defines the coordinates of the centre of the beam spot (i.e. the point from
which transport starts) and the beam direction
See also BEAM, BEAMAXES, SOURCE
WHAT(1) = x-coordinate of the spot centre.
Can be overridden in user routine SOURCE by assigning a value to
variable XBEAM
Default: 0.0
WHAT(2) = y-coordinate of the spot centre.
Can be overridden in user routine SOURCE by assigning a value to
variable YBEAM
Default: 0.0
WHAT(3) = z-coordinate of the spot centre.
Can be overridden in user routine SOURCE by assigning a value to
variable ZBEAM
Default: 0.0
WHAT(4) = direction cosine of the beam with respect to the x-axis.
Can be overridden in user routine SOURCE by assigning a value to
variable UBEAM
Default: 0.0
WHAT(5) = direction cosine of the beam with respect to the y-axis.
Can be overridden in user routine SOURCE by assigning a value to
variable VBEAM
Default: 0.0
WHAT(6) : not used
SDUM = NEGATIVE means that the direction cosine with respect to z-axis
is negative.
The value of the direction cosine with respect to the z-axis can
be overridden in user routine SOURCE by assigning a value to
variable WBEAM (make sure that the three cosines are properly
normalised so that the sum of their squares is 1.0!!)
Deafault: beam directed in the positive z-direction
Default (option BEAMPOS not requested): all the above defaults apply (the
beam starts at point 0., 0., 0. in the z direction)
Notes:
1) To take full advantage of some tracking optimisation features, it is
often a good idea to create a buffer vacuum region containing the
whole geometry, which must itself be contained within the external
(mandatory) blackhole region. It is then suggested that the beam
impact point be chosen in vacuum, slightly upstream of the actual
one on a material boundary. As a general rule, anyway, it is
recommended to never select the impact point EXACTLY on a boundary.
2) The beam spot coordinates and the beam director cosines as defined
with the BEAMPOS card are available to user routines with names
XBEAM, YBEAM, ZBEAM and UBEAM, VBEAM, WBEAM respectively. These
variables, as well as those defining other beam properties, are in
COMMON BEAMCM which can be accessed with the INCLUDE file (BEAMCM).
3) Beam divergence and transversal profile defined by option BEAM, as
well as polarisation defined by option POLARIZAti, are meaningful
only if the beam direction is along the positive z-axis, unless a
command BEAMAXES is issued to establish a beam reference frame
different from the geometry frame
4) When an isotropic source is defined (by setting WHAT(3) > 2000 pi),
any cosines defined by option BEAMPOS become meaningless, although
their values are still reported on standard output.
Examples:
* A beam parallel to the x-axis starting at a point of
* coordinates -0.1, 5.0, 5.0 :
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7...
BEAMPOS -0.1 5.0 5.0 1.0 0.0 0.0
* A beam perpendicular to the x-axis, with director cosines
* 0., 1/sqrt(2), -1/sqrt(2) with respect to x, y and z,
* starting at point 0.0, 0.0, 0.0 :
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
BEAMPOS 0.0 0.0 0.0 0.0 0.7071068 0.0 NEGATIVE
