New Features

• Adds particle tracing capabilities in desc.particles module.
  • Particle tracing is done via desc.particles.trace_particles function.
  • Particles can be initialized in couple different ways:
    • ManualParticleInitializerLab : Initializes particles at given positions in lab coordinates.
    • ManualParticleInitializerFlux : Initializes particles at given positions in flux coordinates.
    • CurveParticleInitializer : Initializes N particles on a given curve.
    • SurfaceParticleInitializer : Initializes N particles on a given surface.
  • Implemented particle trajectory models are:
    • VacuumGuidingCenterTrajectory : Integrates the particle motion by vacuum guiding center ODEs, conserving energy and mu.
  • Particle trajectories can be plotted with desc.plotting.plot_particle_trajectories function.
• Adds new option for loss_function, "sum", which changes an objective to target the sum of the values computed.
• Adds utility functions desc.external.paraview.export_surface_to_paraview, desc.external.paraview.export_volume_to_paraview and desc.external.paraview.export_coils_to_paraview to export Paraview files for surfaces, volume and coils. These functions use an optional dependency pyvista which is not automatically installed.
• The x_scale option for eq.optimize and related functions can now be given as a dictionary mapping individual parameter names to their relevant scales, or if multiple things are being optimized, x_scale can be a list of dict, one for each optimizable thing.
• Adds new option x_scale='ess' to use exponential spectral scaling from (Jang 2025) which has been shown to improve performance and robustness as an alternative to fourier continuation methods.
• Adds x_scale='ess' option for the OmigenousField class.
• Adds "scipy-l-bfgs-b" optimizer option as a wrapper to scipy's "l-bfgs-b" method.
• The x_scale parameter can now be used with stochastic gradient descent type optimizers.
• Adds wrappers for optax optimizers. They can be used by prepending 'optax-' to the name of the optimizer (i.e. optax-adam). Additional arguments to the optimizer such as learning_rate can be pass via options = {'optax-options': {'learning_rate': 0.01}}. Even a custom optax optimizer can be used by specifying the method as 'optax-custom' and passing the optax optimizer via the 'update-rule' key of optax-options in the options dictionary. See the docstring of the optax-custom for details.
• Adds check_intersection flag to desc.magnetic_fields.FourierCurrentPotentialField.to_Coilset, to allow the choice of checking the resulting coilset for intersections or not.
• Changes the import paths for desc.external to require reference to the sub-modules.
• Adds a differentiable utility for finding constant offset toroidal surfaces inside of optimizations. See PR for more details.
• Add support for Python 3.14

Bug Fixes

• No longer uses the full Hessian to compute the scale when x_scale="auto" and using a scipy optimizer that approximates the hessian (e.g. if using "scipy-bfgs", no longer attempts the Hessian computation to get the x_scale).
• SplineMagneticField.from_field() correctly uses the NFP input when given. Also adds this as a similar input option to MagneticField.save_mgrid().
• Fixes some bugs that hampered robustness of desc.geometry.FourierRZToroidalSurface.constant_offset_surface, particularly when the given grid had stellarator symmetry or when NFP=1.
• Fixes possible bug in computing normalizations when both kinetic and pressure profiles are assigned. Also adds warnings whenever an pressure is added to a kinetic-constrained equilibrium and vice-versa to alert user to ambiguous equilibrium setups.
• Adds error in MercierStability to guard against situation where if a grid with a point at rho=0 were used, NaN would be computed, asMercierStability is undefined on-axis.

Performance Improvements

• ProximalProjection.grad uses a single VJP on the objective instead of multiple JVP followed by a manual VJP. This should be more efficient for expensive objectives.

Deprecations

• sgd optimizer is deprecated in favor of optax-sgd, and will be removed in a future release. To achieve the same behavior with optimizer = Optimizer('sgd') and options={'alpha': ..., 'beta': ...} when the optimizer is removed, one can use optimizer = Optimizer('optax-sgd') and options={'optax-options': {'learning_rate': alpha, 'momentum': beta, 'nesterov': True}}.
• Removes FiniteDiffDerivative from the public API. This class was no longer actually usable with the current versions of DESC's optimization framework, as JAX is now required for running any equilibrium or optimization solves.

New Contributors

• @byoungj made their first contribution in https://github.com/PlasmaControl/DESC/pull/1736

Full Changelog: https://github.com/PlasmaControl/DESC/compare/v0.16.0...v0.17.0