Add spin helper mixin class.

Author: cemitch99

src/elements/mixin/spintransport.H

@@ -0,0 +1,74 @@
+/* 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
+ * License: BSD-3-Clause-LBNL
+ */
+#ifndef IMPACTX_ELEMENTS_MIXIN_SPIN_TRANSPORT_H
+#define IMPACTX_ELEMENTS_MIXIN_SPIN_TRANSPORT_H
+
+#include "particles/ImpactXParticleContainer.H"
+
+#include <ablastr/constant.H>
+
+#include <AMReX_Math.H>
+#include <AMReX_Extension.H>
+#include <AMReX_REAL.H>
+
+namespace impactx::elements::mixin
+{
+    /** This is a helper class for applying a spin map generated by a three-component vector
+     *  lambda, whose magnitude defines the angle of rotation and whose direction defines
+     *  the axis of rotation. 
+     */
+    struct SpinTransport
+    {
+        /** Rotate the spin vector (sx,sy,sz).
+         *
+         * @param[in] lambdax generator x-component
+         * @param[in] lambday generator y-component
+         * @param[in] lambdaz generator z-component
+         * @param[inout] sx spin vector x-component
+         * @param[inout] sy spin vector y-component
+         * @param[inout] sz spin vector z-component
+         */
+        template<typename T_Real>
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        void rotate_spin (
+            amrex::ParticleReal lambdax,
+            amrex::ParticleReal lambday,
+            amrex::ParticleReal lambdaz,
+            amrex::ParticleReal sx,
+            amrex::ParticleReal sy,
+            amrex::ParticleReal sz
+        ) const
+        {
+            using namespace amrex::literals;  // for _prt
+
+            // Compute quaternion coefficients
+            amrex::ParticleReal angle = std::sqrt(lambdax*lambdax+lambday*lambday+lambdaz*lambdaz);
+            auto const [sin_half_angle, cos_half_angle] = amrex::Math::sincos(angle/2_prt);
+            amrex::ParticleReal w0 = cos_half_angle;
+            amrex::ParticleReal w1 = (theta==0_prt) ? 0_prt : (lambdax/theta)*sin_half_angle;
+            amrex::ParticleReal w2 = (theta==0_prt) ? 0_prt : (lambday/theta)*sin_half_angle;
+            amrex::ParticleReal w3 = (theta==0_prt) ? 0_prt : (lambdaz/theta)*sin_half_angle;
+
+            // Apply rotation
+            amrex::ParticleReal sxout = (1_prt-2_prt*(w2*w2+w3*w3))*sx + 2_prt*(w1*w2+w0*w3)*sy + 2_prt*(w1*w3-w0*w2)*sz;
+            amrex::ParticleReal syout = 2_prt*(w1*w2-w0*w3)*sx + (1_prt-2_prt*(w1*w1+w3*w3))*sy + 2_prt*(w0*w1+w2*w3)*sz;
+            amrex::ParticleReal szout = 2_prt*(w0*w2+w1*w3)*sx + 2_prt*(w2*w3-w0*w1)*sy + (1_prt-2_prt*(w1*w1+w2*w2))*sz;
+
+            // Update spin vector
+            sx = sxout;
+            sy = syout;
+            sz = szout;
+        }
+
+    };
+
+} // namespace impactx::elements::mixin
+
+#endif // IMPACTX_ELEMENTS_MIXIN_SPIN_TRANSPORT_H