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One-dimensional periodic CIC deposition and adjoint potential gather
Statement
Take , , , and one particle of charge at . Define the one-dimensional cloud-in-cell (CIC) weights by , where is the periodic distance. The only nonzero weights at are and . The deposited density before neutralization is , and subtracting the uniform background gives For every grid potential , the adjoint identity is
The diagram shows the local CIC stencil between nodes and ; indices on the full grid are understood modulo .
\begin{tikzpicture}[x=2cm,y=1cm,>=stealth,every node/.style={font=\small}] \draw[->] (-0.35,0) -- (2.35,0) node[right] {}; \foreach \j in {0,1,2} { \draw (\j,0.07) -- (\j,-0.07); \fill (\j,0) circle (1.4pt); \node[below=3pt] at (\j,-0.07) {node \j}; } \fill[red] (0.333,0.55) circle (1.6pt); \node[above] at (0.333,0.55) {}; \draw[dashed,red] (0.333,0.52) -- (0,0.04); \draw[dashed,red] (0.333,0.52) -- (1,0.04); \node[red,left] at (-0.05,0.45) {}; \node[red,right] at (0.95,0.45) {}; \draw[<->] (0,0.26) -- node[above] {} (1,0.26); \end{tikzpicture}
Facts & Assumptions
Given: The five-node periodic grid, a particle of charge at , the CIC weights, and the assignment/gather conventions.
Periodic PIC deposition conserves charge and satisfies Poisson compatibility: normalized assignment conserves total charge and the neutralized source has zero mean.
Periodic PIC deposition shape and gather operators: an algebraic assignment shape need only be nonnegative and sum to one; smoothness is an additional condition for the smooth fixed- branch.
Proof
Since , the two nonzero assigned densities are and . The background is , so the neutralized values are ; their sum is , hence their cell-weighted sum is zero.
For any point in a periodic cell , with its right endpoint interpreted modulo , the CIC formula gives and ; all other weights vanish. Both are nonnegative and sum to one. At a grid node this reduces to the single weight one. Hence the stated CIC family is a nonnegative normalized assignment shape.
The pre-neutralized cell-weighted sum is , verifying charge conservation directly; [F3] also applies because step 1.2 proves the shape partition. By [F4], the potential pairing equals . Since , the formula includes exactly the contribution of the uniform background.
Near node , the periodic distance formula gives for and for . The one-sided derivatives at the node are and , so the CIC weight is continuous but not differentiable at this grid crossing. Therefore CIC regularity alone does not supply the vector-field hypothesis of the RK4 error theorem; the page's time-error claim uses the separate smooth fixed- branch.
Depends on
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Sources
- R. W. Hockney and J. W. Eastwood, Computer Simulation Using Particles, Sections 2-2-3 and 5-2 (standard reference, not scraped)