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/*******************************************************************************
*
* McStas, neutron ray-tracing package
* Copyright (C) 1997-2008, All rights reserved
* Risoe National Laboratory, Roskilde, Denmark
* Institut Laue Langevin, Grenoble, France
*
* Component: DiskChopper
*
* %I
* Written by: Peter Willendrup
* Date: March 9 2006
* Origin: Risoe
* Based on Chopper (Philipp Bernhardt), Jitter and beamstop from work by
* Kaspar Hewitt Klenoe (jan 2006), adjustments by Rob Bewey (march 2006)
*
* %D
* Models a disc chopper with nslit identical slits, which are symmetrically distributed
* on the disc. At time t=0, the centre of the first slit opening will be situated at the
* vertical axis when phase=0, assuming the chopper centre of rotation is placed <b>BELOW</b> the beam axis.
* If you want to place the chopper <b>ABOVE</b> the beam axis, please use a 180 degree rotation around Z
* (otherwise unexpected beam splitting can occur in combination with the isfirst=1 setting, see
* <a href="https://github.com/McStasMcXtrace/McCode/issues/650">related bug on GitHub</a>)
*
* For more complicated gemometries, see component manual example of DiskChopper GROUPing.
*
* If the chopper is the 1st chopper of a continuous source instrument, you should use the "isfirst" parameter.
* This parameter SETS the neutron time to match the passage of the chooper slit(s), taking into account the
* chopper timing and phasing (thus conserving your simulated statistics).
*
* The isfirst parameter is ONLY relevant for use in continuous source settings.
*
* Example: DiskChopper(radius=0.2, theta_0=10, nu=41.7, nslit=3, delay=0, isfirst=1) First chopper
* DiskChopper(radius=0.2, theta_0=10, nu=41.7, nslit=3, delay=0, isfirst=0)
*
* NOTA BENE wrt. GROUPing and isfirst:
* When setting up a GROUP of DiskChoppers for a steady-state / reactor source, you will need
* to set up
* 1) An initial chopper with isfirst=1, NOT part of the GROUP - and using a "big" chopper opening
* that spans the full angular extent of the openings of the subsequent GROUP
* 2) Add your DiskChopper GROUP setting isfirst=0
*
* %P
* INPUT PARAMETERS:
*
* theta_0: [deg] Angular width of the slits.
* yheight: [m] Slit height (if = 0, equal to radius). Auto centering of beam at half height.
* radius: [m] Radius of the disc
* nu: [Hz] Frequency of the Chopper, omega=2*PI*nu (algebraic sign defines the direction of rotation)
* nslit: [1] Number of slits, regularly arranged around the disk
*
* Optional parameters:
* isfirst: [0/1] Set it to 1 for the first chopper position in a cw source (it then spreads the neutron time distribution)
* n_pulse: [1] Number of pulses (Only if isfirst)
* jitter: [s] Jitter in the time phase
* abs_out: [0/1] Absorb neutrons hitting outside of chopper radius?
* delay: [s] Time 'delay'
* phase: [deg] Angular 'delay' (overrides delay)
* xwidth: [m] Horizontal slit width opening at beam center
* verbose: [1] Set to 1 to display Disk chopper configuration
*
* %E
*******************************************************************************/
DEFINE COMPONENT DiskChopper
SETTING PARAMETERS (theta_0=0, radius=0.5, yheight, nu, nslit=3, jitter=0, delay=0, isfirst=0, n_pulse=1, abs_out=1, phase=0, xwidth=0, verbose=0)
/* Neutron parameters: (x,y,z,vx,vy,vz,t,sx,sy,sz,p) */
DECLARE
%{
double Tg;
double To;
double delta_y;
double height;
double omega;
%}
INITIALIZE
%{
/* If slit height 'unset', assume full opening */
if (yheight == 0) {
