LinearComptonCollisionFunc.H

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/* Copyright 2023 Arianna Formenti
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
 
#ifndef LINEAR_COMPTON_COLLISION_FUNC_H_
#define LINEAR_COMPTON_COLLISION_FUNC_H_
 
#include "SingleLinearComptonCollisionEvent.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 <AMReX_Algorithm.H>
#include <AMReX_DenseBins.H>
#include <AMReX_ParmParse.H>
#include <AMReX_Random.H>
#include <AMReX_REAL.H>
#include <AMReX_Vector.H>

class LinearComptonCollisionFunc
{
    // 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:
    LinearComptonCollisionFunc () = default;

    LinearComptonCollisionFunc (
            const std::string& collision_name,
            MultiParticleContainer const * const /*mypc*/,
            const bool isSameSpecies) :
        m_isSameSpecies(isSameSpecies)
    {
        using namespace amrex::literals;
 
#ifdef AMREX_SINGLE_PRECISION_PARTICLES
        WARPX_ABORT_WITH_MESSAGE("Linear Compton scattering module does not currently work with single precision");
#endif
 
        const amrex::ParmParse pp_collision_name(collision_name);
 
        constexpr auto default_event_multiplier = 1.0_prt;
        m_event_multiplier = default_event_multiplier;
        utils::parser::queryWithParser(
            pp_collision_name, "event_multiplier", m_event_multiplier);
 
        constexpr auto default_probability_threshold = 0.02_prt;
        m_probability_threshold = default_probability_threshold;
        utils::parser::queryWithParser(
            pp_collision_name, "probability_threshold", m_probability_threshold);
 
        constexpr auto default_probability_target_value = 0.002_prt;
        m_probability_target_value = default_probability_target_value;
        utils::parser::queryWithParser(
            pp_collision_name, "probability_target_value",
            m_probability_target_value);
 
        m_exe.m_event_multiplier = m_event_multiplier;
        m_exe.m_probability_threshold = m_probability_threshold;
        m_exe.m_probability_target_value = m_probability_target_value;
        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::Real const /*global_lamdb*/,
            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::ParticleReal* /*p_product_data*/,
            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];
            uint64_t* AMREX_RESTRICT idcpu1 = soa_1.m_idcpu;
 
            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];
            uint64_t* AMREX_RESTRICT idcpu2 = soa_2.m_idcpu;
 
            // 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;
 
            // multiplier ratio to take into account unsampled pairs
            const auto multiplier_ratio = static_cast<int>(
                m_isSameSpecies ? min_N + max_N - 1 : min_N);
 
            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)
            {
                // do not check for collision if a particle's weight was
                // reduced to zero from a previous collision
                if (idcpu1[ I1[i1] ]==amrex::ParticleIdCpus::Invalid ||
                    idcpu2[ I2[i2] ]==amrex::ParticleIdCpus::Invalid ) {
                    p_mask[pair_index] = false;
                }
                else {
                    SingleLinearComptonCollisionEvent(
                        u1x[ I1[i1] ], u1y[ I1[i1] ], u1z[ I1[i1] ],
                        u2x[ I2[i2] ], u2y[ I2[i2] ], u2z[ I2[i2] ],
                        w1[ I1[i1] ], w2[ I2[i2] ],
                        dt, dV, static_cast<int>(pair_index), p_mask, p_pair_reaction_weight,
                        m_event_multiplier, multiplier_ratio,
                        m_probability_threshold,
                        m_probability_target_value,
                        engine);
                    // Remove pair reaction weight from the colliding particles' weights
                    if (p_mask[pair_index]) {
                        BinaryCollisionUtils::remove_weight_from_colliding_particle(
                            w1[ I1[i1] ], idcpu1[ I1[i1] ], p_pair_reaction_weight[pair_index]);
                        BinaryCollisionUtils::remove_weight_from_colliding_particle(
                            w2[ I2[i2] ], idcpu2[ I2[i2] ], p_pair_reaction_weight[pair_index]);
                    }
                }
 
                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_event_multiplier;
        amrex::ParticleReal m_probability_threshold;
        amrex::ParticleReal m_probability_target_value;
        bool m_isSameSpecies;
        bool m_computeSpeciesDensities = false;
        bool m_computeSpeciesTemperatures = false;
        bool m_need_product_data = false;
    };
 
    [[nodiscard]] Executor const& executor () const { return m_exe; }
 
    bool use_global_debye_length() { return false; }
 
private:
    // Factor used to increase the number of collisions by decreasing the weight of the
    // produced particles
    amrex::ParticleReal m_event_multiplier;
    // If the event multiplier is too high and results in a collision probability that approaches
    // 1, there is a risk of underestimating the total yield of electrons and positrons.
    // In these cases, we reduce the event multiplier used in a given collision.
    // m_probability_threshold is the probability above which we reduce the event multiplier.
    // m_probability_target_value is the target probability used to determine by how much
    // the event multiplier should be reduced.
    amrex::ParticleReal m_probability_threshold;
    amrex::ParticleReal m_probability_target_value;
    bool m_isSameSpecies;
 
    Executor m_exe;
};
 
#endif // LINEAR_COMPTON_COLLISION_FUNC_H_