proton-whatever also gives you some proton-proton fusion so now you have deuterium which is not so clean. Let alone p whatever the walls of your chamber are.
Still, if your going to pretend we can do anything harder harder than DT or perhaps DD your might as well go for p+p as it's fairly 'clean' and the fuel is plentiful.
p+p is a lot harder to initiate than any of the reactions listed on the Wikipedia page I linked to. The only reason it happens in stars is that gravitational confinement makes stellar cores much, much denser than plasmas we can produce on Earth. It's not going to happen in an Earthbound reactor using proton (+ something else) fuel.
You're right that there will be some fuel particles that escape confinement and react with the chamber walls; but proper confinement makes this effect very small, much smaller than the numbers you were quoting for radiation exposure, which are based on using fuels that produce neutrons as reaction products. Neutron's can't be confined in a plasma because they're uncharged, so they immediately escape and hit the chamber walls. Protons, and other fuel particles, don't have that problem since they're charged and can be confined (if they couldn't be you wouldn't be able to make the reactor work at all).
It's not just particles directly causing reactions by hitting the walls they also cause sputtering which contaminates the plasma with whatever the wall was made out of. As to PP fusion you get some in any vary high energy plasma probably not enough to be useful for power generation but enough to make some D. Not to mention your fuel is hardly going to be pure in the first place.
Which gets back to my first point you can have low nitron fusion but if your generating useful amount of power your going to be makeing significant amounts of neutrons simply because there so deadly.
> As to PP fusion you get some in any vary high energy plasma
Do you have a reference for this? As I understand it, there aren't significant amounts of protons in our current plasma experiments to begin with, and at the densities we use in those experiments, the cross section for p-p reactions is way too small for them to appear.
> if your generating useful amount of power your going to be makeing significant amounts of neutrons
For current and foreseeable reactors, I agree; but I don't think this is a valid blanket statement about every possible type of fusion reactor that could ever be built, even when our technology has advanced well beyond where it is now.
By some I mean it happens not that it's a significant reaction. PP fusion is in no way a useful approach until you run out of everything else and still want power. Still the sun is 1/10th of ITER's goal temperature so your well in the range for PP fusion based on however much contaminates the plasma.
As to the long term potential I don't think we can rule it out in the longer term, just that when people talk about fusion without neutrons they mean low levels or don't actually know what there talking about.
And I'm asking if you have seen any actual evidence that it happens. I have not, and the information I have seen, which I mentioned, leads me to believe that it should not have happened in any fusion experiments we've done to date. That's why I asked you for a reference.
> the sun is 1/10th of ITER's goal temperature so your well in the range for PP fusion
No, the ITER is not "in the range for PP fusion", because temperature is not the only requirement. You also need sufficient density. The density in the Sun's core is many orders of magnitude larger than the density of plasma in ITER or any other Earthbound fusion experiment. That has a huge effect on the PP reaction cross section.
The rate of fusion changes as the square of density but you still get fusion at vary low density's given sufficient energy it's just less common. You might be thinking of fission?
As to PP fusion from what read. Without a large enough plasma the beta more common proton emission path >99.99% ends up costing more energy than you gain from beta-plus decay <0.01%. "The least stable is 5He, with a half-life of 7.6×10−22 seconds, although it is possible that 2He has an even shorter half-life" http://en.wikipedia.org/wiki/Isotopes_of_helium#Helium-2_.28...
> The rate of fusion changes as the square of density but you still get fusion at vary low density's given sufficient energy it's just less common
How much less common? Again, have you seen any actual evidence that P-P fusion events actually happen in actual Earth-bound plasmas? Because the numbers I've seen indicate that at Earth-bound plasma densities, such events are so unlikely that we should not expect to have observed any.
http://www.sns.ias.edu/~jnb/Papers/Preprints/Solarfusion/pap... "the rate for the fundamental p1p→2 D1e11ne reaction is too small to be measured in the laboratory. Instead, the cross section for the p-p reaction must be calculated from standard weak-interaction theory." Considering the solar density's are ~10^11 times as high as steady state fusion experiments on earth we are looking at ~1/10^22 as many reactions so you might be right that beta-plus decay of deuterium may not have happened in the lab but PP fusion into deuterium is far more common.
PS: And thanks for this, it's good to be called out on something like this. I tossed out the PP comment without thinking though simple contamination is a far larger source of high energy neutrons.
> beta-plus decay of deuterium may not have happened in the lab but PP fusion into deuterium is far more common
It looks to me like the comment about the rate being too small to measure in the lab applies to the PP fusion into deuterium; that's the reaction referred to in the statement you quote. The positron that's produced is not a "beta-plus decay of deuterium"; it's a product of the PP fusion into deuterium, part of the same overall reaction.
The rate of fusion changes as the square of density but you still get fusion at vary low density's it's just less common. You might be thinking of fission?
As to PP fusion from what read. Without a large enough plasma the beta more common proton emission path >99.99% ends up costing more energy than you gain from beta-plus decay <0.01%. "The least stable is 5He, with a half-life of 7.6×10−22 seconds, although it is possible that 2He has an even shorter half-life" http://en.wikipedia.org/wiki/Isotopes_of_helium#Helium-2_.28...
Still, if your going to pretend we can do anything harder harder than DT or perhaps DD your might as well go for p+p as it's fairly 'clean' and the fuel is plentiful.