path: root/Source/Particles/Collision/BackgroundStopping/BackgroundStopping.cpp

/* Copyright 2022 David Grote
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
#include "BackgroundStopping.H"

#include "Utils/Parser/ParserUtils.H"
#include "Utils/ParticleUtils.H"
#include "Utils/WarpXProfilerWrapper.H"
#include "WarpX.H"

#include <AMReX_ParmParse.H>
#include <AMReX_REAL.H>

#include <string>

BackgroundStopping::BackgroundStopping (std::string const collision_name)
    : CollisionBase(collision_name)
{
    using namespace amrex::literals;

    AMREX_ALWAYS_ASSERT_WITH_MESSAGE(m_species_names.size() == 1,
                                     "Background stopping must have exactly one species.");

    const amrex::ParmParse pp_collision_name(collision_name);

    std::string background_type_str;
    pp_collision_name.get("background_type", background_type_str);
    if (background_type_str == "electrons") {
        m_background_type = BackgroundStoppingType::ELECTRONS;
    } else if (background_type_str == "ions") {
        m_background_type = BackgroundStoppingType::IONS;
    } else {
        AMREX_ALWAYS_ASSERT_WITH_MESSAGE(false, "background_type must be either electrons or ions");
    }

    amrex::ParticleReal background_density;
    std::string background_density_str;
    if (utils::parser::queryWithParser(pp_collision_name, "background_density", background_density)) {
        AMREX_ALWAYS_ASSERT_WITH_MESSAGE(background_density > 0_prt,
                 "For background stopping, the background density must be greater than 0");
        m_background_density_parser =
            utils::parser::makeParser(std::to_string(background_density), {"x", "y", "z", "t"});
    } else if (pp_collision_name.query("background_density(x,y,z,t)", background_density_str)) {
        m_background_density_parser =
            utils::parser::makeParser(background_density_str, {"x", "y", "z", "t"});
    } else {
        AMREX_ALWAYS_ASSERT_WITH_MESSAGE(false,
                 "For background stopping, the background density must be specified.");
    }

    amrex::ParticleReal background_temperature;
    std::string background_temperature_str;
    if (utils::parser::queryWithParser(pp_collision_name, "background_temperature", background_temperature)) {
        AMREX_ALWAYS_ASSERT_WITH_MESSAGE(background_temperature > 0_prt,
                 "For background stopping, the background temperature must be greater than 0");
        m_background_temperature_parser =
            utils::parser::makeParser(std::to_string(background_temperature), {"x", "y", "z", "t"});
    } else if (pp_collision_name.query("background_temperature(x,y,z,t)", background_temperature_str)) {
        m_background_temperature_parser =
            utils::parser::makeParser(background_temperature_str, {"x", "y", "z", "t"});
    } else {
        AMREX_ALWAYS_ASSERT_WITH_MESSAGE(false,
                 "For background stopping, the background temperature must be specified.");
    }

    constexpr auto num_parser_args = 4;
    m_background_density_func = m_background_density_parser.compile<num_parser_args>();
    m_background_temperature_func = m_background_temperature_parser.compile<num_parser_args>();

    if (m_background_type == BackgroundStoppingType::ELECTRONS) {
        m_background_mass = PhysConst::m_e;
        utils::parser::queryWithParser(
            pp_collision_name, "background_mass", m_background_mass);
    } else if (m_background_type == BackgroundStoppingType::IONS) {
        utils::parser::getWithParser(
            pp_collision_name, "background_mass", m_background_mass);
        utils::parser::getWithParser(
            pp_collision_name, "background_charge_state", m_background_charge_state);
    }
    AMREX_ALWAYS_ASSERT_WITH_MESSAGE(m_background_mass > 0_prt,
             "For background stopping, the background mass must be greater than 0");

}

void
BackgroundStopping::doCollisions (amrex::Real cur_time, amrex::Real dt, MultiParticleContainer* mypc)
{
    WARPX_PROFILE("BackgroundStopping::doCollisions()");
    using namespace amrex::literals;

    auto& species = mypc->GetParticleContainerFromName(m_species_names[0]);
    amrex::ParticleReal const species_mass = species.getMass();
    amrex::ParticleReal const species_charge = species.getCharge();

    WARPX_ALWAYS_ASSERT_WITH_MESSAGE(species_mass > 0_prt, "Error: With background stopping, the species mass must be > 0");
    WARPX_ALWAYS_ASSERT_WITH_MESSAGE(species_charge != 0_prt, "Error: With background stopping, the species charge must be nonzero");

    const BackgroundStoppingType background_type = m_background_type;

