Re: [fluka-discuss]: DESCRIBING A BEAM

From: Vittorio Boccone <dr.vittorio.boccone_at_ieee.org>
Date: Sun, 22 Jun 2014 14:36:26 +0200

Hi Zhang,
your first problem lies in the definitions of "beam x" "beam y" and "beam z"
"beam x" and "beam y" are the direction orthogonal to the beam. Can call
the X' and Y' if you want and the "beam z" is the direction pointing to
the beam.

With BEAMPOS and BEAMAXES you actually define the direction cosines of
the beam axis system with respect to the geometrical one.
This is clearly stated in the FLUKA introduction in the beginning of the
manual.

in this example BEAMAXES is needed is you want to redefine the axis used
for beam reference in order to be able to correctly specify the
divergence in such a tilted frame.

Apart for merely disputable-aesthetic reasons, one of the leading
convention in accelerator physics lead people to prefer to have a beam
going in the in Z-direction.

I'm not exactly sure about what you want to do. Do you really want to
define the divergence along the y-geometry axis?

What I would do is to redefine with BEAMAXES your beam reference frame
such that beam goes in the z-direction in the beam reference frame. This
is very useful to make sure that you beam dimension are properly
transformed. You would need to specify the correct cosines that move the
z-geometrical axis on on the z-beam axis.

I would like to recall you note 4 of the BEAM manual entry:

    All options governed by WHAT(3){WHAT(5) 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 (see p. 7.5). If the beam is not in the positive z direction
    and no BEAMAXES command has been given,
    WHAT(3){WHAT(5) must be set = 0.0 (unpredictable effects would arise
    otherwise).

Now... you specify a divergence therefore you need to definea new beam
axis and the 6 direction cosines between the geometrical and the beam
frame system with the BEAMAXES.
What you write seems correct if the divergence you specify is really
along the y-geometry axis.

General hint: you can always give it a try and check where the particles
are going with a simple toy simulation scoring particles with a USRBIN
and plotting them with Flair.

Best,
V.
On 6/21/2014 3:53 PM, 张良 wrote:
>
> Dear FLUKA experts:
>
> I’m a new Fluka user, I have a question in describing a beam. I have
> two questions (they could also be found in the attachment).
>
> Firstly I don’t understand the example in page 78 of the user manual
> as follows:
>
> Example:
>
> * The next option cards describe a 10 GeV’ proton beam with a
> divergence of
>
> * 50 mrad and a gaussian profile in the "beam x"-direction and in the
>
> * "beam y"-direction described by standard deviations sigma_x = 1. cm
>
> * (FWHM = 2.36 cm) and sigma_y = 0.5 cm (FWHM = 1.18 cm). The beam starts
>
> * from point (0,0,0) and is directed in a direction perpendicular to the
>
> * "geometry x" axis, at 45 degrees with respect to both "geometry y" and
>
> * "geometry z". The "beam x" axis has cosines 1,0,0 and the "beam z"
>
> * axis has cosines 0, cos(pi/4), cos(pi/4)
>
> *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
>
> BEAM -10.0 0.0 50.0 -2.36 -1.18 1.0 PROTON
>
> BEAMPOS 0.0 0.0 0.0 0.0 0.7071068 0.0
>
> BEAMAXES 1.0 0.0 0.0 0.0 0.7071068 0.7071068
>
> My question is, “beam z” axis is exactly the beam direction, why
> WHAT(5) (direction cosine of the beam with respect to the y-axis of
> the beam reference frame) in “BEAMPOS” is0.7071068? I think it should
> be 0. I’m really confused with it, could you please answer me the
> question?
>
>
> My second question is:
>
> I want to describe a 1GeV proton with a guass profile in thegeometry
> y-direction described by standard deviations sigma_y = 1cm, the beam
> starts from point(0,0,0) and is perpendicular to the "geometry x"
> axis, and at 60 and 30 degrees with respect to "geometry y" and
> "geometry z".
>
> My description is:
>
> BEAM -1 0 0 0 -2.36 1.0 PROTON
>
> BEAMPOS 0.0 0.0 0.0 0.0 0.5 0.0
>
> BEAMAXES 1.0 0.0 0.0 0 0 1
>
> In fact, I define the beam reference frame exactly the same with the
> geometry frame, and then I define the beam direction with BEAMPOS. Is
> it correct? I really need your help. Thank you very much.
>
>
Received on Sun Jun 22 2014 - 19:26:40 CEST