# QUASR

:::{note}
The QUASR interface requires [`simsopt`](https://github.com/hiddenSymmetries/simsopt) to be installed.
:::

The *QUAsi-symmetric Stellarator Repository* [QUASR](https://quasr.flatironinstitute.org/) [^QUASR1] [^QUASR2] [^QUASR3] is a database of curl-free stellarators optimized for volume quasi-symmetry.

A QUASR configuration can be loaded with
```{code} bash
pygvec load-quasr ID
```
where `ID` is replaced with the desired configuration. Alternatively
```{code} bash
pygvec load-quasr -s FILE
```
can be used instead, to load a boundary from a simsopt compatible JSON file (e.g. manually downloaded from QUASR).
With
```{code} bash
pygvec load-quasr -f FILE
```
the cartesian boundary data is read directly from the supplied netCDF file (in this case `simsopt` is also not required).

This script will download the requested QUASR configuration, generate a *G-Frame* [^GFrame] to be used as $h$-map and the boundary representation in that *G-Frame*, as well as a [GVEC parameter file](./gvec-parameter-list.md).

The `--tol` parameter sets the desired tolerance of the boundary representation which directly impacts the necessary degrees of freedom and therefore computational speed.
The `--nt` and `--nz` parameters set the number of points in $\vartheta$ and $\zeta$ respectively for one field period, from which a *G-Frame* as well as the boundary cross-sections are computed. The points exclude the periodic point and should be chosen to be odd.
With `--save-xyz` the cartesian boundary data can be saved as a netCDF file.
Other parameters can be seen with `pygvec load-quasr --help`.

<!--- References -->

[^QUASR1]: A. Giuliani, F. Wechsung, A. Cerfon, G. Stadler, M. Landreman (2022). Single-stage gradient-based stellarator coil design: Optimization for near-axis quasi-symmetry. Journal of Computational Physics, 459, 111147, [DOI: 10.1016/j.jcp.2022.111147](https://doi.org/10.1016/j.jcp.2022.111147).

[^QUASR2]: A. Giuliani, F. Wechsung, G. Stadler, A. Cerfon, M. Landreman (2022). Direct computation of magnetic surfaces in Boozer coordinates and coil optimization for quasisymmetry. Journal of Plasma Physics, 88 (4), 905880401, [DOI: 10.1017/S0022377822000563](https://doi.org/10.1017/S0022377822000563).

[^QUASR3]: A. Giuliani, F. Wechsung, A. Cerfon, M. Landreman, G. Stadler (2023). Direct stellarator coil optimization for nested magnetic surfaces with precise quasi-symmetry. Phys. Plasmas, 30 (4), 042511, [DOI: 10.1063/5.0129716](https://doi.org/10.1063/5.0129716).

[^GFrame]: F. Hindenlang, G. Plunk, O. Maj (2025). Computing MHD equilibria of stellarators with a flexible coordinate frame. Plasma Physics and Controlled Fusion, 67, 045002, [DOI: 10.1088/1361-6587/adba11](https://doi.org/10.1088/1361-6587/adba11).