Correction to proton-boron fusion error check addressing issue #3797

[eebasso]

Changed PhysicalSpecies "helium" to "helium4" and made the error message string mention "helium4" instead of "alpha." It appears that PhysicalSpecies::alpha is currently unused. This is intended to address issue #3797.

[RemiLehe]

Thanks for this PR!

I actually also allowed alpha in addition to helium4. (See here for the difference between alpha and helium4 in WarpX: helium4 is the atom and includes the mass of the electrons, whereas alpha is the nucleus only.)

I also updated the proton-boron fusion test, since it was (incorrectly) using helium instead of helium4/alpha.

[eebasso]

Thanks for this PR!

I actually also allowed alpha in addition to helium4. (See here for the difference between alpha and helium4 in WarpX: helium4 is the atom and includes the mass of the electrons, whereas alpha is the nucleus only.)

I also updated the proton-boron fusion test, since it was (incorrectly) using helium instead of helium4/alpha.

Thank you for the response. I was thinking of doing that change at first, but it seemed to me that the "helium4 OR alpha" check isn't currently done elsewhere in BinaryCollisionUtils, and so I was trying to be consistent. For example, deuterium-helium3 fusion allows helium4 as a product species, but not alpha. I assume that had to do with conserving the number of electrons.

Perhaps every instance of helium4 check needs to have an alpha check alternative? Technically, neither 3 x helium4 nor 3 x alpha conserves the number of electrons from proton + boron11 fusion, but 3*helium4 comes closer. The process of hydrogen1 + boron11 -> 3 x helium4 would conserve charge.

Edit: another alternative I can think of is to make one of alpha or helium4 as an alternative name to other in the map string_to_species defined in SpeciesPhysicalProperties.cpp.

Looking at the map species_to_properties, it seems that helium4 is still considered a +2 charged particle, but it has a larger mass than alpha due to two electrons.

        {PhysicalSpecies::helium4, Properties{
             amrex::Real(4.00260325413) * PhysConst::m_u,
             amrex::Real(2) * PhysConst::q_e}},
        {PhysicalSpecies::alpha, Properties{
             amrex::Real(4.00260325413) * PhysConst::m_u - amrex::Real(2) * PhysConst::m_e,
             amrex::Real(2) * PhysConst::q_e}},

[RemiLehe]

That's a very good point.
There are indeed some inconsistencies between the mass and the charge of the species. I will discuss this with the other WarpX developers.

[eebasso]

Excellent. I am happy to assist you in any way I can.

[RemiLehe]

Superseded by #4480
(as discussed offline in the WarpX developer meeting)