    // Loop over refinement levels
    auto const flvl = species.finestLevel();
    for (int lev = 0; lev <= flvl; ++lev) {

        auto cost = WarpX::getCosts(lev);

        // loop over particles box by box
#ifdef _OPENMP
#pragma omp parallel if (amrex::Gpu::notInLaunchRegion())
#endif
        for (WarpXParIter pti(species, lev); pti.isValid(); ++pti) {
            if (cost && WarpX::load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::Timers)
            {
                amrex::Gpu::synchronize();
            }
            amrex::Real wt = amrex::second();

            if (background_type == BackgroundStoppingType::ELECTRONS) {
                doBackgroundStoppingOnElectronsWithinTile(pti, dt, cur_time, species_mass, species_charge);
            } else if (background_type == BackgroundStoppingType::IONS) {
                doBackgroundStoppingOnIonsWithinTile(pti, dt, cur_time, species_mass, species_charge);
            }

            if (cost && WarpX::load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::Timers)
            {
                amrex::Gpu::synchronize();
                wt = amrex::second() - wt;
                amrex::HostDevice::Atomic::Add(&(*cost)[pti.index()], wt);
            }
        }

    }
}

void BackgroundStopping::doBackgroundStoppingOnElectronsWithinTile (WarpXParIter& pti, amrex::Real dt, amrex::Real t,
                                                                    amrex::ParticleReal species_mass, amrex::ParticleReal species_charge)
{
    using namespace amrex::literals;

    // So that CUDA code gets its intrinsic, not the host-only C++ library version
    using std::sqrt, std::abs, std::log, std::exp;

    // get particle count
    long const np = pti.numParticles();

    // get background particle mass
    amrex::ParticleReal const mass_e = m_background_mass;

    // setup parsers for the background density and temperature
    auto const n_e_func = m_background_density_func;
    auto const T_e_func = m_background_temperature_func;

    // get Struct-Of-Array particle data, also called attribs
    auto& attribs = pti.GetAttribs();
    amrex::ParticleReal* const AMREX_RESTRICT ux = attribs[PIdx::ux].dataPtr();
    amrex::ParticleReal* const AMREX_RESTRICT uy = attribs[PIdx::uy].dataPtr();
    amrex::ParticleReal* const AMREX_RESTRICT uz = attribs[PIdx::uz].dataPtr();

    // May be needed to evaluate the density and/or temperature functions
    auto const GetPosition = GetParticlePosition(pti);

    amrex::ParallelFor(np,
        [=] AMREX_GPU_HOST_DEVICE (long ip)
        {

            amrex::ParticleReal x, y, z;
            GetPosition.AsStored(ip, x, y, z);
            amrex::ParticleReal const n_e = n_e_func(x, y, z, t);
            amrex::ParticleReal const T_e = T_e_func(x, y, z, t)*PhysConst::kb;

            AMREX_ASSERT(n_e > 0_prt);
            AMREX_ASSERT(T_e > 0_prt);

            // This implements the equation 14.12 from Introduction to Plasma Physics,
            // Goldston and Rutherford, the slowing down of beam ions due to collisions with electrons.
            // The equation is written as dV/dt = -alpha*V, and integrated to
            // give V(t+dt) = V(t)*exp(-alpha*dt)

            amrex::ParticleReal constexpr pi = MathConst::pi;
            amrex::ParticleReal constexpr ep0 = PhysConst::ep0;
            amrex::ParticleReal constexpr q_e = PhysConst::q_e;
            amrex::ParticleReal constexpr q_e2 = q_e*q_e;
            amrex::ParticleReal constexpr ep02 = ep0*ep0;

            amrex::ParticleReal const Zb = abs(species_charge/q_e);

            amrex::ParticleReal const vth = sqrt(3_prt*T_e/mass_e);
            amrex::ParticleReal const wp = sqrt(n_e*q_e2/(ep0*mass_e));
            amrex::ParticleReal const lambdadb = vth/wp;
            amrex::ParticleReal const lambdadb3 = lambdadb*lambdadb*lambdadb;
            amrex::ParticleReal const loglambda = log((12_prt*pi/Zb)*(n_e*lambdadb3));

            AMREX_ASSERT(loglambda > 0_prt);

            amrex::ParticleReal const pi32 = pi*sqrt(pi);
            amrex::ParticleReal const q2 = species_charge*species_charge;
            amrex::ParticleReal const T32 = T_e*sqrt(T_e);

            amrex::ParticleReal const alpha = sqrt(2_prt)*n_e*q2*q_e2*sqrt(mass_e)*loglambda/(12_prt*pi32*ep02*species_mass*T32);

            ux[ip] *= exp(-alpha*dt);
            uy[ip] *= exp(-alpha*dt);
            uz[ip] *= exp(-alpha*dt);

