What is TORAX?

TORAX is a differentiable tokamak core transport simulator aimed for fast and accurate forward modelling, pulse-design, trajectory optimization, and controller design workflows. TORAX is written in Python-JAX, with the following motivations:

• Open-source and extensible, aiding with flexible workflow coupling
• JAX provides auto-differentiation capabilities and code compilation for fast runtimes. Differentiability allows for gradient-based nonlinear PDE solvers for fast and accurate modelling, and for sensitivity analysis of simulation results to arbitrary parameter inputs, enabling applications such as trajectory optimization and data-driven parameter identification for semi-empirical models. Auto-differentiability allows for these applications to be easily extended with the addition of new physics models, or new parameter inputs, by avoiding the need to hand-derive Jacobians
• Python-JAX is a natural framework for the coupling of ML-surrogates of physics models

For more comprehensive documentation, see our readthedocs page.

TORAX now has the following physics feature set:

• Coupled PDEs of ion and electron heat transport, electron particle transport, and current diffusion
  • Finite-volume-method
  • Multiple solver options: linear with Pereverzev-Corrigan terms, nonlinear with Newton-Raphson, nonlinear with optimization using the jaxopt library
• Ohmic power, ion-electron heat exchange, fusion power, bootstrap current with the analytical Sauter model
• Time dependent boundary conditions and sources
• Coupling to the QLKNN10D [van de Plassche et al, Phys. Plasmas 2020] QuaLiKiz-neural-network surrogate for physics-based turbulent transport
• General geometry, provided via CHEASE equilibrium files
  • For testing and demonstration purposes, a single CHEASE equilibrium file is available in the data/geo directory. It corresponds to an ITER hybrid scenario equilibrium based on simulations in [Citrin et al, Nucl. Fusion 2010], and was obtained from PINT. A PINT license file is available in data/geo.

Additional heating and current drive sources can be provided by prescribed formulas, or user-provided analytical models.

Model implementation was verified through direct comparison of simulation outputs to the RAPTOR [Felici et al, Plasma Phys. Control. Fusion 2012] tokamak transport simulator.

This is not an officially supported Google product.

Feature roadmap

Short term development plans include:

• Time dependent geometry
• More flexible initial conditions
• Implementation of forward sensitivity calculations w.r.t. control inputs and parameters
• Implementation of persistent compilation cache for CPU
• More extensive documentation and tutorials
• Visualisation improvements

Longer term desired features include:

• Sawtooth model (Porcelli + reconnection)
• Neoclassical tearing modes (modified Rutherford equation)
• Radiation sinks
  • Cyclotron radiation
  • Bremsstrahlung
  • Line radiation
• Neoclassical transport + multi-ion transport, with a focus on heavy impurities
• IMAS coupling
• Stationary-state solver
• Momentum transport

Contributions in line with the roadmap are welcome. In particular, TORAX is envisaged as a natural framework for coupling of various ML-surrogates of physics models. These could include surrogates for turbulent transport, neoclassical transport, heat and particle sources, line radiation, pedestal physics, and core-edge integration, MHD, among others.

Installation guide

Requirements

Install Python 3.10 or greater.

Make sure that tkinter is installed:

sudo apt-get install python3-tk

How to install

Install virtualenv (if not already installed):

pip install --upgrade pip

pip install virtualenv

Create a code directory where you will install the virtual env and other TORAX dependencies.

mkdir /path/to/torax_dir && cd "$_"

Where /path/to/torax_dir should be replaced by a path of your choice.

Create a TORAX virtual env:

python3 -m venv toraxvenv

Activate the virtual env:

source toraxvenv/bin/activate

Download QLKNN dependencies:

git clone https://gitlab.com/qualikiz-group/qlknn-hyper.git

export TORAX_QLKNN_MODEL_PATH="$PWD"/qlknn-hyper

It is recommended to automate the environment variable export. For example, if using bash, run:

echo export TORAX_QLKNN_MODEL_PATH="$PWD"/qlknn-hyper >> ~/.bashrc

The above command only needs to be run once on a given system.

Download and install the TORAX codebase via http:

git clone https://github.com/google-deepmind/torax.git

or ssh (ensure that you have the appropriate SSH key uploaded to github).

git clone [email protected]:google-deepmind/torax.git

Enter the TORAX directory and pip install the dependencies.

cd torax; pip install -e .

If you want to install with the dev dependencies (useful for running pytest and installing pyink for lint checking), then run with the [dev]:

cd torax; pip install -e .[dev]

Optional: Install additional GPU support for JAX if your machine has a GPU: https://jax.readthedocs.io/en/latest/installation.html#supported-platforms

Running an example

The following command will run TORAX using the default configuration file examples/basic_config.py.

python3 run_simulation_main.py \
   --config='torax.examples.basic_config' --log_progress

To run more involved, ITER-inspired simulations, run:

python3 run_simulation_main.py \
   --config='torax.examples.iterhybrid_rampup' --log_progress

and

python3 run_simulation_main.py \
   --config='torax.examples.iterhybrid_predictor_corrector' --log_progress

Additional configuration is provided through flags which append the above run command, and environment variables:

Set environment variables

Path to the QuaLiKiz-neural-network parameters. Note: if installation instructions above were followed, this may already be set.

$ export TORAX_QLKNN_MODEL_PATH="<myqlknnmodelpath>"

Path to the geometry file directory. This prefixes the path and filename provided in the geometry_file geometry constructor argument in the run config file. If not set, TORAX_GEOMETRY_DIR defaults to the relative path torax/data/third_party/geo.

$ export TORAX_GEOMETRY_DIR="<mygeodir>"

If true, error checking is enabled in internal routines. Used for debugging. Default is false since it is incompatible with the persistent compilation cache.

$ export TORAX_ERRORS_ENABLED=<True/False>

If false, JAX does not compile internal TORAX functions. Used for debugging. Default is true.

$ export TORAX_COMPILATION_ENABLED=<True/False>

Set flags

Output simulation time, dt, and number of stepper iterations (dt backtracking with nonlinear solver) carried out at each timestep.

python3 run_simulation_main.py \
   --config='torax.examples.iterhybrid_predictor_corrector' \
   --log_progress

Live plotting of simulation state and derived quantities.

python3 run_simulation_main.py \
   --config='torax.examples.iterhybrid_predictor_corrector' \
   --plot_progress

Combination of the above.

python3 run_simulation_main.py \
   --config='torax.examples.iterhybrid_predictor_corrector' \
   --log_progress --plot_progress

Post-simulation

Once complete, the time history of a simulation state and derived quantities is written to state_history.nc. The output path is written to stdout.

To take advantage of the in-memory (non-persistent) cache, the process does not end upon simulation termination. It is possible to modify the runtime_params, toggle the log_progress and plot_progress flags, and rerun the simulation. Only the following modifications will then trigger a recompilation:

• Grid resolution
• Evolved variables (equations being solved)
• Changing internal functions used, e.g. transport model, or time_step_calculator

Cleaning up

You can get out of the Python virtual env by deactivating it:

deactivate

Simulation tutorials

Under construction

Citing TORAX

A technical report is in preparation and will soon be made available for citing.