PROP_NO_G_ functions

docs/manuals/mcstas/kernelcalls.tex

@@ -79,6 +79,14 @@ \subsection{Neutron propagation}
 \item \textbfMCRH{PROP\_GRAV\_DT}$(dt,Ax,Ay,Az)$. Like \textbf{PROP\_DT}, but it also
   includes gravity using the acceleration $(Ax,Ay,Az)$. In addition
   to adjusting $(x,y,z)$ and $t$, also $(vx,vy,vz)$ is modified.
+\item \textbfMCRH{PROP\_NO\_G\_DT}$(dt)$. Propagates the neutron through the
+  time interval $dt$ using straight-line motion, regardless of the global
+  gravitation setting. The negative-time and back-propagation rules are the
+  same as for \textbf{PROP\_DT}.
+\item \textbfMCRH{PROP\_NO\_G\_X0}, \textbfMCRH{PROP\_NO\_G\_Y0}, and
+  \textbfMCRH{PROP\_NO\_G\_Z0}. Propagate the neutron to the corresponding
+  local coordinate plane using straight-line motion, regardless of the global
+  gravitation setting.
 \item \textbfMCRH{ALLOW\_BACKPROP}. Indicates that the next propagation routine
   will not remove the neutron ray, even if negative propagation times
   are found. Subsequent propagations are not affected.

mccode/nlib/share/mcstas-r.h

@@ -201,6 +201,30 @@ void SCATTER_func(_class_particle *_particle); /* provides function to SCATTER f
   } while(0)
 
 
+/* Propagation variants that always use straight-line motion. */
+#define PROP_NO_G_DT(dt) \
+  do { \
+    if(dt < 0 && allow_backprop == 0) { RESTORE=1; ABSORB; }; \
+    mcPROP_DT(dt); \
+    DISALLOW_BACKPROP; \
+  } while(0)
+
+#define PROP_NO_G_Z0 \
+  do { \
+    mcPROP_Z0; \
+  } while(0)
+
+#define PROP_NO_G_X0 \
+  do { \
+    mcPROP_X0; \
+  } while(0)
+
+#define PROP_NO_G_Y0 \
+  do { \
+    mcPROP_Y0; \
+  } while(0)
+
+
 #ifdef DEBUG
 
 #define DEBUG_STATE() if(!mcdotrace); else \

mcstas-comps/examples/Tests_grammar/Unittest_PROP_NO_G/Unittest_PROP_NO_G.instr

@@ -0,0 +1,51 @@
+/*******************************************************************************
+*         McStas, neutron ray-tracing package
+*
+* Instrument: Unittest_PROP_NO_G
+*
+* %Identification
+* Written by: Mads Bertelsen
+* Date: September 2026
+* Origin: ESS
+* %INSTRUMENT_SITE: Tests_grammar
+*
+* Test for the PROP_NO_G_DT propagation macro.
+*
+* %Description
+* A long-wavelength beam is focused onto a small detector 10 m away. The
+* intermediate Arm uses PROP_NO_G_DT, so the beam should remain focused even
+* when gravity is enabled.
+*
+* %Example: -g Detector: PropNoGMonitor_I=7.95774e-8
+*
+* %End
+*******************************************************************************/
+DEFINE INSTRUMENT Unittest_PROP_NO_G()
+
+TRACE
+
+COMPONENT Origin = Progress_bar()
+  AT (0, 0, 0) ABSOLUTE
+
+COMPONENT Source = Source_simple(
+    radius = 0.01,
+    dist = 10,
+    focus_xw = 0.01,
+    focus_yh = 0.01,
+    lambda0 = 30,
+    dlambda = 0,
+    flux = 0)
+  AT (0, 0, 0) RELATIVE Origin
+
+COMPONENT PropNoG = Arm()
+  AT (0, 0, 0) RELATIVE Source
+EXTEND %{
+  PROP_NO_G_DT(10/vz);
+%}
+
+COMPONENT PropNoGMonitor = Monitor(
+    xwidth = 0.01,
+    yheight = 0.01)
+  AT (0, 0, 10) RELATIVE PropNoG
+
+END

support/common/editors/mccode.lang

@@ -397,9 +397,13 @@
         <keyword>RESTORE_NEUTRON</keyword>
         <keyword>PROP_GRAV_DT</keyword>
         <keyword>PROP_DT</keyword>
+        <keyword>PROP_NO_G_DT</keyword>
         <keyword>PROP_Z0</keyword>
         <keyword>PROP_X0</keyword>
         <keyword>PROP_Y0</keyword>
+        <keyword>PROP_NO_G_Z0</keyword>
+        <keyword>PROP_NO_G_X0</keyword>
+        <keyword>PROP_NO_G_Y0</keyword>
         <keyword>vec_prod</keyword>
         <keyword>scalar_prod</keyword>
         <keyword>NORM</keyword>

support/common/editors/mccode.vim

@@ -65,10 +65,14 @@
 	\ RESTORE_NEUTRON 
 	\ PROP_GRAV_DT 
 	\ PROP_DT 
+	\ PROP_NO_G_DT
         \ PROP_DL
 	\ PROP_Z0 
 	\ PROP_X0 
 	\ PROP_Y0 
+	\ PROP_NO_G_Z0
+	\ PROP_NO_G_X0
+	\ PROP_NO_G_Y0
 	\ vec_prod 
 	\ scalar_prod 
 	\ NORM