GSFit: Grad-Shafranov Fit

GSFit is a modern tokamak equilibrium reconstruction tool for post-shot experimental analysis and scientific interpretation. Developed by Tokamak Energy Ltd. for use on ST40, the world's highest field spherical tokamak.

The Grad-Shafranov equation is the equilibrium solution to the ideal, single-fluid, MHD equations, assuming axisymmetry. The Grad-Shafranov equation has two free degrees of freedom, often referred to as the source functions: the pressure gradient p_prime and ff_prime, where f gives rise to the poloidal plasma current. GSFit uses diagnostics, most importantly the magnetics, to simultaneously solve the non-linear Grad-Shafranov equation while optimising the p_prime and ff_prime degrees of freedom to minimise the difference between the experimental "measured" values and the "calculated" values. More details about the algorithms used can be found in the excellent paper J.-M. Moret, et. al., "Tokamak equilibrium reconstruction code LIUQE and its real time implementation", Fusion Eng. Design, 91, 2015.

GSFit's numerical solver is written in Rust for speed and robustness, and the user interface / set-up is written in Python for its convenience.

The goal of GSFit is high accuracy, not performance; when something can be calculated accurately, it should be calculated accurately regardless of computational cost.

GSFit uses the COCOS 13 coordinate system, as described in O. Sauter and S. Y. Medvedev, "Tokamak Coordinate Conventions: COCOS", Comput. Phys. Commun. 184, 2013. Where, in summary, flux is measured in weber, and the poloidal angle increases counterclockwise, starting from the outboard mid-plane.

Why Rust and Python?

Rust is a compiled, high-performance language, nearly as fast as C and Fortran. It includes modern tooling such as a package manager and centralised registry (cargo and crates.io), autoformatting (rustfmt), and has testing built into the language. Rust enforces strict ownership rules, making programs extremely memory efficient without requiring any extra effort. These rules also eliminate entire classes of memory issues. Additionally, Rust has zero-cost abstractions, allowing object-oriented programming (implementations) to be used without a performance penalty. For more information, the official Rust book gives a complete introduction to the language with examples.

Python is ubiquitous within the fusion industry. Using Python allows easy integration into existing workflows. For example, in examples/example_02_mastu_with_synthetic_data_from_freegsnke.ipynb, we reconstruct an equilibrium using synthetic "measured" data produced by the open source forward Grad-Shafranov solver FreeGSNKE.

1. Installation and environment

1.1 Python environment

GSFit can be run on Python 3.11, 3.12, 3.13, and 3.14.

uv is both an environment and package manager and is recommended. There are several other Python virtual environment providers which should work, but we have not tested against them.

# Install the `uv` environment and package manager
python -m pip install uv

# Creating and activating a virtual environment, called `venv_gsfit`
uv venv --python=3.13
source .venv/bin/activate

1.2 Installing from PyPI

GSFit is available on the PyPI package registry as a pre-compiled binary. The binary is compiled for Linux, macOS, and Windows. For Linux the "manylinux2014" build system is used, which typically can be run on any Linux machine newer than 2014.

All of the packages GSFit depends on are listed in the pyproject.toml. These are divided into essential packages which are always required for any run and optional packages which can be installed for different purposes. An example of optional packages are machine-specific database readers.

# Install GSFit from the PyPI package registry, with only the essential packages
uv pip install gsfit
# or install GSFit from the PyPI package registry, with the "developer" packages
uv pip install gsfit[dev]
# or install GSFit with reading/writing to/from ST40's experimental database
uv pip install gsfit[with_st40_mdsplus]  # (this will only work within Tokamak Energy's network)
# or any combination
uv pip install gsfit[dev,with_st40_mdsplus]

1.3 Compiling and installing from source code on Linux

# Install the Rust compiler (only needs to be done once)
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

# Clone a copy of GSFit from GitHub
git clone git@github.com:tokamak-energy/gsfit.git
cd gsfit

# Install GSFit
uv pip install --reinstall --editable .
# or install with the "developer" packages, such as `pytest` and `mypy`
uv pip install --reinstall --editable .[dev]

1.4 Compiling and installing from source code on macOS

# Install the Rust compiler (only needs to be done once)
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

# Clone a copy of GSFit from GitHub
git clone git@github.com:tokamak-energy/gsfit.git
cd gsfit

# Install GSFit
uv pip install --reinstall --editable .
# or install with the "developer" packages, such as `pytest` and `mypy`
uv pip install --reinstall --editable .[dev]

1.5 Compiling and installing from source code on Windows

# Download and install Rust compiler
# run rustup-init.exe from https://www.rust-lang.org/tools/install

# Clone a copy of GSFit from GitHub
git clone git@github.com:tokamak-energy/gsfit.git
cd gsfit

# Install GSFit
uv pip install --reinstall --editable .
# or install with the "developer" packages, such as `pytest` and `mypy`
uv pip install --reinstall --editable .[dev]

1.6 IDE

VS Code is the recommended IDE for GSFit.

A particularly useful extension is rust-analyzer, which "is a part of a larger rls-2.0 effort to create excellent IDE support for Rust."

The PyO3 crate requires Python>=3.7. On machines where the python3 command links to an unsupported Python version, you can change the following file: ~/.vscode-server/data/Machine/settings.json to point to your desired Python's "standard library files", which will be similar to:

{
    "rust-analyzer.server.extraEnv": {
        "PYO3_CROSS_LIB_DIR": "/home/<user.name>/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib"
    }
}

and Python's "standard library files" directory can be found by:

python -c "import sysconfig; print(sysconfig.get_config_var('LIBDIR'))"

2. GSFit program layout and flow

2.1 Initialisation

GSFit's solver and numerics are programmed in Rust, with the initialisation done within Python.

