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/* Copyright 2021 Luca Fedeli
*
* This file is part of WarpX.
*
* License: BSD-3-Clause-LBNL
*/
#ifndef PARTICLES_KINETIC_ENERGY_H_
#define PARTICLES_KINETIC_ENERGY_H_
#include "Utils/WarpXConst.H"
#include "AMReX_Extension.H"
#include "AMReX_GpuQualifiers.H"
#include "AMReX_REAL.H"
#include <cmath>
namespace Algorithms{
/**
* \brief Computes the kinetic energy of a particle.
* This method should not be used with photons.
*
* @param[in] ux x component of the particle momentum (code units)
* @param[in] uy y component of the particle momentum (code units)
* @param[in] uz z component of the particle momentum (code units)
* @param[in] mass mass of the particle (in S.I. units)
*
* @return the kinetic energy of the particle (in S.I. units)
*/
AMREX_GPU_HOST_DEVICE AMREX_INLINE
amrex::ParticleReal KineticEnergy(
const amrex::ParticleReal ux, const amrex::ParticleReal uy, const amrex::ParticleReal uz,
const amrex::ParticleReal mass)
{
using namespace amrex;
constexpr auto inv_c2 = 1.0_prt/(PhysConst::c * PhysConst::c);
// The expression used is derived by reducing the expression
// (gamma - 1)*(gamma + 1)/(gamma + 1)
const auto u2 = ux*ux + uy*uy + uz*uz;
const auto gamma = std::sqrt(1.0_prt + u2*inv_c2);
return 1.0_prt/(1.0_prt + gamma)*mass*u2;
}
/**
* \brief Computes the kinetic energy of a photon.
*
* @param[in] ux x component of the particle momentum (code units)
* @param[in] uy y component of the particle momentum (code units)
* @param[in] uz z component of the particle momentum (code units)
*
* @return the kinetic energy of the photon (in S.I. units)
*/
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 // PARTICLES_ALGORITHMS_H_
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