PhotonCreationFunc.H

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/* Copyright 2024 David Grote
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
 
#ifndef WARPX_PHOTON_CREATION_FUNC_H_
#define WARPX_PHOTON_CREATION_FUNC_H_
 
#include "Particles/Collision/BinaryCollision/BinaryCollisionUtils.H"
 
#include "Particles/ParticleCreation/SmartCopy.H"
#include "Particles/MultiParticleContainer.H"
#include "Particles/WarpXParticleContainer.H"
#include "Utils/ParticleUtils.H"
 
#include <AMReX_DenseBins.H>
#include <AMReX_GpuAtomic.H>
#include <AMReX_GpuDevice.H>
#include <AMReX_GpuContainers.H>
#include <AMReX_INT.H>
#include <AMReX_Random.H>
#include <AMReX_REAL.H>
#include <AMReX_Vector.H>

class PhotonCreationFunc
{
    // Define shortcuts for frequently-used type names
    using ParticleType = typename WarpXParticleContainer::ParticleType;
    using ParticleTileType = typename WarpXParticleContainer::ParticleTileType;
    using ParticleTileDataType = typename ParticleTileType::ParticleTileDataType;
    using ParticleBins = amrex::DenseBins<ParticleTileDataType>;
    using index_type = typename ParticleBins::index_type;
    using SoaData_type = typename WarpXParticleContainer::ParticleTileType::ParticleTileDataType;
 
public:
    PhotonCreationFunc () = default;

    PhotonCreationFunc (const std::string& collision_name, MultiParticleContainer const * mypc);

    AMREX_INLINE
    amrex::Vector<int> operator() (
                    const index_type& n_total_pairs,
                    ParticleTileType& ptile1, ParticleTileType& ptile2,
                    const amrex::Vector<WarpXParticleContainer*>& pc_products,
                    ParticleTileType** AMREX_RESTRICT tile_products,
                    const amrex::ParticleReal& m1, const amrex::ParticleReal& m2,
                    const amrex::Vector<amrex::ParticleReal>& /*products_mass*/,
                    const index_type* AMREX_RESTRICT p_mask,
                    const amrex::Vector<index_type>& products_np,
                    const SmartCopy* AMREX_RESTRICT copy_species1,
                    const SmartCopy* /*copy_species2*/,
                    const index_type* AMREX_RESTRICT p_pair_indices_1,
                    const index_type* AMREX_RESTRICT p_pair_indices_2,
                    const amrex::ParticleReal* AMREX_RESTRICT p_pair_reaction_weight,
                    const amrex::ParticleReal* AMREX_RESTRICT p_product_data
                    ) const
    {
        using namespace amrex::literals;
 
        if (n_total_pairs == 0) { return amrex::Vector<int>(m_num_product_species, 0); }
 
        // Compute offset array and allocate memory for the produced species
        amrex::Gpu::DeviceVector<index_type> offsets(n_total_pairs);
        const index_type* AMREX_RESTRICT p_offsets = offsets.dataPtr();
 
        int total;
        if (m_create_photons) {
            total = amrex::Scan::ExclusiveSum(n_total_pairs, p_mask, offsets.data());
        } else {
            total = 0;
        }
 
        amrex::Vector<int> num_added_vec(m_num_product_species);
        for (int i = 0; i < m_num_product_species; i++)
        {
            const index_type num_added = total;
            num_added_vec[i] = static_cast<int>(num_added);
            tile_products[i]->resize(products_np[i] + num_added);
        }
 
        const auto soa_1 = ptile1.getParticleTileData();
        const auto soa_2 = ptile2.getParticleTileData();
 
        // Create necessary GPU vectors, that will be used in the kernel below
        amrex::Vector<SoaData_type> soa_products;
        for (int i = 0; i < m_num_product_species; i++)
        {
            soa_products.push_back(tile_products[i]->getParticleTileData());
        }
#ifdef AMREX_USE_GPU
        amrex::Gpu::DeviceVector<SoaData_type> device_soa_products(m_num_product_species);
        amrex::Gpu::DeviceVector<index_type> device_products_np(m_num_product_species);
 
        amrex::Gpu::copyAsync(amrex::Gpu::hostToDevice, soa_products.begin(),
                              soa_products.end(),
                              device_soa_products.begin());
        amrex::Gpu::copyAsync(amrex::Gpu::hostToDevice, products_np.begin(),
                              products_np.end(),
                              device_products_np.begin());
 
