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/* Copyright 2020 Luca Fedeli, Neil Zaim
*
* This file is part of WarpX.
*
* License: BSD-3-Clause-LBNL
*/
#ifndef WARPX_schwinger_process_wrapper_h_
#define WARPX_schwinger_process_wrapper_h_
#include "QedWrapperCommons.H"
#include <picsar_qed/physics/schwinger/schwinger_pair_engine_core.hpp>
#include <AMReX_Random.H>
#include <cmath>
/**
* This function is a wrapper around a PICSAR function which calculates
* the number of Schwinger pairs created at a given timestep and in a given
* cell as a function of the EM field in that cell.
*
* @param[in] dV Volume of the cell.
* @param[in] dt temporal step.
* @param[in] Ex x-component of the electric field on the cell.
* @param[in] Ey y-component of the electric field on the cell.
* @param[in] Ez z-component of the electric field on the cell.
* @param[in] Bx x-component of the magnetic field on the cell.
* @param[in] By y-component of the magnetic field on the cell.
* @param[in] Bz z-component of the magnetic field on the cell.
* @param[in] PoissonToGaussianThreshold If the expected number of created
* pairs is below this parameter, a Poisson distribution is used
* to draw the number of created pairs. Otherwise a Gaussian
* distribution is used.
* @return the number of pairs generated via the Schwinger process
*/
AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
amrex::Real
getSchwingerProductionNumber (const amrex::Real dV, const amrex::Real dt,
const amrex::ParticleReal Ex, const amrex::ParticleReal Ey, const amrex::ParticleReal Ez,
const amrex::ParticleReal Bx, const amrex::ParticleReal By, const amrex::ParticleReal Bz,
const amrex::ParticleReal PoissonToGaussianThreshold,
amrex::RandomEngine const& engine)
{
using namespace amrex;
namespace pxr_p = picsar::multi_physics::phys;
namespace pxr_sh = picsar::multi_physics::phys::schwinger;
const auto expectedPairNumber =
pxr_sh::expected_pair_number<amrex::Real, pxr_p::unit_system::SI>(
Ex, Ey, Ez, Bx, By, Bz, dV, dt);
if (expectedPairNumber <= PoissonToGaussianThreshold) {
return amrex::RandomPoisson(expectedPairNumber, engine);
} else {
const auto numpairs =
amrex::RandomNormal(expectedPairNumber, std::sqrt(expectedPairNumber), engine);
return numpairs > 0._rt ? numpairs : 0._rt;
}
}
#endif // WARPX_schwinger_process_wrapper_h_
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