testing MAX_FIELD_STEP

[zhaozhiwen]

I did some tests with different MAX_FIELD_STEP values using jlabce:2.5 container which has gemc 2.9 and geant4.10.7.p03
The field map used has 1cm grid and -FIELD_PROPERTIES="solenoid_CLEOv8, 1*mm, G4ClassicalRK4,linear"

1st test
high energy tracks of chargedgeantino in vacuum shows a few percent difference between default and 1mm as follows, while 100mm,10mm,1mm don't differ much

• (51, 14) px: 2283.68152336 2280.41070037 2276.14836298 2271.14774281 2265.8469355
• (51, 15) py: -142.361192324 -192.090911539 -238.98160889 -281.501300474 -316.780474258
• (51, 16) pz: 10759.3927334 10759.3135524 10759.276761 10759.3050402 10759.4417976
• (51, 14) px: 2283.90165767 2280.74337534 2276.58981228 2271.68982116 2266.47485493
• (51, 15) py: -137.000983078 -186.735916606 -233.634107317 -276.163262947 -311.452802157
• (51, 16) pz: 10759.4155951 10759.3373101 10759.3008105 10759.3289386 10759.4650835
  check details at ifarm by
  cd /group/solid/solid_github/JeffersonLab/solid_gemc_test/analysis/field_test
  diff solidgem.txt_max1 solidgem.txt_maxdefault
  repeat the test by
  cp /group/solid/solid_github/JeffersonLab/solid_gemc_test/analysis/field_test/solidgem.gcard ./
  cp /group/solid/solid_github/JeffersonLab/solid_gemc_test/analysis/field_test/solidgem__geometry_Original.txt ./
  gemc solidgem.gcard -BEAM_P="chargedgeantino,11GeV,12deg,0deg" -BEAM_V="(0,0,-350)cm" -FIELD_DIR="/group/solid/solid_github/JeffersonLab/solid_gemc_test/field" -HALL_FIELD="solenoid_CLEOv8" -FIELD_PROPERTIES="solenoid_CLEOv8, 1mm, G4ClassicalRK4,linear" -MAX_FIELD_STEP=1
  more solidgem.txt

2nd test
testing speed of gemc for 1e6 events and SoLID PVDIS full geometry and 1cm3 grid map at different MAX_FIELD_STEP
0-11GeV electrons entering solid at all angles, the speed is 1:1 for MAX_FIELD_STEP default:10mm
11GeV electrons shooting at 40cm LD2 target, the speed is 1:2.5:15 for MAX_FIELD_STEP default:100mm:10mm

my thoughts
MAX_FIELD_STEP in gemc is fLargestAcceptableStep in geant4 (geometry/navigation/src/G4PropagatorInField.cc) whose default 1000m is too large for both high energy particles shown in my test and low energy particles in clas12 test ("moller electrons were jumping un-physically to unshielded areas")
Maybe it's best for our application with 1cm map grid to use MAX_FIELD_STEP 1cm to balance speed and accuracy.