        amrex::Gpu::streamSynchronize();
        SoaData_type* AMREX_RESTRICT soa_products_data = device_soa_products.data();
        const index_type* AMREX_RESTRICT products_np_data = device_products_np.data();
#else
        SoaData_type* AMREX_RESTRICT soa_products_data = soa_products.data();
        const index_type* AMREX_RESTRICT products_np_data = products_np.data();
#endif
 
        bool const create_photons = m_create_photons;
 
        amrex::ParallelForRNG(n_total_pairs,
        [=] AMREX_GPU_DEVICE (int i, amrex::RandomEngine const& engine) noexcept
        {
            if (p_mask[i])
            {
                constexpr auto c2 = PhysConst::c2;
 
                int const i1 = static_cast<int>(p_pair_indices_1[i]);
                int const i2 = static_cast<int>(p_pair_indices_2[i]);
 
                // Move to the ion rest frame and calculate the momentum and energy of the three particles
                // This is done here to avoid having to save the photon momentum components
                amrex::ParticleReal & u1x = soa_1.m_rdata[PIdx::ux][i1];
                amrex::ParticleReal & u1y = soa_1.m_rdata[PIdx::uy][i1];
                amrex::ParticleReal & u1z = soa_1.m_rdata[PIdx::uz][i1];
                amrex::ParticleReal const & w1 = soa_1.m_rdata[PIdx::w][i1];
                amrex::ParticleReal & u2x = soa_2.m_rdata[PIdx::ux][i2];
                amrex::ParticleReal & u2y = soa_2.m_rdata[PIdx::uy][i2];
                amrex::ParticleReal & u2z = soa_2.m_rdata[PIdx::uz][i2];
                amrex::ParticleReal const & w2 = soa_2.m_rdata[PIdx::w][i2];
 
                amrex::ParticleReal u1x_rel = u1x;
                amrex::ParticleReal u1y_rel = u1y;
                amrex::ParticleReal u1z_rel = u1z;
                ParticleUtils::doLorentzTransformWithU(u1x_rel, u1y_rel, u1z_rel, u2x, u2y, u2z);
 
                amrex::ParticleReal const u1sq_rel = (u1x_rel*u1x_rel + u1y_rel*u1y_rel + u1z_rel*u1z_rel);
                amrex::ParticleReal const gamma1_rel = std::sqrt(1.0_prt + u1sq_rel/c2);
 
                amrex::ParticleReal const Ephoton = p_product_data[i];
                amrex::ParticleReal const wp = p_pair_reaction_weight[i];
 
                // Calculate electron and ion energy and momentum using conservation
                amrex::ParticleReal const u1_rel = std::sqrt(u1sq_rel);
                amrex::ParticleReal const A = u1_rel/PhysConst::c - Ephoton*wp/(w1*m1*c2);
                amrex::ParticleReal const B = gamma1_rel + w2*m2/(w1*m1) - Ephoton*wp/(w1*m1*c2);
 
                // Expanding A**2 - B**2 cancels some terms and reduces the round off error
                amrex::ParticleReal const AAmBB = -1.0_prt - 2.0_prt*gamma1_rel*w2*m2/(w1*m1) - w2*m2/(w1*m1)*w2*m2/(w1*m1)
                                                  +2.0_prt*(gamma1_rel + w2*m2/(w1*m1) - u1_rel/PhysConst::c)*Ephoton*wp/(w1*m1*c2);
                amrex::ParticleReal const D = w2*m2*w2*m2/(w1*m1*w1*m1) + AAmBB - 1.0_prt;
 
                amrex::ParticleReal const a = 4.0_prt*(-AAmBB);
                amrex::ParticleReal const b = 4.0_prt*A*D;
                amrex::ParticleReal const c = 4.0_prt*B*B - D*D;
 
                // c could also be expanded but it doesn't seem to improve the round off
                // u = u1_rel/PhysConst::c
                // E = Ephoton*wp/(w1*m1*c2)
                // w = w2*m2/(w1*m1)
                // g = gamma1_rel
                // cc1 = w*w*(- 4.0_prt*u*u + 8.0_prt*E*g - 4.0_prt*E*E)
                // cc2 = w*(+ 8.0_prt*E*u*u - 8.0_prt*E*u*g + 8.0_prt*E + 8.0_prt*E*E*u - 8.0_prt*E*E*g)
                // cc3 = (- 8.0_prt*E*u + 4.0_prt*u*u - 8.0_prt*E*E*u*u + 8.0_prt*E*E*u*g)
                // c = cc1 + cc2 + cc3
 