        }
        );
}

void BackgroundStopping::doBackgroundStoppingOnIonsWithinTile (WarpXParIter& pti, amrex::Real dt, amrex::Real t,
                                                               amrex::ParticleReal species_mass, amrex::ParticleReal species_charge)
{
    using namespace amrex::literals;

    // So that CUDA code gets its intrinsic, not the host-only C++ library version
    using std::sqrt, std::abs, std::log, std::exp, std::pow;

    // get particle count
    long const np = pti.numParticles();

    // get background particle mass
    amrex::ParticleReal const mass_i = m_background_mass;
    amrex::ParticleReal const charge_state_i = m_background_charge_state;

    // setup parsers for the background density and temperature
    auto const n_i_func = m_background_density_func;
    auto const T_i_func = m_background_temperature_func;

    // get Struct-Of-Array particle data, also called attribs
    auto& attribs = pti.GetAttribs();
    amrex::ParticleReal* const AMREX_RESTRICT ux = attribs[PIdx::ux].dataPtr();
    amrex::ParticleReal* const AMREX_RESTRICT uy = attribs[PIdx::uy].dataPtr();
    amrex::ParticleReal* const AMREX_RESTRICT uz = attribs[PIdx::uz].dataPtr();

    // May be needed to evaluate the density function
    auto const GetPosition = GetParticlePosition(pti);

    amrex::ParallelFor(np,
        [=] AMREX_GPU_HOST_DEVICE (long ip)
        {

            amrex::ParticleReal x, y, z;
            GetPosition.AsStored(ip, x, y, z);
            amrex::ParticleReal const n_i = n_i_func(x, y, z, t);
            amrex::ParticleReal const T_i = T_i_func(x, y, z, t)*PhysConst::kb;

            AMREX_ASSERT(n_i > 0_prt);
            AMREX_ASSERT(T_i > 0_prt);

            // This implements the equation 14.20 from Introduction to Plasma Physics,
            // Goldston and Rutherford, the slowing down of beam ions due to collisions with electrons.
            // The equation is written with energy, W, as dW/dt = -alpha/W**0.5, and integrated to
            // give W(t+dt) = (W(t)**1.5 - 3./2.*alpha*dt)**(2/3)

            amrex::ParticleReal constexpr pi = MathConst::pi;
            amrex::ParticleReal constexpr q_e = PhysConst::q_e;
            amrex::ParticleReal constexpr q_e2 = q_e*q_e;
            amrex::ParticleReal constexpr ep0 = PhysConst::ep0;
            amrex::ParticleReal constexpr ep02 = ep0*ep0;

            amrex::ParticleReal const qi2 = charge_state_i*charge_state_i*q_e2;
            amrex::ParticleReal const qb2 = species_charge*species_charge;
            amrex::ParticleReal const Zb = abs(species_charge/q_e);

            amrex::ParticleReal const vth = sqrt(3_prt*T_i/mass_i);
            amrex::ParticleReal const wp = sqrt(n_i*q_e2/(ep0*mass_i));
            amrex::ParticleReal const lambdadb = vth/wp;
            amrex::ParticleReal const lambdadb3 = lambdadb*lambdadb*lambdadb;
            amrex::ParticleReal const loglambda = log((12_prt*pi/Zb)*(n_i*lambdadb3));

            AMREX_ASSERT(loglambda > 0_prt);

            amrex::ParticleReal const alpha = sqrt(2_prt)*n_i*qi2*qb2*sqrt(species_mass)*loglambda/(8_prt*pi*ep02*mass_i);

            amrex::ParticleReal const W0 = 0.5_prt*species_mass*(ux[ip]*ux[ip] + uy[ip]*uy[ip] + uz[ip]*uz[ip]);
            amrex::ParticleReal const f1 = pow(W0, 1.5_prt) - 1.5_prt*alpha*dt;
            // If f1 goes negative, the particle has fully stopped, so set W1 to 0.
            amrex::ParticleReal const W1 = pow((f1 > 0_prt ? f1 : 0_prt), 2_prt/3_prt);
            amrex::ParticleReal const vscale = (W0 > 0_prt ? std::sqrt(W1/W0) : 0_prt);

            ux[ip] *= vscale;
            uy[ip] *= vscale;
            uz[ip] *= vscale;

        }
        );
}