spintransport.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
 * License: BSD-3-Clause-LBNL
 */
#ifndef IMPACTX_ELEMENTS_MIXIN_SPIN_TRANSPORT_H
#define IMPACTX_ELEMENTS_MIXIN_SPIN_TRANSPORT_H
 
#include "particles/ImpactXParticleContainer.H"
 
#include <ablastr/constant.H>
 
#include <AMReX_Math.H>
#include <AMReX_Extension.H>
#include <AMReX_REAL.H>
#include <AMReX_SIMD.H>
 
#include <cmath>
 
 
namespace impactx::elements::mixin
{
    struct SpinTransport
    {
        template<typename T_Real=amrex::ParticleReal>
        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
        void rotate_spin (
            T_Real const & AMREX_RESTRICT lambdax,
            T_Real const & AMREX_RESTRICT lambday,
            T_Real const & AMREX_RESTRICT lambdaz,
            T_Real & AMREX_RESTRICT sx,
            T_Real & AMREX_RESTRICT sy,
            T_Real & AMREX_RESTRICT sz
        ) const
        {
            using namespace amrex::literals;  // for _prt
            using namespace std; // for cmath(float)
 
            // Compute quaternion coefficients
            T_Real const angle = sqrt(lambdax*lambdax + lambday*lambday + lambdaz*lambdaz);
 
            auto const [sin_half_angle, cos_half_angle] = amrex::Math::sincos(angle*0.5_prt);
            T_Real const w0 = cos_half_angle;
            T_Real const c_sin_half = -sin_half_angle / angle;
 
            auto const non_zero_angle = angle!=0_prt;
#ifdef AMREX_USE_SIMD
            T_Real w1 = 0_prt;
            amrex::simd::stdx::where(non_zero_angle, w1) = lambdax * c_sin_half;
            T_Real w2 = 0_prt;
            amrex::simd::stdx::where(non_zero_angle, w2) = lambday * c_sin_half;
            T_Real w3 = 0_prt;
            amrex::simd::stdx::where(non_zero_angle, w3) = lambdaz * c_sin_half;
#else
            T_Real w1 = non_zero_angle ? lambdax * c_sin_half : 0_prt;
            T_Real w2 = non_zero_angle ? lambday * c_sin_half : 0_prt;
            T_Real w3 = non_zero_angle ? lambdaz * c_sin_half : 0_prt;
#endif
 
            // Apply rotation
            T_Real const sxout = (1_prt-2_prt*(w2*w2+w3*w3))*sx + 2_prt*(w1*w2+w0*w3)*sy + 2_prt*(w1*w3-w0*w2)*sz;
            T_Real const syout = 2_prt*(w1*w2-w0*w3)*sx + (1_prt-2_prt*(w1*w1+w3*w3))*sy + 2_prt*(w0*w1+w2*w3)*sz;
            T_Real const szout = 2_prt*(w0*w2+w1*w3)*sx + 2_prt*(w2*w3-w0*w1)*sy + (1_prt-2_prt*(w1*w1+w2*w2))*sz;
 
            // Update spin vector
            sx = sxout;
            sy = syout;
            sz = szout;
        }

        template<typename T_Real=amrex::ParticleReal>
        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
        void tbmt_precession_vector (
            T_Real const & AMREX_RESTRICT x,
            T_Real const & AMREX_RESTRICT ux,
            T_Real const & AMREX_RESTRICT uy,
            T_Real const & AMREX_RESTRICT uz,
            T_Real const & AMREX_RESTRICT gamma,
            amrex::ParticleReal const & AMREX_RESTRICT h,
            amrex::ParticleReal const & AMREX_RESTRICT gyro_anomaly,
            T_Real const & AMREX_RESTRICT Bx,
            T_Real const & AMREX_RESTRICT By,
            T_Real const & AMREX_RESTRICT Bz,
            T_Real const & AMREX_RESTRICT Ex,
            T_Real const & AMREX_RESTRICT Ey,
            T_Real const & AMREX_RESTRICT Ez,
            T_Real & AMREX_RESTRICT Omegax,
            T_Real & AMREX_RESTRICT Omegay,
            T_Real & AMREX_RESTRICT Omegaz
        ) const
        {
            using namespace amrex::literals;  // for _prt
            using namespace std; // for cmath(float)
 
            // Evaluation of the full Thomas-BMT precession vector in the Frenet-Serret frame.
            T_Real const beta_gamma = sqrt(gamma*gamma - 1_prt);
            T_Real const beta_gamma_z = uz * beta_gamma;
            T_Real const leading_factor = -(1_prt + h * x) / beta_gamma_z;
            T_Real const udotB = ux*Bx + uy*By + uz*Bz;
            T_Real const ucrossE_x = uy*Ez - uz*Ey;
            T_Real const ucrossE_y = uz*Ex - ux*Ez;
            T_Real const ucrossE_z = ux*Ey - uy*Ex;
            T_Real const c1 = (1_prt + gyro_anomaly*gamma);
            T_Real const c2 = gyro_anomaly * (gamma - 1_prt) * udotB;
            T_Real const c3 = (gyro_anomaly + 1_prt / (1_prt + gamma)) * beta_gamma;
            Omegax = leading_factor * (c1 * Bx - c2 * ux - c3 * ucrossE_x);
            Omegay = leading_factor * (c1 * By - c2 * uy - c3 * ucrossE_y) + h;
            Omegaz = leading_factor * (c1 * Bz - c2 * uz - c3 * ucrossE_z);
        }
 
    };
 
} // namespace impactx::elements::mixin
 
#endif // IMPACTX_ELEMENTS_MIXIN_SPIN_TRANSPORT_H