KineticEnergy.H

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/* Copyright 2021 Luca Fedeli
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
 
#ifndef WARPX_PARTICLES_KINETIC_ENERGY_H_
#define WARPX_PARTICLES_KINETIC_ENERGY_H_
 
#include "Utils/WarpXConst.H"
 
#include "AMReX_Extension.H"
#include "AMReX_GpuQualifiers.H"
#include "AMReX_Math.H"
#include "AMReX_REAL.H"
 
#include <cmath>
 
namespace Algorithms{

    template <typename T = amrex::ParticleReal>
    AMREX_GPU_HOST_DEVICE AMREX_INLINE
    T KineticEnergy(
        const amrex::ParticleReal ux, const amrex::ParticleReal uy, const amrex::ParticleReal uz, const amrex::ParticleReal mass)
    {
        using namespace amrex;
 
        constexpr auto one = static_cast<T>(1.0);
        constexpr auto inv_c2 = one/Math::powi<2>(static_cast<T>(PhysConst::c));
 
        // The expression used is derived by reducing the expression
        // (gamma - 1)*(gamma + 1)/(gamma + 1)
 
        const auto u2 = static_cast<T>(ux*ux + uy*uy + uz*uz);
        const auto gamma = std::sqrt(one + u2*inv_c2);
        return one/(one + gamma)*static_cast<T>(mass)*u2;
    }

    AMREX_GPU_HOST_DEVICE AMREX_INLINE
    amrex::ParticleReal KineticEnergyPhotons(
        const amrex::ParticleReal ux, const amrex::ParticleReal uy, const amrex::ParticleReal uz)
    {
        // Photons have zero mass, but ux, uy and uz are calculated assuming a mass equal to the
        // electron mass. Hence, photons need a special treatment to calculate the total energy.
        constexpr auto me_c = PhysConst::m_e * PhysConst::c;
 
        return me_c * std::sqrt(ux*ux + uy*uy + uz*uz);
    }
 
}
 
#endif // WARPX_PARTICLES_KINETIC_ENERGY_H_