SingleNuclearFusionEvent.H

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/* Copyright 2021 Neil Zaim
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
 
#ifndef WARPX_SINGLE_NUCLEAR_FUSION_EVENT_H_
#define WARPX_SINGLE_NUCLEAR_FUSION_EVENT_H_
 
#include "BoschHaleFusionCrossSection.H"
#include "ProtonBoronFusionCrossSection.H"
 
#include "Particles/Collision/BinaryCollision/BinaryCollisionUtils.H"
#include "Utils/WarpXConst.H"
 
#include <AMReX_Algorithm.H>
#include <AMReX_Random.H>
#include <AMReX_REAL.H>
 
#include <cmath>
 

template <typename index_type>
AMREX_GPU_HOST_DEVICE AMREX_INLINE
void SingleNuclearFusionEvent (const amrex::ParticleReal& u1x, const amrex::ParticleReal& u1y,
                               const amrex::ParticleReal& u1z, const amrex::ParticleReal& u2x,
                               const amrex::ParticleReal& u2y, const amrex::ParticleReal& u2z,
                               const amrex::ParticleReal& m1, const amrex::ParticleReal& m2,
                               amrex::ParticleReal w1, amrex::ParticleReal w2,
                               const amrex::Real& dt, const amrex::ParticleReal& dV, const int& pair_index,
                               index_type* AMREX_RESTRICT p_mask,
                               amrex::ParticleReal* AMREX_RESTRICT p_pair_reaction_weight,
                               const amrex::ParticleReal& fusion_multiplier,
                               const int& multiplier_ratio,
                               const amrex::ParticleReal& probability_threshold,
                               const amrex::ParticleReal& probability_target_value,
                               const NuclearFusionType& fusion_type,
                               const amrex::RandomEngine& engine)
{
    amrex::ParticleReal E_coll, v_coll, lab_to_COM_factor;
 
    BinaryCollisionUtils::get_collision_parameters(
        m1*u1x, m1*u1y, m1*u1z, m2*u2x, m2*u2y, m2*u2z, m1, m2,
        E_coll, v_coll, lab_to_COM_factor);
 
    using namespace amrex::literals;
 
    const amrex::ParticleReal w_min = amrex::min(w1, w2);
    const amrex::ParticleReal w_max = amrex::max(w1, w2);
 
    // Compute fusion cross section as a function of kinetic energy in the center of mass frame
    auto fusion_cross_section = amrex::ParticleReal(0.);
    if (fusion_type == NuclearFusionType::ProtonBoronToAlphas)
    {
        fusion_cross_section = ProtonBoronFusionCrossSection(E_coll);
    }
    else if (BinaryCollisionUtils::is_two_product_fusion_type(fusion_type))
    {
        fusion_cross_section = BoschHaleFusionCrossSection(E_coll, fusion_type, m1, m2);
    }
 
    // First estimate of probability to have fusion reaction
    amrex::ParticleReal probability_estimate = multiplier_ratio * fusion_multiplier *
                                lab_to_COM_factor * w_max * fusion_cross_section * v_coll * dt / dV;
 
    // Effective fusion multiplier
    amrex::ParticleReal fusion_multiplier_eff = fusion_multiplier;
 
    // If the fusion probability is too high and the fusion multiplier greater than one, we risk to
    // systematically underestimate the fusion yield. In this case, we reduce the fusion multiplier
    // to reduce the fusion probability
    if (probability_estimate > probability_threshold)
    {
        // We aim for a fusion probability of probability_target_value but take into account
        // the constraint that the fusion_multiplier cannot be smaller than one
        fusion_multiplier_eff  = amrex::max(fusion_multiplier *
                                         probability_target_value / probability_estimate , 1._prt);
        probability_estimate *= fusion_multiplier_eff/fusion_multiplier;
    }
 
    // Compute actual fusion probability that is always between zero and one
    // In principle this is obtained by computing 1 - exp(-probability_estimate)
    // However, the computation of this quantity can fail numerically when probability_estimate is
    // too small (e.g. exp(-probability_estimate) returns 1 and the computation returns 0).
    // std::expm1 is used since it maintains correctness for small exponent.
    const amrex::ParticleReal probability = -std::expm1(-probability_estimate);
 
    // Get a random number
    const amrex::ParticleReal random_number = amrex::Random(engine);
 
    // If we have a fusion event, set the mask the true and fill the product weight array
    if (random_number < probability)
    {
        p_mask[pair_index] = true;
        p_pair_reaction_weight[pair_index] = w_min/fusion_multiplier_eff;
    }
    else
    {
        p_mask[pair_index] = false;
    }
 
}
#endif // WARPX_SINGLE_NUCLEAR_FUSION_EVENT_H_