C_Gather

概览

C_Gather 收集 PIC 方法中“网格到粒子”的插值例程。它位于场求解器和粒子推进器之间：先从网格场量或形函数模板中得到粒子位置处的局部信息，再把这些信息交给 pusher 或其他粒子算子。

C_Gather 模块
ID	功能	核心接口	典型用途
C01	输入 cell-centered 三维电磁场，输出粒子位置处的 E/B	`sub_C01_gather_3Dxyz` / `sub_C01_gather_3Dxyz_push`	将 cell-centered 三维直角坐标电磁场插值到粒子位置，或在同一循环中完成 gather 和 Boris push。
C02	输入 cell-centered 三维电磁场和粒子位置，输出 B-spline gather 得到的 E/B	`sub_C02_gather_3Dxyz_bspline`	按运行时 `order` 生成 centered B-spline 张量积权重，并直接插值 `Ex/Ey/Ez/Bx/By/Bz`。
C03	输入非均匀柱坐标交错场，输出粒子位置处的 E/B	`sub_C03_gather_3Draz_nonuniform` / `sub_C03_gather_3Draz_nonuniform_point`	按物理面坐标生成 face / center 模板，将静电交错 E 与节点 B 插值到粒子位置。

数值约定

C01/C02 粒子坐标使用网格指标单位。C01 假定电磁场存储在 cell-centered 网格上，并使用 par(1:3,p)+0.5 映射到插值坐标。C02_gather_3Dxyz_bspline 直接对六个电磁场分量执行 B-spline gather，阶数由运行时参数 order 指定。

测试状态

tests/007_gather 提供独立测试，覆盖 C01 的三线性 gather / fused gather-push，以及 C02 的直接 B-spline gather。C03 的独立测试覆盖非均匀交错场插值、边界、周期和错误输入。见 007_gather 测试总览。

Overview

C_Gather contains grid-to-particle interpolation routines used by PIC workflows. These routines sit between field solvers and particle pushers: they obtain local particle-position data from grid fields or shape-function stencils before that information is consumed by a pusher or another particle operator.

C_Gather Modules
ID	Function	Core interface	Typical use
C01	Input cell-centered 3D fields and output particle-position E/B	`sub_C01_gather_3Dxyz` / `sub_C01_gather_3Dxyz_push`	Interpolate cell-centered 3D Cartesian fields to particle positions, or fuse gather with Boris push in one loop.
C02	Input cell-centered 3D fields and a particle position, output B-spline-gathered E/B	`sub_C02_gather_3Dxyz_bspline`	Build centered B-spline tensor-product weights from runtime `order` and directly interpolate `Ex/Ey/Ez/Bx/By/Bz`.
C03	Input nonuniform cylindrical staggered fields and output particle-position E/B	`sub_C03_gather_3Draz_nonuniform` / `sub_C03_gather_3Draz_nonuniform_point`	Build face / center stencils from physical coordinates and gather staggered electrostatic E and nodal B.

Numerical Conventions

C01/C02 use particle coordinates in grid-index units. C01 assumes electromagnetic fields on a cell-centered grid and maps positions with par(1:3,p)+0.5 before interpolation. C02_gather_3Dxyz_bspline directly gathers six electromagnetic-field components with a runtime order argument.

Test Status

tests/007_gather provides standalone tests for C01 trilinear gather / fused gather-push, C02 direct B-spline gather, and C03 nonuniform staggered-field gather, boundaries, periodicity, and invalid inputs. See the 007_gather test overview.