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/* Copyright 2019 David Grote, Maxence Thevenet, Remi Lehe
* Weiqun Zhang
*
* This file is part of WarpX.
*
* License: BSD-3-Clause-LBNL
*/
#ifndef WARPX_PARTICLES_PUSHER_UPDATEMOMENTUM_BORIS_H_
#define WARPX_PARTICLES_PUSHER_UPDATEMOMENTUM_BORIS_H_
#include <AMReX_REAL.H>
/** \brief Push the particle's positions over one timestep,
* given the value of its momenta `ux`, `uy`, `uz` */
AMREX_GPU_HOST_DEVICE AMREX_INLINE
void UpdateMomentumBoris(
amrex::ParticleReal& ux, amrex::ParticleReal& uy, amrex::ParticleReal& uz,
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::Real q, const amrex::Real m, const amrex::Real dt )
{
using namespace amrex::literals;
const amrex::Real econst = 0.5_rt*q*dt/m;
// First half-push for E
ux += econst*Ex;
uy += econst*Ey;
uz += econst*Ez;
// Compute temporary gamma factor
constexpr amrex::Real inv_c2 = 1._rt/(PhysConst::c*PhysConst::c);
const amrex::Real inv_gamma = 1._rt/std::sqrt(1._rt + (ux*ux + uy*uy + uz*uz)*inv_c2);
// Magnetic rotation
// - Compute temporary variables
const amrex::Real tx = econst*inv_gamma*Bx;
const amrex::Real ty = econst*inv_gamma*By;
const amrex::Real tz = econst*inv_gamma*Bz;
const amrex::Real tsqi = 2._rt/(1._rt + tx*tx + ty*ty + tz*tz);
const amrex::Real sx = tx*tsqi;
const amrex::Real sy = ty*tsqi;
const amrex::Real sz = tz*tsqi;
const amrex::Real ux_p = ux + uy*tz - uz*ty;
const amrex::Real uy_p = uy + uz*tx - ux*tz;
const amrex::Real uz_p = uz + ux*ty - uy*tx;
// - Update momentum
ux += uy_p*sz - uz_p*sy;
uy += uz_p*sx - ux_p*sz;
uz += ux_p*sy - uy_p*sx;
// Second half-push for E
ux += econst*Ex;
uy += econst*Ey;
uz += econst*Ez;
}
#endif // WARPX_PARTICLES_PUSHER_UPDATEMOMENTUM_BORIS_H_
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