Editing Input syntax manual (section)

=== wwin (weight window mesh definition)<span id="wwin"></span> ===

 '''wwin''' ''NAME'' 
    [ [[#wwin_wf|'''wf''']] ''FILE'' ''FMT'' ]
    [ [[#wwin_wn|'''wn''']] ''F'' ''X'' ''Y'' ''Z'' ''E'' ] 
    [ [[#wwin_wx|'''wx''']] ''C'' ''G'' ] 
    [ [[#wwin_wt|'''wt''']] ''SB'' ''TYPE'' ''MIN'' ''MAX'' ]
    [ [[#wwin_wi1|'''wi''']] ''ITP'' ''NI'' ''WWG<sub>1</sub>'' ''DF<sub>1</sub>'' ''WWG<sub>2</sub>'' ''DF<sub>2</sub>'' ... ]
    [ [[#wwin_wi2|'''wi''']] ''ITP'' ''NI'' ''WWG'' ''NX'' ''NY'' ''NZ'' ''NLOOP'' ''NTRK'' ''ISPL'' ''NSPL'' ''DSPL<sub>1</sub>'' ''SX<sub>1</sub>'' ''SY<sub>1</sub>'' ''SZ<sub>1</sub>'' ''DSPL<sub>2</sub>'' ''SX<sub>2</sub>'' ''SY<sub>2</sub>'' ''SZ<sub>2</sub>'' ...]

Defines a weight window mesh for variance reduction. The first parameter:

{|
| <tt>''NAME''</tt>
| : a unique name to identify the mesh
|}

The remaining parameters are defined by separate key words followed by the input values.

<u>Notes:</u>

*Only works in external source simulation mode.
*Importance (weight window) meshes can be generated by running the [[#wwgen|response matrix based solver]], or read in MCNP WWINP format<ref>Kulesza, J. A. (ed.), ''“MCNP code version 6.3.0 Theory & User Manual: Appendix A Mesh-Based WWINP, WWOUT, and WWONE File Format,”'' LA-UR-22-30006, Rev. 1, Los Alamos National Laboratory [https://mcnp.lanl.gov/pdf_files/TechReport_2022_LANL_LA-UR-22-30006Rev.1_KuleszaAdamsEtAl.pdf (2022)].</ref>.
*Importance maps can be visualized using the [[#plot|geometry plotter]].
*See also [[#set wwb|set wwb]] and [[#set maxsplit|set maxsplit]] for setting options for weight windows, splitting and Russian roulette.
*See also practical examples on [[Variance reduction]].


<u>Weight-window mesh paramters:</u>

Mesh file (<tt>'''wf'''</tt>):<span id="wwin_wf"></span>

{|
| <tt>''FILE''</tt>
| : file path and name of the importance mesh file
|-
| <tt>''FMT''</tt> 
| : file format (1 = mesh produced by Serpent importance map generator, 2 = MCNP WWINP format weight window mesh file)
|}

<u>Notes:</u>

*By default the importance map is read from the mesh file and used as-is, the additional options are provided for adjustments.
*Currently the MCNP format only supports simple mesh types (no sub-mesh).


Mesh normalization (<tt>'''wn'''</tt>):<span id="wwin_wn"></span>

{|
| <tt>''F''</tt> 
| : importance for renormalization
|-
| <tt>''X,Y,Z''</tt> 
| : coordinates of point used for renormalization
|-
| <tt>''E''</tt> 
| : energy used for renormalization [in MeV]
|}

<u>Notes:</u>

*The importances can be renormalized by fixing the value at a given position and energy.


Mesh adjustment (<tt>'''wx'''</tt>):<span id="wwin_wx"></span>

{|
| <tt>''C''</tt> 
| : constant multiplier for adjusting importances
|-
| <tt>''G''</tt> 
| : exponential for adjusting importances
|}

<u>Notes:</u>

*The importances can be adjusted by constant multiplier <tt>''C''</tt> and exponential factor <tt>''G''</tt> such that <math>F' = CF^G</math>.


