Add NetCDF results output with Cartesian positions for heat load analysis

[krystophny]

Summary

Add structured NetCDF output (results.nc) for particle tracing results, replacing/supplementing ASCII times_lost.dat. Key addition: Cartesian positions for wall heat load analysis.

Motivation

• Efficiency: Binary NetCDF is 10-100x smaller than ASCII for large runs
• Decoupling: Cartesian positions (xstart_cart, xend_cart) enable post-processing without coordinate transformation machinery
• Self-describing: Config stored as attributes for reproducibility
• Heat loads: Direct 3D wall positions for engineering optimization

Proposed Structure

results.nc

Dimensions:
  particle, xyz(3), phase(5), iclass_dim(3)

Variables:
  # Phase space (reference coordinates, 5D)
  zstart(phase, particle)     float64   # (rho, theta, zeta, p, v_par/v)
  zend(phase, particle)       float64
  
  # Cartesian position only (3D)
  xstart_cart(xyz, particle)  float64   # (x, y, z) in meters
  xend_cart(xyz, particle)    float64   # (x, y, z) at loss/end
  
  # Scalars
  times_lost(particle)        float64
  trap_par(particle)          float64
  perp_inv(particle)          float64
  iclass(iclass_dim, particle) int32
  class_lost(particle)        int8

Global Attributes:
  # All namelist config params
  ntestpart, trace_time, facE_al, v0, integmode, relerr, ...
  coord_input, field_input, netcdffile
  coordinate_type   ("vmec" | "chartmap")
  # etc.

Implementation

1. New module netcdf_results_output.f90 (~150 lines)
2. Add output_results_netcdf to namelist (default .false. initially)
3. Compute xstart_cart, xend_cart via ref_coords%evaluate_cart()
4. Write at end of simulation

Backwards Compatibility

• times_lost.dat unchanged (ASCII legacy preserved)
• New output is opt-in via namelist parameter

Use Case: Wall Heat Loads

import xarray as xr
ds = xr.open_dataset('results.nc')

# Wall hits (chartmap: rho >= 1)
lost = ds.where((ds.times_lost > 0) & (ds.zend[0] >= 0.99), drop=True)

# 3D positions and energy
x, y, z = lost.xend_cart[0], lost.xend_cart[1], lost.xend_cart[2]
energy_eV = lost.zend[3]**2 * ds.attrs['facE_al'] * 3.5e6

Related Issues

• Add Cartesian wall position to output for chartmap tracing #285 (Cartesian wall position output) - superseded by this
• Add wall heat load visualization and port placement optimization #284 (Wall heat load visualization) - enabled by this
• Enable SIMPLE until wall #72 (SIMPLE until wall) - output format for wall hits