                // Make sure that the root term is always non-negative.
                amrex::ParticleReal const bac = std::max(b*b - 4.0_prt*a*c, 0._prt);
 
                // Use the "plus" solution since it keeps the electon motion forward
                amrex::ParticleReal const u1_rel_after = (-b + std::sqrt(bac))/(2.0_prt*a)*PhysConst::c;
 
                // The terms inside the sqrt can also be expanded but doesn't seem to improve the round off error
                // u1_rel_after = (-b + 4.0_prt*std::sqrt(B*B*D*D + 4.0_prt*A*A*B*B - 4.0_prt*B*B*B*B))/(2.0_prt*a)*PhysConst::c
 
                amrex::ParticleReal const u2_rel_after = (A*PhysConst::c - u1_rel_after)*w1*m1/(w2*m2);
 
                u1x_rel *= u1_rel_after/u1_rel;
                u1y_rel *= u1_rel_after/u1_rel;
                u1z_rel *= u1_rel_after/u1_rel;
                amrex::ParticleReal u2x_rel = u2_rel_after*u1x_rel/u1_rel_after;
                amrex::ParticleReal u2y_rel = u2_rel_after*u1y_rel/u1_rel_after;
                amrex::ParticleReal u2z_rel = u2_rel_after*u1z_rel/u1_rel_after;
                amrex::ParticleReal p3x_rel = (Ephoton/PhysConst::c)*u1x_rel/u1_rel_after;
                amrex::ParticleReal p3y_rel = (Ephoton/PhysConst::c)*u1y_rel/u1_rel_after;
                amrex::ParticleReal p3z_rel = (Ephoton/PhysConst::c)*u1z_rel/u1_rel_after;
 
                // Lorentz transform electron and ion back to lab frame
                ParticleUtils::doLorentzTransformWithU(u1x_rel, u1y_rel, u1z_rel, -u2x, -u2y, -u2z);
                ParticleUtils::doLorentzTransformWithU(u2x_rel, u2y_rel, u2z_rel, -u2x, -u2y, -u2z);
                ParticleUtils::doLorentzTransformWithP(p3x_rel, p3y_rel, p3z_rel, 0.0_prt, -u2x, -u2y, -u2z);
                u1x = u1x_rel;
                u1y = u1y_rel;
                u1z = u1z_rel;
                u2x = u2x_rel;
                u2y = u2y_rel;
                u2z = u2z_rel;
 
                if (create_photons) {
 
                    // Create product particle at position of particle 1
                    const auto product_index = products_np_data[0] + p_offsets[i];
                    copy_species1[0](soa_products_data[0], soa_1, i1,
                                     static_cast<int>(product_index), engine);
 
                    // Set the weight of the new particle to p_pair_reaction_weight[i]
                    soa_products_data[0].m_rdata[PIdx::w][product_index] = wp;
 
                    amrex::ParticleReal & upx = soa_products_data[0].m_rdata[PIdx::ux][product_index];
                    amrex::ParticleReal & upy = soa_products_data[0].m_rdata[PIdx::uy][product_index];
                    amrex::ParticleReal & upz = soa_products_data[0].m_rdata[PIdx::uz][product_index];
 
                    // Lorentz transform to lab frame (from ion rest frame)
                    upx = p3x_rel/PhysConst::m_e;
                    upy = p3y_rel/PhysConst::m_e;
                    upz = p3z_rel/PhysConst::m_e;
                }
            }
        });
 
        // Initialize the user runtime components
        for (int i = 0; i < m_num_product_species; i++)
        {
            const auto start_index = int(products_np[i]);
            const auto stop_index  = int(products_np[i] + num_added_vec[i]);
            ParticleCreation::DefaultInitializeRuntimeAttributes(*tile_products[i],
                                                 *pc_products[i], start_index, stop_index);
        }
 
        amrex::Gpu::synchronize();
 
        return num_added_vec;
    }
 
private:
    // How many different type of species the collision produces
    int m_num_product_species;
 
    // Whether photons are created
    bool m_create_photons;
 
    CollisionType m_collision_type;
 
};
 
#endif // WARPX_PHOTON_CREATION_FUNC_H_