Flux surface mapping and error field measurement for MUSE table top stellarator*

MUSE is the world's first quasi-axisymmetric stellarator constructed with planar circular coils and permanent magnets[1]. System assembly has finished[2] and electron beam mapping of flux surfaces[3] has been conducted to confirm its magnetic topology.

 Good nested flux surfaces have been confirmed at different toroidal field(TF) coil currents. Close agreement of the flux surfaces shapes with flux surfaces from VMEC and FIELDLINES at the corresponding TF value has been demonstrated. The $iota$ profile has been measured and 

 compared to our model by observing low order resonant structures at different TF values.

 Magnetic error field Fourier components will be quantified by measuring flux surface shape deviations and low order islands widths. Attempts will be made

 to correct these error fields by adjusting the PM holder positions accordingly.

[1] T. Qian, et al, Nucl. Fusion 084001 (2022)

[2] T. Qian, et al, Design and Construction of the MUSE Permanent Magnet Stellarator,  accepted for publication by J. Plasma Physics

[3] S. A. Lazerson, et al, Nucl. Fusion 106005 (2016)