SpinvMF.H

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/* Copyright 2022-2026 The Regents of the University of California, through Lawrence
 *           Berkeley National Laboratory (subject to receipt of any required
 *           approvals from the U.S. Dept. of Energy). All rights reserved.
 *
 * This file is part of ImpactX.
 *
 * Authors: Chad Mitchell, Axel Huebl
 * License: BSD-3-Clause-LBNL
 */
#ifndef IMPACTX_DISTRIBUTION_SPINVMF
#define IMPACTX_DISTRIBUTION_SPINVMF
 
#include <ablastr/constant.H>
 
#include <AMReX_Math.H>
#include <AMReX_Random.H>
#include <AMReX_REAL.H>
 
#include <cmath>
 
 
namespace impactx::distribution
{
    struct SpinvMF
    {
        SpinvMF (
            amrex::ParticleReal mux,
            amrex::ParticleReal muy,
            amrex::ParticleReal muz
        )
        : m_muX(mux), m_muY(muy), m_muZ(muz)
        {
            // magnitude of the polarization vector (= polarization magnitude)
            amrex::ParticleReal const pmag = std::sqrt(
                mux * mux +
                muy * muy +
                muz * muz
            );
 
            m_kappa = inverse_Langevin(pmag);
        }

        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
        void operator() (
            amrex::ParticleReal & AMREX_RESTRICT sx,
            amrex::ParticleReal & AMREX_RESTRICT sy,
            amrex::ParticleReal & AMREX_RESTRICT sz,
            amrex::RandomEngine const & engine
        ) const
        {
            using namespace amrex::literals;
            using amrex::Math::powi;
            using ablastr::constant::math::pi;
 
            // Normalize the mean direction vector just in case:
            amrex::ParticleReal mu_norm = std::sqrt(powi<2>(m_muX)+powi<2>(m_muY)+powi<2>(m_muZ));
            amrex::ParticleReal mux = m_muX/mu_norm;
            amrex::ParticleReal muy = m_muY/mu_norm;
            amrex::ParticleReal muz = m_muZ/mu_norm;
 
            // Evaluate constants independent of particle:
            amrex::ParticleReal const muperp = std::sqrt(powi<2>(mux)+powi<2>(muy));
            amrex::ParticleReal const a = (muperp==0) ? 0_prt : mux/muperp;
            amrex::ParticleReal const b = (muperp==0) ? 1_prt : muy/muperp;
 
            // Sample two iid random variables:
            amrex::ParticleReal x1 = amrex::Random(engine);
            amrex::ParticleReal x2 = amrex::Random(engine);
 
            // Basic transformation of the random variables:
            amrex::ParticleReal c = std::cos(2_prt*pi*x1);
            amrex::ParticleReal s = std::sin(2_prt*pi*x1);
            amrex::ParticleReal t;
            // The form of the function below is obtained from eq (30) of:
            // D. Frisch and U. D. Hanebeck, "Deterministic Von Mises-Fisher Sampling on the Sphere Using
            // Fibonacci Lattices", IEEE, 2023, DOI: 10.1109/SDF-MFI59545.2023.10361396
            if(m_kappa > 0) {
               t = 1_prt + std::log1p(x2 * std::expm1(-2_prt * m_kappa)) / m_kappa;
            } else {
               t = 1_prt - 2_prt * x2;
            }
 
            // Evaluation of the spin components
            amrex::ParticleReal u = std::sqrt(1_prt-powi<2>(t));
            sx = u * (b*c + a*muz*s) + t*mux;
            sy = u * (-a*c + b*muz*s) + t*muy;
            sz = u * (-muperp*s) + t*muz;
 
        }
 

        AMREX_GPU_HOST AMREX_FORCE_INLINE
        static amrex::ParticleReal
        inverse_Langevin (amrex::ParticleReal pmag)
        {
            using namespace amrex::literals; // for _rt and _prt
            using amrex::Math::powi;
 
            AMREX_ASSERT_WITH_MESSAGE(pmag >= 0_prt, "Polarization magnitude must be greater or equal 0.");
            AMREX_ASSERT_WITH_MESSAGE(pmag < 1_prt, "Polarization magnitude must be less than 1.");
 
            // Evaluation of the inverse Langevin function.  Here, we use the approximate expression
            // due to R. Petrosyan.  See:
            // R. Petrosyan, Rheologica Acta 56, 21-26, 2016; doi:10.1007/s00397-016-0977-9
            // R. Jedynak, Mathematics and Mechanics of Solids 24, 1-25, 2018; doi:10.1177/1081286518811395
 
            amrex::ParticleReal concentration =
                3_prt * pmag +
                powi<2>(pmag) / 5_prt * std::sin(7_prt * pmag * 0.5_prt) +
                powi<3>(pmag) / (1_prt - pmag);
 
            return concentration;
        }
 
        amrex::ParticleReal m_muX, m_muY, m_muZ;  
        amrex::ParticleReal m_kappa;  
    };
 
} // namespace impactx::distribution
 
#endif // IMPACTX_DISTRIBUTION_SPINVMF