This small demo reuses the validated Cartesian TDm/RBSL magnetic rope used by the magnetic-topology examples, but runs the independent MPI finite-volume relative-helicity backend. It deliberately contains only the user module and two parameter files; setup.pl generates build files locally and output/ is only a runtime directory.

Build and generate a snapshot

From this directory, set AMRVAC_DIR to the repository root and build a private three-dimensional executable:

export AMRVAC_DIR=/absolute/path/to/amrvac
cd "$AMRVAC_DIR/tests/demo4/MagneticHelicity_Cart"
"$AMRVAC_DIR/setup.pl" -d=3 -v=3
make AMRVAC_DIR="$AMRVAC_DIR"
mkdir -p output
 
OMP_NUM_THREADS=1 OMP_DYNAMIC=false \
OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1 \
mpirun --bind-to none --map-by slot -np 1 ./amrvac \
  -i amrvac_generate.par

The light default mesh is 64 x 48 x 64. For a u128-style run, change the three domain_nx* values to 128, 96, 128 and the block sizes to 32 in amrvac_generate.par.

Run the helicity conversion

Use any MPI size; the backend is not the OpenMP field-line topology backend. The macOS Open MPI form below avoids core-binding failures:

OMP_NUM_THREADS=1 OMP_DYNAMIC=false \
OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1 \
mpirun --bind-to none --map-by slot -np 2 ./amrvac -convert \
  -if output/tdm_0000.dat -i amrvac_generate.par amrvac_helicity.par

The result is output/tdm_helicity.csv. It contains code and physical-unit Hm, HJ, HPJ, magnetic energy E, potential energy Ep, free energy Efree, the ratio abs_HJ_over_abs_Hm, and quality fields for flux balance, boundary normals, curl reconstruction, decomposition closure, divergence, and multigrid residual/cycles. mh_write_debug_vti is off by default; enable it only when a streamed B_p, A, A_p, and density-field VTI is needed for numerical inspection.

The six-face Neumann reference solve requires a uniform, unstretched, three-dimensional Cartesian mesh and a compatible net boundary flux. AMR snapshots must first be remapped with level_io to a fixed uniform level. ‘convert_type='magnetic_topology’remains a separate single-MPI-rank, OpenMP field-line workflow. The sampled TDm/RBSL faces have a measured discrete relative flux imbalance of about1.54e-5; the demo therefore uses mh_max_flux_imbalance=1.d-4and keeps the measured value in the CSV quality columns. Analytic or cleaned input data can use the stricter1.d-6` guard.