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Question Why error in advancing propagation in a toroidal magnetic field ?  

Forum: Fields: Magnetic and Otherwise
Date: 13 Sep, 2016
From: Mengting Li <Mengting Li>

Hello,everyone.

I'm a beginner of Geant4. I am running a program but come up with some troubles. I put a tungsten cylinder along the Z-axis.Around it is a toroidal magnetic field whose intensity is proportional to 1/r,(r^2 = x^2+y^2). There is no field in the body of cylinder. 10 thousands of electrons are shoot from the top surface to the body of the cylinder by gps. Class PhysListEmStandard is applied in this program.

However,when through the process,the message related to Intersection Point will come up occasionally and core dump.

-------- EEEE ------- G4Exception-START -------- EEEE -------
*** G4Exception : GeomNav0003
      issued by : G4MultiLevelLocator::EstimateIntersectionPoint()
Location: G4MultiLevelLocator::EstimateIntersectionPoint()- After EndIf(Intersects_AF)
       Current Position  and  Direction 
Step#       s        X(mm)      Y(mm)      Z(mm)    N_x     N_y     N_z   Delta|N|   StepLen  StartSafety   PhsStep 
Step taken was -8.64432e-08 out of PhysicalStep= -1
Final safety is: 0.130552
Chord length = 0.156986

Error in advancing propagation.
        Point A (start) is  (  X= 0.331215996 1.08094566 -3.4741878  P= -131.728134 -167.725763 -327.111142  Pmag= 390.494471 Ekin= 389.983807 l= 5.58268473056 m0= 0.510999 (Pdir-1)= 0 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
        Point B (end)   is  (  X= 0.326517902 0.924737709 -3.45930456  P= -47.0535408 100.897103 514.15904  Pmag= 526.07393 Ekin= 525.563179 l= 5.58268464411 m0= 0.510999 (Pdir-1)= -1.11022e-16 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
        Curve distance is -8.64432e-08

The final curve point is not further along than the original!
 Value of fEpsStep= 5e-05
 Point A (Curve start) is  (  X= 0.464250573 0.914462415 -3.36521426  P= -145.55947 -196.43762 202.363436  Pmag= 317.374004 Ekin= 316.863416 l= 3.69842595121 m0= 0.510999 (Pdir-1)= 0 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
 Point B (Curve   end)   is  (  X= 0.326517902 0.924737709 -3.45930456  P= -47.0535408 100.897103 514.15904  Pmag= 526.07393 Ekin= 525.563179 l= 5.58268464411 m0= 0.510999 (Pdir-1)= -1.11022e-16 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
 Point A (Current start) is  (  X= 0.331215996 1.08094566 -3.4741878  P= -131.728134 -167.725763 -327.111142  Pmag= 390.494471 Ekin= 389.983807 l= 5.58268473056 m0= 0.510999 (Pdir-1)= 0 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
 Point B (Current end)   is  (  X= 0.326517902 0.924737709 -3.45930456  P= -47.0535408 100.897103 514.15904  Pmag= 526.07393 Ekin= 525.563179 l= 5.58268464411 m0= 0.510999 (Pdir-1)= -1.11022e-16 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
 Point S (Sub start)     is  (  X= 0.333625816 1.02129804 -3.34673714  P= -20.9478231 68.3420992 -14.1914734  Pmag= 72.8755908 Ekin= 72.3663834 l= 4.68247052465 m0= 0.510999 (Pdir-1)= 0 t_lab= 0 t_proper= 0 PolV= (0,0,0)  )  Point E (Trial Point)   is (0.32712672103104140087,0.94498048042670135249,-3.4612332593349210974)
 Point F (Intersection)  is  (  X= 0.331215996 1.08094566 -3.4741878  P= -131.728134 -167.725763 -327.111142  Pmag= 390.494471 Ekin= 389.983807 l= 5.58268473056 m0= 0.510999 (Pdir-1)= 0 t_lab= 0 t_proper= 0 PolV= (0,0,0)  ) 
 LocateIntersection parameters are : 
      Substep no (total) = 5
      Substep (depth= 05
*** Fatal Exception *** core dump ***
-------- EEEE -------- G4Exception-END --------- EEEE -------

The message tell "The final curve point is not further along than the original",but I don't understand the real mean of the words.Does it imply that I should tweak the value "delta_intersection"?

I've followed John's advice to update my Geant4 to release 10.02.p02. But the error still occurred. http://hypernews.slac.stanford.edu/HyperNews/geant4/get/emprocess/1433/1.html

Any advices about fixing the error? Thanks so much.

Amanda.

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