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

# The `FermiChopper` Component

Fermi Chopper with rotating frame.

## Identification

- Author: M. Poehlmann, C. Carbogno, H. Schober
- Origin: ILL Grenoble / TU Muenchen
- Date: May 2002
- Version: 1.9
- Modification history:
  - K.Lieutenant, June 2005: added phase parameter. Comp validation.

## Description

    Models a fermi chopper with optional supermirror coated blades
    supermirror facilities may be disabled by setting m = 0, R0=0
    Slit packages are straight. Chopper slits are separated by an infinitely
    thin absorbing material. The effective transmission (resulting from fraction
    of the transparent material and its transmission) may be specified.
    The chopper slit package width may be specified through the total width 'width'
    of the full package or the width 'w' of each single slit. The other parameter
    is calculated by: width = Nslit*w.
    The chopper needs a default velocity to calculate the initial value for an
    iterative calculation of TOF values. This can be specified by its velocity,
    energy, wavelength or wavevector.

    Example:
    FermiChopper(phase=-50.0, radius=0.04, nu=100,
      ymin=-0.04, ymax=0.04, w=0.00022475, Nslit=200.0, R0=0.0,
      Qc=0.02176, alpha=2.33, m=0.0, length=0.012, eff=0.95)

    Markus Poehlmann
    Christian Carbogno
    and Helmut Schober

    %VALIDATION
    Apr 2005: extensive external test, most problems solved (cf. 'Bugs')
    Validated by: K. Lieutenant

    limitations:
    no blade width used

    %BUGS
    - overestimates peak width for long wavelengths
    - may not give the right pulse position, shape and width for slit widths below 0.1 mm

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default
--- | --- | --- | ---
phase | deg | chopper phase at t=0 | 0
radius | m | chopper cylinder radius | 0.04
nu | Hz | chopper frequency | 100
ymin | m | lower y bound of cylinder | -0.04
ymax | m | upper y bound of cylinder | 0.04
w | m | width of one chopper slit | 0.00022475
Nslit | 1 | number of chopper slits | 200
R0 | 1 | low-angle reflectivity | 0.0
Qc | AA-1 | critical scattering vector | 0.02176
alpha | AA | slope of reflectivity | 2.33
m | 1 | m-value of material. Zero means completely absorbing. | 0.0
W | AA-1 | width of supermirror cut-off | 2e-3
length | m | channel length of the Fermi chopper | 0.012
eff | 1 | efficiency = transmission x fraction of transparent material | 0.95
zero_time | 1 | set time to zero: 0: no 1: once per half cycle | 0
width | m | optional total width of slit package | 0
verbose | 1 | set to 1 or 2 gives debugging information | 0
rad | - | , slit, alpham,Wi,dist,Vi | 0
slit |  |  | 0
alpham |  |  | 0
Wi |  |  | 0
dist |  |  | 0
Vi |  |  | 0
curvature | m-1 | Curvature of slits (1/radius of curvature). | 0

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
FCVars | - | structure |

## Links

- Source code for `FermiChopper.comp`.
- Vitess_ChopperFermi<\a> component by G. Zsigmond, imported from Vitess by K. Lieutenant.

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