NuclearFusionFunc.H

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/* Copyright 2021 Neil Zaim
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
 
#ifndef WARPX_NUCLEAR_FUSION_FUNC_H_
#define WARPX_NUCLEAR_FUSION_FUNC_H_
 
#include "SingleNuclearFusionEvent.H"
 
#include "Particles/Collision/BinaryCollision/BinaryCollisionUtils.H"
#include "Particles/Pusher/GetAndSetPosition.H"
#include "Particles/MultiParticleContainer.H"
#include "Particles/WarpXParticleContainer.H"
#include "Utils/Parser/ParserUtils.H"
#include "Utils/TextMsg.H"
#include "WarpX.H"
 
#include <AMReX_Algorithm.H>
#include <AMReX_DenseBins.H>
#include <AMReX_ParmParse.H>
#include <AMReX_Random.H>
#include <AMReX_REAL.H>
#include <AMReX_Vector.H>
 
class NuclearFusionFunc
{
    // Define shortcuts for frequently-used type names
    using ParticleType = WarpXParticleContainer::ParticleType;
    using ParticleTileType = WarpXParticleContainer::ParticleTileType;
    using ParticleTileDataType = ParticleTileType::ParticleTileDataType;
    using ParticleBins = amrex::DenseBins<ParticleTileDataType>;
    using index_type = ParticleBins::index_type;
    using SoaData_type = WarpXParticleContainer::ParticleTileType::ParticleTileDataType;
 
public:
    NuclearFusionFunc () = default;
 
    NuclearFusionFunc (const std::string& collision_name, MultiParticleContainer const * const mypc,
                       const bool isSameSpecies):
        m_fusion_multiplier{amrex::ParticleReal{1.0}}, // default fusion multiplier
        m_probability_threshold{amrex::ParticleReal{0.02}}, // default fusion probability threshold
        m_probability_target_value{amrex::ParticleReal{0.002}}, // default fusion probability target_value
        m_fusion_type{BinaryCollisionUtils::get_nuclear_fusion_type(collision_name, mypc)},
        m_isSameSpecies{isSameSpecies}
    {
 
#ifdef AMREX_SINGLE_PRECISION_PARTICLES
        WARPX_ABORT_WITH_MESSAGE("Nuclear fusion module does not currently work with single precision");
#endif
 
        const amrex::ParmParse pp_collision_name(collision_name);
        utils::parser::queryWithParser(
            pp_collision_name, "fusion_multiplier", m_fusion_multiplier);
        utils::parser::queryWithParser(
            pp_collision_name, "fusion_probability_threshold", m_probability_threshold);
        utils::parser::queryWithParser(
            pp_collision_name, "fusion_probability_target_value",
            m_probability_target_value);
 
        m_exe.m_fusion_multiplier = m_fusion_multiplier;
        m_exe.m_probability_threshold = m_probability_threshold;
        m_exe.m_probability_target_value = m_probability_target_value;
        m_exe.m_fusion_type = m_fusion_type;
        m_exe.m_isSameSpecies = m_isSameSpecies;
    }
 
    struct Executor {
        AMREX_GPU_HOST_DEVICE AMREX_INLINE
        void operator() (
            index_type const I1s, index_type const I1e,
            index_type const I2s, index_type const I2e,
            index_type const* AMREX_RESTRICT I1,
            index_type const* AMREX_RESTRICT I2,
            const SoaData_type& soa_1, const SoaData_type& soa_2,
            GetParticlePosition<PIdx> /*get_position_1*/, GetParticlePosition<PIdx> /*get_position_2*/,
            amrex::ParticleReal const /*n1*/, amrex::ParticleReal const /*n2*/,
            amrex::ParticleReal const /*T1*/, amrex::ParticleReal const /*T2*/,
            amrex::ParticleReal const  /*q1*/, amrex::ParticleReal const  /*q2*/,
            amrex::ParticleReal const  m1, amrex::ParticleReal const  m2,
            amrex::Real const  dt, amrex::Real const dV, index_type coll_idx,
            index_type const cell_start_pair, index_type* AMREX_RESTRICT p_mask,
            index_type* AMREX_RESTRICT p_pair_indices_1, index_type* AMREX_RESTRICT p_pair_indices_2,
            amrex::ParticleReal* AMREX_RESTRICT p_pair_reaction_weight,
            amrex::RandomEngine const& engine) const
        {
            amrex::ParticleReal * const AMREX_RESTRICT w1 = soa_1.m_rdata[PIdx::w];
            amrex::ParticleReal * const AMREX_RESTRICT u1x = soa_1.m_rdata[PIdx::ux];
            amrex::ParticleReal * const AMREX_RESTRICT u1y = soa_1.m_rdata[PIdx::uy];
            amrex::ParticleReal * const AMREX_RESTRICT u1z = soa_1.m_rdata[PIdx::uz];
 
            amrex::ParticleReal * const AMREX_RESTRICT w2 = soa_2.m_rdata[PIdx::w];
            amrex::ParticleReal * const AMREX_RESTRICT u2x = soa_2.m_rdata[PIdx::ux];
            amrex::ParticleReal * const AMREX_RESTRICT u2y = soa_2.m_rdata[PIdx::uy];
            amrex::ParticleReal * const AMREX_RESTRICT u2z = soa_2.m_rdata[PIdx::uz];
 
