[mcstas-users] Disk chopper angular definitions

[Peter Kjær Willendrup]

Dear Andrew,


Thanks for raising the issue!

I have had a closer look and also written a small test-instrument myself, see attached. This instrument contains a source that fully illuminates a chopper disk in the time-interval of [0 1]s, and with exposed parameters to set phase, delay, nu etc.

I have the following findings:

1) Looking at the outputs, setting the default input-parameters it does in fact look like the order of rotation is left-handed rather than right-handed, see attached plot1.png, run with a 1Hz chopper

2) Putting phase to 90 degrees and parking the chopper there (nu=0), it further looks like the geometry as visualised using mcdisplay is opposite to what the resulting simulation output shows, see mcdisplay.png  and plot2.png



I will file a us a GitHub issue so that we remember to do something about this, likely the below solution as a number of existing instrument-models use the DiskChopper exactly as it looks today:

1) Fix the documentation so that it is clear that the default “sign” of rotation is left-handed

2) Fix mcdisplay so that it reflects what the neutrons will “see” at t=0

3) Potentially add a sign-parameter to choose the “handedness”


Best and thanks for reporting,
Peter



[cid:c797dbdc-e501-44d5-81ca-f5c579d3efff at win.dtu.dk]
[cid:90b68f6b-918a-4e1d-be66-4257549c305f at win.dtu.dk]
[cid:5f66804a-bc37-432f-aa15-8f259a2b5e85 at win.dtu.dk]



> On 11 Jun 2021, at 04.35, Andrew Nelson <andyfaff at gmail.com> wrote:
>
> Dear Markus,
> thank you for the reply. Perhaps naively I would expect the trace to correspond to what happens with the actual simulation.
>
> I have done something along the lines you suggest, see end of email. The TOF monitor is shifted along the +ve x direction, which is to the right when looking from the detector to the source (it'd be roughly where the 'h' label is in fig 6.1)
>
> When nu is positive the TOF monitor has a pulse distribution whose offset is shifted to positive times. I think this means that the midpoint of the disk window has already crossed the y-axis before it gets to the monitor. This must mean that the direction of rotation is clockwise when looking along z from +ve to -ve.
>
> When nu is negative the TOF monitor has a pulse distribution whose offset is shifted to negative times. This must mean that the direction of rotation is anticlockwise when looking along z from +ve to -ve.
>
> Is this assessment correct?
>
> If so, then this is not what I expect from a right hand coordinate system. The direction of rotation from this concept experiment is the opposite of what's displayed in figure 6.1 of the component manual.
>
>
>
>
> ===============================================================
> DEFINE INSTRUMENT template_simple(Par1=1)
>
> DECLARE
> %{
> %}
>
> INITIALIZE
> %{
> %}
>
> TRACE
>
> COMPONENT origin = Progress_bar()
> AT (0, 0, 0) RELATIVE ABSOLUTE
>
> // insert components here (e.g. Insert -> Source -> ...)
> COMPONENT source_div = Source_div(
>     xwidth=0.02,
>     yheight=0.05,
>     focus_aw=1,
>     focus_ah=1,
>     lambda0=2.8,
>     dlambda=0.1)
> AT (0, 0, 0) RELATIVE PREVIOUS
>
> COMPONENT diskchopper = DiskChopper(
>     theta_0=5,
>     yheight=0.1,
>     nu=24,
> isfirst=1)
> AT (0, 0, 0) RELATIVE PREVIOUS
>
> COMPONENT mon = TOF_monitor(
>     filename="mon",
>     xmin=0.005,
>     xmax=0.01,
>     ymin=-0.025,
>     ymax=0.025,
>     tmin=-500,
>     tmax=500,
> restore_neutron=1)
> AT (0, 0, 0) RELATIVE PREVIOUS
>
> FINALLY
> %{
> %}
>
> END
> _______________________________________________
> mcstas-users mailing list
> mcstas-users at mcstas.org
> https://mailman2.mcstas.org/mailman/listinfo/mcstas-users

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