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McStas: Vitess_ChopperFermi Component

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

The Vitess_ChopperFermi Component

Fermi chopper with absorbing walls using the VITESS module 'chopper_fermi'


  • Author: Geza Zsigmond
  • Origin: VITESS module 'chopper_fermi'
  • Date: Sep 2004
  • Version:(Unknown)
  • Modification history:


This component simulates a Fermi chopper with absorbing walls. The rotation axis is
vertical (y-axis), i.e. the path length through the channels is given by the length
'depth' along the z-axis.
The shape of the channels can be straight, curved with circular, or curved with ideal
(i.e. close to a parabolic) shape. This is determined by the parameter 'GeomOption'.

Geometry for straight and circular channels:
The geometry of the chopper consists of a rectangular shaped object with a channel
system. In transmission position, there are 'Nchannels' slits along the x-axis,
separated by absorbing walls of thickness 'wallwidth' giving a total width 'width'.
The rectangular channel system is surrounded by a so-called shadowing cylinder
(cf component manual).

Geometry for parabolic channels:
In this case, the Fermi chopper is supposed to be a full cylinder, i.e. the central
channels are longer than those on the edges (cf. figure in the component manual).
The other features are the same as for the other options.

Apart from the frequency of rotation, the phase of the chopper at t=0 has to be given;
phase = 0 means transmission orientation.

The option 'zerotime' may be used to reset the time at the chopper position. The
consequence is that only 1 pulse is generated instead of several.

NOTE: This component must NOT be located at the same position as the previous one.
      This also stands for monitors and Arms. A non zero distance must be defined.

 straight Fermi chopper, 18000 rpm, 20 channels a 0.9 mm separated by 0.1 mm walls,
 16 mm channel length, minimal shadowing cylinder, phased to be open at 1 ms,
 generation of only 1 pulse, normal precision (for short wavelength neutrons)
Vitess_ChopperFermi(GeomOption=0, zerotime=1,  Nchannels=20,  Ngates=4,
                    freq=300.0,   height=0.06, width=0.0201,
                    depth=0.016,  r_curv=0.0,  diameter=0.025691, Phase=-108.0,
                    wallwidth=0.0001, sGeomFileName="FC_geom_str.dat")

 Fermi chopper with circular channels, 12000 rpm, optimized for 6 Ang, several pulses,
 highest accuracy (because of long wavelength neutrons used), rest as above
Vitess_ChopperFermi(GeomOption=2, zerotime=0,    Nchannels=20,  Ngates=8,
                    freq=200.0,   height=0.06,   width=0.0201,
                    depth=0.016,  r_curv=0.2623, diameter=0.025691, Phase=-72.0,
                    wallwidth=0.0001, sGeomFileName="FC_geom_circ.dat")

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

limitations: slow (10 times slower than FermiChopper), especially for a high number of channels

reduction of transmission by a large shadowing cylinder underestimated

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
sGeomFileName str name of output file for geometry information 0
GeomOption 1 option: 0:straight 1:parabolic 2:circular 0
zerotime 1 option: 1:'set time to zero' 0: 'do not' 0
Nchannels 1 number of channels of the Fermi chopper 20
Ngates 1 number of gates defining the channel: 4=default, 6 or 8 for long wavelengths 4
freq Hz number of rotations per second 300.0
height m height of the Fermi chopper 0.05
width m total width of the Fermi chopper 0.04
depth m channel length of the Fermi chopper 0.03
r_curv m radius of curvature of the curved Fermi chopper 0.5
diameter m diameter of the shadowing cylinder 0.071
Phase deg dephasing angle at zero time 0.0
wallwidth m thickness of walls separating the channels 0.0002

Output parameters

Name Unit Description Default
Option - 1: straight FC, 2: curved FC  
CurvGeomOption - 1: ideal shape (nearly parabolic) 2: circular  
TOF ms TOF of neutron under consideration  
WL Ang wavelength of neutron  
radius_of_curv curved FC [cm] radius of curvature  
main_depth cm max. channel length due to diameter and total_width  
shift_y cm shift to channel actually written to geometry file  
angle_channel rad half of the curvature of a curved Fermi chopper  
phase0 rad chopper phase at TOF of neutron to chopper centre  
y_ch cm position of gates perpendicular to flight direction  
x_ch cm position of gates along flight direction  
coef_pi - number of half-rotation to reach identical state  
GeomFilePtr - pointer to geometry file name  
Pos cm position of neutron  
Dir - flight direction of neutron  
Neutrons - neutron parameter set at end of chopper  
pos_ch cm centre position of the Fermi chopper  
omega 1/s angular frequency of the chopper  
optimal_wl Ang wavelength of highest transmission of curved FC  


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

Generated automatically by McDoc, Peter Willendrup <> / Fri Sep 23 18:47:27 2022

Last Modified: Friday, 23-Sep-2022 20:47:27 CEST
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