[mcstas-users] Antwort: Re: Twisted guide simulations

Jean-Francois.Moulin at hzg.de Jean-Francois.Moulin at hzg.de
Mon Jun 8 11:11:36 CEST 2015

Dear Peter, Peter, Kim and all others,

I discussed the problem a long time ago with Emmanuel, he even helped me starting to write a special component which would describe our twisted guide (at refsans a full 90 degree twist of the guide is used to go from vertical to horizontal beam) taking gravity into account. The prototype code did not work -yet- however. I also looked into writing the exact solution for the problem but that required very intensive numerical calculation and was clearly not a viable option.
Using guide anyshape and the description of the surface with *many* polygons proved also non trivial (I do not remember exactly what the problem was, I think the full 90 degrees rotation brought some funky effects in the description of the normal). Moreover if I remember correctly gravity might be a problem with this component.

So, this project unfortunately never came to completion... it stays on my todo list though and I would be happy to read about your opinion/ideas. I'll dig into archives to find the code for the component (which I remember we called twist_again, a sure sign of Emmanuel's touch ;0)


Helmholtz Zentrum Geesthacht
Institut für Werkstoffforschung
Abteilung WPN, Instrument REFSANS
Lichtenbergstr. 1
85747 Garching FRM II
Tel.: +49 (0)89 289 10762
Internet: http://www.frm2.tum.de

-----"mcstas-users" <mcstas-users-bounces at mcstas.org> schrieb: -----
An: mcstas-users at mcstas.org
Von: Peter Link 
Gesendet von: "mcstas-users" 
Datum: 08.06.2015 09:26
Betreff: Re: [mcstas-users] Twisted guide simulations

Dear Peter and Kim,
since REFSANS at FRMII falls under my regime I had several times thought of
this problem. To my opinion and tests a segmentation into piecewise
straight elements with whatever Guide_xx component does not give the
correct result. One way could be to use Guide_anyshape from Emmanuel. I
have not tested this, but I think its worth a try.
Best regards,

Am 07.06.2015 um 17:20 schrieb Kim Lefmann:
> Dear Peter,
> I simulated a very long curved guide recently, using piecewise straight
> segments. I realized that I had propagating uncertainties that blew up
> divergence and hence reduced intensity down the guide. While I believe
> this is a correct physical effect (after some thought), I found that to
> emulate a continuously curved guide I had to go to ridiculously short
> guide pieces, like 2-5 cm. Then, intensity was conserved... Perhaps this
> works for you?
> best, Kim
>  On 06/07/2015 10:42 AM, Петр Коник wrote:
>> Dear McStas colleagues,
>> I try to simulate twisted neutron guide (as RefSANS at FRM-II). I've made
>> it of number of straight section rotated relative common axis, but
>> that gives me significantly lower flux than expected. Is there any
>> natural route to model that guide in McStas?
>> Best regards,
>> Peter Konik
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Dr. Peter Link
Leiter Neutronenoptik
Forschungs-Neutronenquelle Heinz Maier-Leibnitz
Technische Universität München
Lichtenbergstr. 1
85747 Garching
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