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McStas: FermiChopper Component

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

The FermiChopper Component

Fermi Chopper with rotating frame.


  • Author: M. Poehlmann, C. Carbogno, H. Schober, E. Farhi
  • Origin: ILL Grenoble / TU Muenchen
  • Date: May 2002
  • Version:(Unknown)
  • Modification history:
    • K.Lieutenant, June 2005: added phase parameter. Comp validation.
    • EF, Nov 2005: completely rewrote comp.
    • EF, Oct 2008: fix chopper orientation
    • EF, Mar 2009: improved intersection algorithm


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 'xwidth'
of the full package or the width 'w' of each single slit. The other parameter
is calculated by: xwidth = nslit*w. The slit package may be made curved and use
super-mirror coating.

FermiChopper(phase=-50.0, radius=0.04, nu=100, yheight=0.08, w=0.00022475, nslit=200.0, R0=0.0, Qc=0.02176, alpha=2.33, m=0.0, length=0.012, eff=0.95)

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

May 2017: Issues in use of supermirror slit packages are being investigated. Is NOT solved for McStas 2.4. The component works correctly when not using supermirrors.
We suggest to instead use Guide_channeled with the rotate/derotate option shown in the test instrument Test_Fermi.instr. 

no absorbing blade width used

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. Omega=2*PI*nu in rad/s, nu*60 in rpm. Positive value corresponds to a counter-clockwise rotation around y. 100
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, 2=auto adjust phase 0
xwidth m optional total width of slit package 0
verbose 1 set to 1,2 or 3 gives debugging information 0
yheight m height of slit package 0.08
curvature m-1 Curvature of slits (1/radius of curvature). 0
delay s sets phase so that transmision is centered on 'delay' 0

Output parameters

Name Unit Description Default


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

Generated automatically by McDoc, Peter Willendrup <> / Mon May 22 14:59:10 2017

Last Modified: Monday, 22-May-2017 16:59:10 CEST
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