Types and options (<tt>'''wt'''</tt>):<span id="wwin_wt"></span>

{|
| <tt>''SB''</tt> 
| : option to set source biasing on (1/yes) or off (0/no) with Serpent-generated importance maps
|-
| <tt>''TYPE''</tt> 
| :  bounds type for Serpent-generated weight-windows (1 = averaged, 2 = segment-wise)
|-
| <tt>''MIN''</tt>
| : minimum truncation limit for importances
|-
| <tt>''MAX''</tt>
| : maximum truncation limit for importances
|}

<u>Notes:</u>

*Source biasing is currently not available


Weight-window iterations, fixed mesh (<tt>'''wi'''</tt>):<span id="wwin_wi1"></span>

{|
| <tt>''ITP''</tt>
| : iteration type (1 = fixed mesh)
|-
| <tt>''NI''</tt>
| : number of iterations between Monte Carlo simulation and the response matrix solver
|-
| <tt>''WWG<sub>i</sub>''</tt>
| :  name of the WWG-structure used in the iteration
|-
| <tt>''DF<sub>i</sub>''</tt>
| :  global density factor
|}

<u>Notes:</u>

*The fixed mesh option (<tt>''ITP''</tt> = 1) allows performing iterations using a single or multiple meshes generated using the [[#wwgen|response matrix based solver]].
*The global density factor is a multiplier applied to all material densities.


Weight-window iterations, adaptive mesh (<tt>'''wi'''</tt>):<span id="wwin_wi2"></span>

{|
| <tt>''ITP''</tt>
| : iteration type (2 = geometry-based adaptation, 3 = tracking-based adaptation)
|-
| <tt>''NI''</tt>
| : number of iterations between Monte Carlo simulation and the response matrix solver
|-
| <tt>''WWG''</tt>
| :  name of the WWG-structure used in the iteration
|-
| <tt>''NX''</tt>
| : number of x-divisions for the adaptive mesh
|-
| <tt>''NY''</tt>
| : number of y-divisions for the adaptive mesh
|-
| <tt>''NZ''</tt>
| : number of z-divisions for the adaptive mesh
|-
| <tt>''NLOOP''</tt>
| : number of outer iteration loops in generation of adaptive mesh
|-
| <tt>''NTRK''</tt>
| : number of tracks per loop in generation of adaptive mesh
|-
| <tt>''ISPL''</tt>
| : importance split criterion
|-
| <tt>''NSPL''</tt>
| : neighbor split criterion
|-
| <tt>''DSPL<sub>i</sub>''</tt>
| : density split criterion (positive value = atomic density [in b<sup>-1</sup>cm<sup>-1</sup>], negative values = mass density [in g/cm<sup>3</sup>])
|-
| <tt>''SZ<sub>i</sub>''</tt>
| : minimum cell dimension [in cm]
|}

<u>Notes:</u>

*The adaptive mesh option (<tt>''ITP''</tt> = 2 or 3) starts with a coarse base mesh, and refines the resolution iteratively.
*There are two adaptive mesh options:
**In the geometry-based option (<tt>''ITP''</tt> = 2) Serpent covers the geometry with <tt>''NTRK''</tt> random tracks and splits cells according to density criteria. 
**In the tracking-based option (<tt>''ITP''</tt> = 3) the tracks are started from the source instead. The procedure is repeated <tt>''NLOOP''</tt> times.
*Cell splitting is defined using the <tt>''NX'', ''NY''</tt> and <tt>''NZ''</tt> options. 
**For example <tt>''NX''</tt> = 2, <tt>''NY''</tt> = 2, <tt>''NZ''</tt> = 2 results in each cell being split to 8 sub-cells (octree mesh). 
**For 2D meshes the <tt>''NZ''</tt> parameter must be set to "1".
*Splitting is carried out recursively, until limiting criteria are met. 
**The importance split criterion defines the maximum relative difference between the importances of two adjacent cells. 
***If the criterion is not met, both cells are split.
**The neighbor split criterion defines the maximum number of neighbor allowed for a cell. 
***If the criterion is not met, the cell is split.
**The <tt>''DSPL''</tt> and <tt>''SZ<sub>i</sub>''</tt> parameters define upper density boundaries and minimum cell sizes for stopping the splits.