height=radius;
} else {
height=yheight;
}
delta_y = radius-height/2; /* radius at beam center */
omega=2.0*PI*nu; /* rad/s */
if (xwidth && !theta_0 && radius) theta_0 = 2*RAD2DEG*asin(xwidth/2/delta_y);
if (nslit<=0 || theta_0 <= 0 || radius <=0)
{ fprintf(stderr,"DiskChopper: %s: nslit, theta_0 and radius must be > 0\n", NAME_CURRENT_COMP);
exit(-1); }
if (nslit*theta_0 >= 360)
{ fprintf(stderr,"DiskChopper: %s: nslit * theta_0 exceeds 2PI\n", NAME_CURRENT_COMP);
exit(-1); }
if (yheight && yheight>radius) {
fprintf(stderr,"DiskChopper: %s: yheight must be < radius\n", NAME_CURRENT_COMP);
exit(-1); }
if (isfirst && n_pulse <=0)
{ fprintf(stderr,"DiskChopper: %s: wrong First chopper pulse number (n_pulse=%g)\n", NAME_CURRENT_COMP, n_pulse);
exit(-1); }
if (!omega) {
fprintf(stderr,"DiskChopper: %s WARNING: chopper frequency is 0!\n", NAME_CURRENT_COMP);
omega = 1e-15; /* We should actually use machine epsilon here... */
}
if (!abs_out) {
fprintf(stderr,"DiskChopper: %s WARNING: chopper will NOT absorb neutrons outside radius %g [m]\n", NAME_CURRENT_COMP, radius);
}
theta_0*=DEG2RAD;
/* Calulate delay from phase and vice versa */
if (phase) {
if (delay) {
fprintf(stderr,"DiskChopper: %s WARNING: delay AND phase specified. Using phase setting\n", NAME_CURRENT_COMP);
}
phase*=DEG2RAD;
/* 'Delay' should always be a delay, taking rotation direction into account: */
delay=phase/fabs(omega);
} else {
phase=delay*omega; /* rad */
}
/* Time from opening of slit to next opening of slit */
Tg=2.0*PI/fabs(omega)/nslit;
/* How long can neutrons pass the Chopper at a single point */
To=theta_0/fabs(omega);
if (!xwidth) xwidth=2*delta_y*sin(theta_0/2);
if (verbose && nu) {
printf("DiskChopper: %s: frequency=%g [Hz] %g [rpm], time frame=%g [s] phase=%g [deg]\n",
NAME_CURRENT_COMP, nu, nu*60, Tg, phase*RAD2DEG);
printf(" %g slits, angle=%g [deg] height=%g [m], width=%g [m] at radius=%g [m]\n",
nslit, theta_0*RAD2DEG, height, xwidth, delta_y);
}
%}
TRACE
%{
double toff;
double yprime;
PROP_Z0;
yprime = y+delta_y;
/* Is neutron outside the vertical slit range and should we absorb? */
if (abs_out && (x*x+yprime*yprime)>radius*radius) {
ABSORB;
}
/* Does neutron hit inner solid part of chopper in case of yheight!=radius? */
if ((x*x+yprime*yprime)<(radius-height)*(radius-height)) {
ABSORB;
}
if (isfirst)
{
/* all events are put in the transmitted time frame */
t=atan2(x,yprime)/omega + To*randpm1()/2.0 + delay + (jitter ? jitter*randnorm():0) + (n_pulse > 1 ? floor(n_pulse*rand01())*Tg : 0);
/* correction: chopper slits transmission opening/full disk */
p *= nslit*theta_0/2.0/PI;
}
else
{
toff=fabs(t-atan2(x,yprime)/omega - delay - (jitter ? jitter*randnorm():0));
/* does neutron hit outside slit? */
if (fmod(toff+To/2.0,Tg)>To) ABSORB;
}
SCATTER;
%}
MCDISPLAY
%{
int j;
/* Arrays for storing geometry of slit/beamstop */
circle("xy", 0, -delta_y, 0, radius);
/* Drawing the slit(s) */
for (j=0; j<nslit; j++) {
/* Angular start/end of slit */
double tmin = j*(2.0*PI/nslit) - theta_0/2.0 + phase;
double tmax = tmin+theta_0;
/* Draw lines for each slit. */
line(
radius*sin(tmin), radius*cos(tmin)-delta_y, 0,
(radius-height)*sin(tmin), (radius-height)*cos(tmin)-delta_y, 0
);
line(
(radius-height)*sin(tmin), (radius-height)*cos(tmin)-delta_y, 0,
(radius-height)*sin(tmax), (radius-height)*cos(tmax)-delta_y, 0);
line(
(radius-height)*sin(tmax), (radius-height)*cos(tmax)-delta_y, 0,
radius*sin(tmax), radius*cos(tmax)-delta_y, 0);
}
%}
END
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