achieve a low neutron output- some times labeled as aneutronic fusion which is defined as less than 1% of the fusion energy coming from the neutrons.
P-B11 can do much better than D-He3 in this regard. I have heard of neutron contributions as low as ~ one part per many millions. But, as mentioned this requires highly purified protium and B11.
There are other considerations about penetrating radiation from ideal P-B11 fusion reactions. A side reaction that produces a high energy gamma ray is rare, but much more common than neutron producing side reactions. This gamma ray in a high powered Polywell P-B11 reactor could be dangerous if not accounted for and shielded against. Penetrating radiation can be greatly reduced but it cannot be reduced to trivial amounts. The health considerations are still important though tremendously reduced. Secondary radiation from neutron transmutation of various elements is also greatly reduced, but again not completely eliminated. Careful selection of first wall materials and a several week 'cool down' may be necessary. Longer cool down periods of up to ~ 100 years may be necessary for high neutron producing fusion reactors (the components of the reactor that accumulate the highest neutron doses). Compare this to ~ 100,000 years for fission reactors.