The communication between Python and Rust is handled by the PyO3 crate. From the Python side the integration is seamless, and a general user would not know if a function / class is written in Python or Rust. In a similar way, a user interacting with numpy does not know if they are calling Python, C or Fortran.

GSFit has the following implementations (which are similar to classes):

• coils for the PF and TF coils. This object contains the geometry; measured values; and reconstructed values
• passives for the toroidally conducting structures, such as the vacuum vessel and passive plates. passives also contains the degrees of freedom allowed for the passive conductors
• plasma contains a $(R, Z)$ grid and the reconstructed equilibrium
• bp_probes contains the Mirnov coils: geometry; measured values; which sensors to use in the reconstruction and their weights; and the reconstructed values
• flux_loops the toroidal flux loops. Note the units are in weber.
• rogowski_coils measuring toroidal currents
• isoflux contains two time-dependent $(R, Z)$ coordinates which have equal poloidal flux
• isoflux_boundary contains a time-dependent $(R, Z)$ coordinate which has the same poloidal flux as the plasma boundary. This can be used if we have measurements of where the strike point is, for example if there is an IR camera

All of these implementations are initialised from Python. As a result, all of the database access is contained within Python, and any pre-processing such as filtering or smoothing experimental data can also be done within Python.

Below is a reproduction of examples/example_01_st40_with_experimental_data.py.

# Note, this example will only run inside Tokamak Energy's network

from gsfit import Gsfit

# Construct the GSFit object
gsfit_controller = Gsfit(
    pulseNo=12050,
    run_name="TEST01",
    run_description="Test run",
    write_to_mds=True,
    settings_path="default",
)

# Run (solve the inverse Grad-Shafranov equation, and if requested write to database)
gsfit_controller.run()

To improve reliability and traceability, the number of arguments needed to initialise the gsfit_controller object is deliberately kept to a minimum. The settings_path="default" argument tells GSFit to use the settings (JSON files) from this directory python/gsfit/settings/default/. By storing all the settings needed to run GSFit in JSON files allows changes to be tracked through Git.

Initial plasma-current guess

The initial toroidal current density is a smooth quadratic distribution centred at r_cur, z_cur:

j_phi(R, Z) = C * max(1 - ((R - r_cur) / minor_radius)^2
                          - ((Z - z_cur) / (minor_radius * kappa))^2, 0)

C is calculated on the discrete plasma grid so that the integrated current equals the configured initial ip. The edge of this current-density support is an initial numerical guess, not an LCFS or an inverse-problem constraint. The configured ellipse must lie inside the plasma grid and vessel and must not contain a limiter point.

Custom settings directories must add minor_radius in metres and dimensionless kappa to the initial_guess section of GSFIT_code_settings.json. Direct callers of gsfit_rs.Plasma(...) must likewise pass initial_guess_minor_radius and initial_guess_elongation; these are required constructor arguments.

2.2 Adding a new experimental device or coupling to a new forward Grad-Shafranov code

The information needed to run GSFit comes from two sources:

1. python/gsfit/settings/: Contains parameters in JSON files, such as:
   • the maximum number of iterations,
   • which magnetic sensors to use in the reconstruction,
   • the degrees of freedom given to p_prime and ff_prime,
   • and which database_reader to use.
2. python/gsfit/database_readers/: Connects to machine-specific databases to read experimental results in, such as:
   • geometry and current in poloidal field coils
   • measured signals from magnetic sensors
   • isoflux constraints

These two sources of information are combined to initialise the Rust objects needed to run GSFit (as described in section 2.1).

Writing to the database is done through python/gsfit/database_writers/, in a similar format to database_readers.

Included are the settings and readers for ST40's experimental data, and synthetic data produced by FreeGS and FreeGSNKE simulations. We welcome and are happy to include configurations for other devices.

2.3 Where are the Green's tables stored?

We can classify objects as either "current sources" or "sensors".

• coils, passives, and plasma are "current sources".
• bp_probes, flux_loops, rogowski_coils, isoflux, and plasma are "sensors".

Note, plasma is both a current source and a sensor; this is because it contains the plasma current and the 2D (R, Z) grid where we want to take measurements on.

Principles about where data should be:

• "sensors" contain the Green's tables.
• "current sources" contain the currents.

The sensors and currents are linked by names; every magnetic probe and PF coil must have a unique name.

2.4 Decision on coils vs PSUs

On many tokamaks, including ST40, several PF coils are connected to a single PSU. For ST40 we have the BVLT and BVLB PF coils, both connected to the BVL PSU.

Several equilibrium reconstruction codes treat the PSU current as a measurement with an associated degree of freedom. To allow for this degree of freedom we would need to tell GSFit which PF coil is connected to which PSU. This is quite burdensome and to keep GSFit simple to initialise, we have decided to only consider "coils". The work-around, if you want to have the PSU currents as a degree of freedom, is to combine all of the PF coils connected to a single power supply into a single coil. On ST40 we could create a PF coil called BVL_combined which contains both BVLT and BVLB.

3. Advanced building

GSFit is intended to be a Python library, where the Rust code is called from Python (Python is not called from within Rust). The advantage of this is that the Rust binary does not need to link agains libpython-dev which is not always installed on all machines.

However, for testing, where the Rust code is run as it's own executable, not from within Python, we must link to Python!

4. Planned future development

• Interfacing to and from IMAS & OMAS.
• More kinetic sensors, such as MSE, polarimeters, and interferometers.

5. Citing GSFit

We intend on publishing a paper on GSFit. While GSFit is unpublished, please cite it as "P. F. Buxton, GSFit, https://github.com/tokamak-energy/gsfit, 2025"

Please use the GSFit nomenclature, not GS-Fit, GSFIT, or g/s fit.