[ Identification | Description | Input parameters | Output parameters | Links ]

# The `Source_Optimizer` Component

A component that optimizes the neutron flux passing through the Source_Optimizer in order to have the maximum flux at the Monitor_Optimizer position.

## Identification

- Author: Emmanuel Farhi
- Origin: ILL (France)
- Date: 17 Sept 1999
- Version: 1.7
- Modification history:
  - (v 0.06) EF, Feb 2000;
  - (v.0.07) EF, Mar 10th 2000; (smoothed, parse options). struct
  - (v.0.08) EF, Oct 12th 2000; optim divergence for v and s
  - (v.0.09) EF, Mar 13th 2001; bug on div s (SIGFPE /0 )

## Description

      Principle: The optimizer first (step 1) computes neutron state parameter
    limits passing in the Source_Optimizer, and then (step 2) records a Reference
    source as well as the state (at Source_Optimizer position) of neutrons
    reaching Monitor. The optimized source is defined as a fraction of the
    Reference source plus the distribution of 'good' neutrons reaching the
    Monitor. The optimization then starts (step 3), and focuses new neutrons on
    the Monitor_Optimizer. In fact it changes 'bad' neutrons into 'good' ones
    (that reach the Monitor), acting on their position, spin and divergence or
    velocity. The overall Monitor flux is kept during process. The energy and
    polarisation distributions are kept during optimization as far as possible
    during optimisation. The optimization method considers that all neutron
    parameters - (x,y), (vx,vy,vz) or (vx/v2,vy/v2,v2), (sx,sy,sz) or
    (sx/s2,sy/s2,s2) - are independent.

      Options: The optimized source can be computed regularly ('continuous'
    option) or only once ('not continuous'). The time spent in steps 1 and 2 can
    be reduced for a shorter optimization ('auto').  The neutrons passing during
    steps 1 and 2 can be smoothed for a better neutron weight distribution
    ('smooth' option).

    Source_optimizer can be placed at any position where you want to act on the
    flux, for instance just after the source.
    Monitor_Optimizer should be placed at position(s) to optimize.
    I prefer to put one just before the sample.

    Default parameters bins, step, and keep are 10, 10%  and 10% respectively.
    The option string can be empty (""), which stands for default configuration
    that works fine in usual cases:

     options="continuous optimization, auto mode, smooth, SetXY+SetDivV+SetDivS"

    Possible options are
        continuous      for continuous source optimization (default).
        verbose         displays optimization process (debug purpose).
        auto            uses the shortest possible 'step 1' and 'step 2'
                        and sets 'step' value as required (default).
        smooth          remove possible spikes generated in
                        steps 1 and 2 (default is smooth).
        unactivate      to unactivate the Optimizer.
        no or not       revert next option
        bins=[value=10] set the Number of cells for sampling neutron states
        step=[value=10] Optimizer step in % of simulation.
        keep=[value=10] Percentage of initial source distribution that is kept
        file=[name]     Filename where to save optimized source distributions
                        (no file is generated if not given. Default ext. is .src)
        SetXY           Keywords to indicate what may be changed during
        SetV            optimisation. Default is position, divergence and spin
        SetS            direction ("SetXY+SetDivV+SetdivS"). Choosing the speed
        SetDivV         or spin optimization (SetV or SetS) may modify the energy
        SetDivS         or polarisation distribution (norm of V and S)
                        as the three components are then independent.

    Parameters bins, step and keep can also be entered as optional parameters.

    EXAMPLE: I use the following settings

     optim_s = Source_Optimizer(options="please be clever") (same as empty)
        (...)
     Monitor_Optimizer(xmin=-0.05, xmax=0.05, ymin=-0.05, ymax=0.05,
                optim_comp = optim_s)

    A good optimization needs to record enough non optimized neutrons on Monitor
    during step 2. Typical enhancement in computation speed is by a factor 20.
    This component usually works well.

    NOTE: You must be aware that in some cases (SetV and SetS),
    the optimization might sligtly afect the energy or spin distribution of the
    source. The optimizer tries to do its best anyway.
    Also, some 'spikes' may sometime appear in monitor signals in the course of
    the optimization, coming from non-optimized neutrons with original weight.
    The 'smooth' option minimises this effect (on by default).

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default
--- | --- | --- | ---
options | str | string of options. See Description | 0
bins | 1 | Number of cells for sampling neutron states. | 10
step | 0-100 | Optimizer step in percent of simulation. | 0.1
keep | 0-100 | Percentage of initial source distribution that is kept. | 0.1

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
DEFS | struct | a set of constant values used in the component |
Vars | struct | structure that contains variables used in the component |

## Links

- Source code for `Source_Optimizer.comp`.
- McStas at ILL
- Monitor_Optimizer

[ Identification | Description | Input parameters | Output parameters | Links ]