            // Number of macroparticles of each species
            const index_type NI1 = I1e - I1s;
            const index_type NI2 = I2e - I2s;
            const index_type max_N = amrex::max(NI1,NI2);
            const index_type min_N = amrex::min(NI1,NI2);
 
            index_type pair_index = cell_start_pair + coll_idx;
 
            // Because the number of particles of each species is not always equal (NI1 != NI2
            // in general), some macroparticles will be paired with multiple macroparticles of the
            // other species and we need to decrease their weight accordingly.
            // c1 corresponds to the minimum number of times a particle of species 1 will be paired
            // with a particle of species 2. Same for c2.
            // index_type(1): https://github.com/AMReX-Codes/amrex/pull/3684
            const index_type c1 = amrex::max(NI2/NI1, index_type(1));
            const index_type c2 = amrex::max(NI1/NI2, index_type(1));
 
            // multiplier ratio to take into account unsampled pairs
            const auto multiplier_ratio = static_cast<int>(
                m_isSameSpecies ? 2*max_N - 1 : max_N);
 
#if (defined WARPX_DIM_RZ)
            amrex::ParticleReal * const AMREX_RESTRICT theta1 = soa_1.m_rdata[PIdx::theta];
            amrex::ParticleReal * const AMREX_RESTRICT theta2 = soa_2.m_rdata[PIdx::theta];
#endif
            index_type i1 = I1s + coll_idx;
            index_type i2 = I2s + coll_idx;
            // we will start from collision number = coll_idx and then add
            // stride (smaller set size) until we do all collisions (larger set size)
            for (index_type k = coll_idx; k < max_N; k += min_N)
            {
                // c1k : how many times the current particle of species 1 is paired with a particle
                // of species 2. Same for c2k.
                const index_type c1k = (k%NI1 < max_N%NI1) ? c1 + 1: c1;
                const index_type c2k = (k%NI2 < max_N%NI2) ? c2 + 1: c2;
 
#if (defined WARPX_DIM_RZ)
                /* In RZ geometry, macroparticles can collide with other macroparticles
                 * in the same *cylindrical* cell. For this reason, collisions between macroparticles
                 * are actually not local in space. In this case, the underlying assumption is that
                 * particles within the same cylindrical cell represent a cylindrically-symmetry
                 * momentum distribution function. Therefore, here, we temporarily rotate the
                 * momentum of one of the macroparticles in agreement with this cylindrical symmetry.
                 * (This is technically only valid if we use only the m=0 azimuthal mode in the simulation;
                 * there is a corresponding assert statement at initialization.) */
                amrex::ParticleReal const theta = theta2[I2[i2]]-theta1[I1[i1]];
                amrex::ParticleReal const u1xbuf = u1x[I1[i1]];
                u1x[I1[i1]] = u1xbuf*std::cos(theta) - u1y[I1[i1]]*std::sin(theta);
                u1y[I1[i1]] = u1xbuf*std::sin(theta) + u1y[I1[i1]]*std::cos(theta);
#endif
 
                SingleNuclearFusionEvent(
                    u1x[ I1[i1] ], u1y[ I1[i1] ], u1z[ I1[i1] ],
                    u2x[ I2[i2] ], u2y[ I2[i2] ], u2z[ I2[i2] ],
                    m1, m2, w1[ I1[i1] ]/c1k, w2[ I2[i2] ]/c2k,
                    dt, dV, static_cast<int>(pair_index), p_mask, p_pair_reaction_weight,
                    m_fusion_multiplier, multiplier_ratio,
                    m_probability_threshold,
                    m_probability_target_value,
                    m_fusion_type, engine);
 
#if (defined WARPX_DIM_RZ)
                amrex::ParticleReal const u1xbuf_new = u1x[I1[i1]];
                u1x[I1[i1]] = u1xbuf_new*std::cos(-theta) - u1y[I1[i1]]*std::sin(-theta);
                u1y[I1[i1]] = u1xbuf_new*std::sin(-theta) + u1y[I1[i1]]*std::cos(-theta);
#endif
 
                p_pair_indices_1[pair_index] = I1[i1];
                p_pair_indices_2[pair_index] = I2[i2];
                if (max_N == NI1) {
                    i1 += min_N;
                }
                if (max_N == NI2) {
                    i2 += min_N;
                }
                pair_index += min_N;
            }
        }
 
        amrex::ParticleReal m_fusion_multiplier;
        amrex::ParticleReal m_probability_threshold;
        amrex::ParticleReal m_probability_target_value;
        NuclearFusionType m_fusion_type;
        bool m_computeSpeciesDensities = false;
        bool m_computeSpeciesTemperatures = false;
        bool m_isSameSpecies;
    };
 
    [[nodiscard]] Executor const& executor () const { return m_exe; }
 
private:
    // Factor used to increase the number of fusion reaction by decreasing the weight of the
    // produced particles
    amrex::ParticleReal m_fusion_multiplier;
    // If the fusion multiplier is too high and results in a fusion probability that approaches
    // 1, there is a risk of underestimating the total fusion yield. In these cases, we reduce
    // the fusion multiplier used in a given collision. m_probability_threshold is the fusion
    // probability threshold above which we reduce the fusion multiplier.
    // m_probability_target_value is the target probability used to determine by how much
    // the fusion multiplier should be reduced.
    amrex::ParticleReal m_probability_threshold;
    amrex::ParticleReal m_probability_target_value;
    NuclearFusionType m_fusion_type;
    bool m_isSameSpecies;
 
    Executor m_exe;
};
 
#endif // WARPX_NUCLEAR_FUSION_FUNC_H_