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Forum: Fields: Magnetic and Otherwise
Re: Question Stepsize Underflow in Stepper (Evan Adamek)
Date: 16 Jul, 2013
From: John Apostolakis <John Apostolakis>

Dear Evan,

What you report is indeed abnormal - but it is not clear though what is causing it.  More information is needed to try to understand its origin.

Potentially what you are seeing was triggered by a move to a different version of Geant4 - with which Geant4UCN has not been tested.  Can you check and let us know what version of Geant4 you were using on your previous computer - from the output you attached it is clear that you are now using Geant4 9.6-patch-02.

It is possible to get a primitive verbose output from G4PropagatorInField (which calls G4MagIntegratorDriver through other classes) by recompiling the Geometry module with the flag "G4DEBUG_FIELD", or simply by commenting out the "#ifdef G4DEBUG_FIELD as in the revised lines below (from :

// #ifdef G4DEBUG_FIELD
    if( fNoZeroStep > fActionThreshold_NoZeroSteps ) {
      printStatus( SubStepStartState,  // or OriginalState,
                   CurrentState,  CurrentProposedStepLength, 
                   NewSafety,     do_loop_count,  pPhysVol );
    if( (fVerboseLevel > 1) && (do_loop_count > fMax_loop_count-10 )) {
      if( do_loop_count == fMax_loop_count-9 ){
        G4cout << " G4PropagatorInField::ComputeStep(): " << G4endl
               << "  Difficult track - taking many sub steps." << G4endl;
      printStatus( SubStepStartState, CurrentState, CurrentProposedStepLength, 
                   NewSafety, do_loop_count, pPhysVol );
// #endif

One possibility is that the magnetic field or the force have unreasonable values.    I suggest to print out their values.  I am not familiar with Geant4UCN, so I cannot suggest where to add printout for the 'force' - it is in whatever equation of motion is used.

I note that in Geant4 9.5 an extension to G4Transportation was made to allow use of a Gravity field directly in Geant4.   This can be accessed from G4Transportation - without UCNTransportation.  I do not think that this could have caused a problem - but there is a small chance that this is the case.

John Apostolakis, SFT (SoFTware) Group, PH Department, CERN
Email:       Office Tel:  +41-22-767-7239

On Jul 15, 2013, at 8:35 PM, Evan Adamek wrote:

> *** Discussion title: Fields: Magnetic and Otherwise
> Hi all,
> I have been working on a neutron simulation using Geant4UCN. I have the code working on an older machine, but when I tried to move it to the one I am now using I run into problems. The program compiles without any issues, but when I run the executable, I get the following error at the very start of the zeroth event:
> G4MagIntegratorDriver::OneGoodStep:
>  Stepsize underflow in Stepper
>  Step's start x=2e-08 and end x= 2e-08 are equal !!
>  Due to step-size= 3.98979e-25 . Note that input step was 3.98979
> This repeats about ten thousand times (with marginally different step
> start/end points) before aborting with a
> G4MultiLevelLocator::EstimateIntersectionPoint()/Too many substeps
> error. I found the following post:
> on this forum, but the solution proposed involves smoothing out field
> discontinuities, and I still get this issue even when running with only
> gravity enabled (there shouldn't be any non-graviational fields at the
> generation location anyway).
> I had my advisor send me her (working) version of the code, and have encountered the same issue when running it on my own computer. I have run the first 4 novice examples as well as the first two persistency/gdml examples and all of them have worked fine. Does anyone know what I might do to fix, or at least troubleshoot, this error?
> I have attached a full version of the output that I get, in case that
> helps.
>   Attachment:
> -------------------------------------------------------------
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1 None: Re: Stepsize Underflow in Stepper   (Evan Adamek - 16 Jul, 2013)
(_ Feedback: Re: Stepsize Underflow in Stepper   (Gumplinger Peter - 16 Jul, 2013)
(_ Note: Re: Stepsize Underflow in Stepper   (Evan Adamek - 17 Aug, 2013)
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