The `Vitess_ChopperFermi` Component

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

Identification

Author: Geza Zsigmond
Origin: VITESS module 'chopper_fermi'
Date: Sep 2004
Version: 1.10
Modification history:
- adapted for McStas by K. Lieutenant, Mar 2005;

Description

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.

Examples:
 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")

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

limitations:
slow, especially for a high number of channels

%BUGS
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
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
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
XFILEName
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

The Vitess_ChopperFermi Component

Identification

Description

Input parameters

Output parameters

Links

The `Vitess_ChopperFermi` Component