Buncher.H

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/* Copyright 2022-2023 The Regents of the University of California, through Lawrence
 *           Berkeley National Laboratory (subject to receipt of any required
 *           approvals from the U.S. Dept. of Energy). All rights reserved.
 *
 * This file is part of ImpactX.
 *
 * Authors: Chad Mitchell, Axel Huebl
 * License: BSD-3-Clause-LBNL
 */
#ifndef IMPACTX_BUNCHER_H
#define IMPACTX_BUNCHER_H
 
#include "particles/ImpactXParticleContainer.H"
#include "mixin/alignment.H"
#include "mixin/beamoptic.H"
#include "mixin/thin.H"
#include "mixin/lineartransport.H"
#include "mixin/named.H"
#include "mixin/nofinalize.H"
 
#include <AMReX_Extension.H>
#include <AMReX_Math.H>
#include <AMReX_REAL.H>
#include <AMReX_SIMD.H>
 
#include <cmath>
#include <stdexcept>
 
 
namespace impactx::elements
{
    struct Buncher
    : public mixin::Named,
      public mixin::BeamOptic<Buncher>,
      public mixin::LinearTransport<Buncher>,
      public mixin::Thin,
      public mixin::Alignment,
      public mixin::NoFinalize
      // At least on Intel AVX512, there is a small overhead to vectorize this element, see
      // https://github.com/BLAST-ImpactX/impactx/pull/1002
      // public amrex::simd::Vectorized<amrex::simd::native_simd_size_particlereal>
    {
        static constexpr auto type = "Buncher";
        using PType = ImpactXParticleContainer::ParticleType;

        Buncher (
            amrex::ParticleReal V,
            amrex::ParticleReal k,
            amrex::ParticleReal dx = 0,
            amrex::ParticleReal dy = 0,
            amrex::ParticleReal rotation_degree = 0,
            std::optional<std::string> name = std::nullopt
        )
        : Named(std::move(name)),
          Alignment(dx, dy, rotation_degree),
          m_V(V), m_k(k)
        {
        }

        using BeamOptic::operator();

        void compute_constants (RefPart const & refpart)
        {
            using namespace amrex::literals; // for _rt and _prt
            using amrex::Math::powi;
 
            Alignment::compute_constants(refpart);
 
            // find beta*gamma^2
            amrex::ParticleReal const betgam2 = powi<2>(refpart.pt) - 1.0_prt;
 
            m_neg_kV = -m_k * m_V;
            m_kV_r2bg2 = -m_neg_kV / (2.0_prt * betgam2);
        }

        template<typename T_Real=amrex::ParticleReal, typename T_IdCpu=uint64_t>
        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
        void operator() (
            T_Real & AMREX_RESTRICT x,
            T_Real & AMREX_RESTRICT y,
            T_Real & AMREX_RESTRICT t,
            T_Real & AMREX_RESTRICT px,
            T_Real & AMREX_RESTRICT py,
            T_Real & AMREX_RESTRICT pt,
            [[maybe_unused]] T_IdCpu & AMREX_RESTRICT idcpu,
            [[maybe_unused]] RefPart const & AMREX_RESTRICT refpart
        ) const
        {
            using namespace amrex::literals; // for _rt and _prt
 
            // shift due to alignment errors of the element
            shift_in(x, y, px, py);
 
            // initialize output values of momenta
            T_Real pxout = px;
            T_Real pyout = py;
            T_Real ptout = pt;
 
            // advance position and momentum
            pxout = px + m_kV_r2bg2 * x;
            pyout = py + m_kV_r2bg2 * y;
            ptout = pt + m_neg_kV   * t;
 
            // assign updated momenta
            px = pxout;
            py = pyout;
            pt = ptout;
 
            // undo shift due to alignment errors of the element
            shift_out(x, y, px, py);
        }

        using Thin::operator();

        using LinearTransport::operator();

        AMREX_GPU_HOST AMREX_FORCE_INLINE
        Map6x6
        transport_map ([[maybe_unused]] RefPart const & AMREX_RESTRICT refpart) const
        {
            using namespace amrex::literals; // for _rt and _prt
            using amrex::Math::powi;
 
            // find beta*gamma^2
            amrex::ParticleReal const betgam2 = powi<2>(refpart.pt) - 1.0_prt;
 
            amrex::ParticleReal const neg_kV = -m_k * m_V;
            amrex::ParticleReal const kV_r2bg2 = -neg_kV / (2.0_prt * betgam2);
 
            // initialize linear map matrix elements
            Map6x6 R = Map6x6::Identity();
 
            // off-identity matrix elements
            R(2,1) = kV_r2bg2;
            R(4,3) = kV_r2bg2;
            R(6,5) = neg_kV;
 
            return R;
        }
 
        amrex::ParticleReal m_V; 
        amrex::ParticleReal m_k; 
 
      private:
        // constants that are independent of the individually tracked particle,
        // see: compute_constants() to refresh
        amrex::ParticleReal m_neg_kV;    
        amrex::ParticleReal m_kV_r2bg2;  
    };
 
} // namespace impactx
 
#endif // IMPACTX_BUNCHER_H