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Dear Lukas Vogl,
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<div>Thank you for reporting this issue. </div>
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<div>With my current understanding of the precession algorithm, there shouldn’t be any explicit limitation preventing relatively high-frequency fields, but you may have run into an implicit one. As you point out, there are thresholds for “field change” and
time-resolution available which “should” allow to circumvent such resolution issues.</div>
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<div>/*Threshold below which two magnetic fields are considered to be</div>
<div> * in the same direction.*/</div>
<div>#ifndef mc_pol_angular_accuracy</div>
<div>#define mc_pol_angular_accuracy (1.0*DEG2RAD)</div>
<div>#endif</div>
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</div>
<div>/*The maximal timestep taken by neutrons in a const field*/</div>
<div>#ifndef mc_pol_initial_timestep</div>
<div>#define mc_pol_initial_timestep 1e-5;</div>
<div>#endif</div>
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<div>I will looking further into the details within the next coming weeks - and potentially contact you off-list for more detailed discussions.</div>
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<div>Best,</div>
<div>Peter Willendrup </div>
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<blockquote type="cite">
<div>On 18 Oct 2023, at 22.44, Lukas Vogl <Lukas.Vogl@frm2.tum.de> wrote:</div>
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<div>Hi McStas Team,<br>
<br>
At the neutron resonance spin echo instrument RESEDA (FRM II), we are working on implementing a virtual clone of the instrument, which combines our instrument control software with a McStas simulation in the background.<br>
<br>
At the heart of the instrument are radio frequency spin flippers in a longitudinal field geometry. The static field is parallel for the neutron fight path and the rotating RF-field perpendicular to it.<br>
The build-in McStas functionality for neutron spin evolution seems to work up to circular frequencies of 2pi*500 1/s. We want to perform simulations with frequencies in the MHz range.<br>
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<br>
The particular problem we see, is that the spin is no longer flipped by Pi for higher frequencies, and only a partial polarisation in the opposite direction after the flipper is seen.<br>
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I have attached the necessary files to reproduce this problem.<br>
pol-lib.c, pol-lib.h and Pol_Bfield.comp have to be placed in the corresponding folders (share, resp. optics) of the McStas installation version 3.4.<br>
<br>
pol-lib.* and Pol_Bfield.comp are modified to implement a rotating B-field, which is not yet present in the current McStas version.<br>
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I already have tried to change the threshold condition for the time step calculation to allow smaller time steps but this did not help.<br>
Changing the length of the flipper does seem to influence the performance which should also not happen.<br>
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Thus our question is, if the current McStas spin propagation code is at all capable to handle RF-fields with such frequencies and if so what we have to change in our implementation.<br>
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Best Regards<br>
<br>
Lukas Vogl<br>
<br>
Working student at<br>
RESEDA - FRM II<br>
<span id="cid:E4B8DD9D-7D20-41FA-BFFE-3AA9C917CD3E"><pol-lib.h></span><span id="cid:40AEE2FB-CF44-41BB-8620-044C11EFBDEA"><pol-lib.c></span><span id="cid:B8A6D7DE-23E4-4162-8F0F-5D90D92F9670"><Pol_Bfield.comp></span><span id="cid:638362EA-8CB0-4728-91D1-83E2749783D8"><rf_flipper.instr></span>_______________________________________________<br>
mcstas-users mailing list<br>
mcstas-users@mcstas.org<br>
https://mailman2.mcstas.org/mailman/listinfo/mcstas-users<br>
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<div><br>
Peter Kjær Willendrup<br>
Forskningsingeniør, Specialkonsulent<br>
<br>
DTU Physics<br>
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