alphastell

A Rust CAD generator for stellarator fusion reactors, built on top of OpenCASCADE (via cadrum) and inspired by parastell.

Cutaway of the six in-vessel layers (chamber → vacuum vessel) with the 40-filament magnet coil set, produced by make showcase.

Overview

alphastell reads a VMEC magnetic equilibrium (wout_*.nc, NetCDF3) and produces solid CAD geometry for a full stellarator in-vessel build plus its modular coil assembly. It is intended as a small, fast, statically-linked companion tool for reactor-design studies — write a VMEC surface, get a STEP file you can drop into a CAD viewer or a neutronics pipeline.

Key outputs:

• STEP for CAD (chamber, first_wall, breeder, back_wall, shield, vacuum_vessel, magnet_set)
• SVG projected bird's-eye renderings for reports and docs
• CSV x,y,z point clouds for quick verification and plotting

One command reproduces the hero image above:

make showcase

Relationship to parastell

alphastell is a Rust reimplementation of the core in-vessel and magnet geometry generation from parastell (Python, MIT, maintained by the Svalinn group at UW-Madison). It borrows:

• the VMEC Fourier evaluation recipe for R(θ, φ) and Z(θ, φ)
• the six-layer material stack (first wall, breeder, back wall, shield, vacuum vessel) and the standard thicknesses (5 / 50 / 5 / 50 / 10 cm)
• the wall_s = 1.08 offset convention and the Planar in-cross-section normal offset method
• the coils.example MAKEGRID format for magnet filaments

Differences: the kernel is Rust + OCCT (bundled statically through cadrum), outputs can be cross-checked with validate against reference parastell STEP files (vendored under parastell/examples/alphastell_full/), and boolean-subtract-based shell construction is used instead of Shell::offset.

The parastell/ directory is a vendored snapshot (not a git submodule). All credit for the underlying approach goes to the parastell authors; bugs in the Rust port are mine.

Subcommands

Run cargo run --release -- <subcommand> --help for the full flag set.

Getting started

git clone https://github.com/lzpel/alphastell
cd alphastell

cargo build --release

make run              # vessel + validate against the bundled parastell reference
make showcase         # reproduce figure/image.png as out/showcase.step + .svg
make points           # 3D scatter of every out/*.csv (needs `uv`)

Prerequisites: stable rustc with edition 2024 support, GNU make, and uv for the Python viewer scripts under tools/ (optional). OCCT is statically linked through the cadrum crate, so no separate install is required.

Example usage

# 6 in-vessel layers (parastell default: wall_s=1.08, scale=100 → cm output)
cargo run --release -- vessel --input parastell/examples/wout_vmec.nc --output out/

# 40-coil magnet set
cargo run --release -- magnet --input parastell/examples/coils.example --output out/magnet_set.step

# Keep half the torus of first_wall (sector [-1/4, +1/4] of τ = [-90°, +90°])
cargo run --release -- cut --cut -i out/first_wall.step -o out/fw_half.step -s -1/4 -e 1/4

# Merge vessel layers + a magnet sector into one colored STEP + SVG
cargo run --release -- compound \
    -i out/chamber.step -i out/vacuum_vessel.step \
    --input-magnet parastell/examples/coils.example \
    -o out/merged.step

The make showcase target wires this together: each vessel layer is cut with a progressively wider window (i · τ/36 half-span, i = 0..6), then all six layers and the −τ/6..τ/6-complementary coil set are compounded. Vessel layers get a linear RGB gradient from #EE7800 (chamber) to #FFFFFF (vacuum vessel); coils keep their per-filament rainbow color from magnet::build_sector.

Repository layout

src/             Rust source for each subcommand
tools/           Python viewers (view_points.py, etc.)
parastell/       Vendored parastell snapshot — reference algorithms and example data
parastell/examples/    wout_vmec.nc, coils.example, alphastell_full/*.step
figure/          Rendered showcase images
notes/           Design notes (Japanese)
examples/        Small cadrum usage examples (seam-dent repro etc.)

Known limitations

• cadrum#120 — the periodic B-spline seam in Solid::bspline(grid, periodic=true) leaves mm-scale dents on chamber-like surfaces; the grid is deliberately kept at M=128, N=48 to keep the artifact small.
• cadrum#122 — round-tripping a multi-solid compound STEP (40 magnet coils) through read_step currently returns zero solids. compound --input-magnet bypasses this by building the coils in-memory.
• The STEP header declares SI_UNIT(.MILLI., .METRE.), but vessel --scale defaults to cm. Viewers therefore render everything at 1/10 of the intended physical size. Relative dimensions are still correct.

Contact

If any of this is useful to your group — or if you just want to compare notes on stellarator CAD / VMEC pipelines — feel free to reach out: Satoshi Misumi on LinkedIn.

License

Released under the MIT License — see LICENSE. The vendored parastell/ tree keeps its upstream MIT license (parastell/LICENSE.md).