An Experimental Study of the Vacuum Magnetic Surfaces of Heliotron E

The structure of the vacuum magnetic surfaces of Heliotron E was studied numerically and experimentally. A pulsed electron beam method was used to map out the magnetic surfaces up to the rotational transforms of 1. An electron emissive probe method was adopted to investigate the position of the boundary. The experiments were performed under a reduced operation field of 500 G, where the geomagnetic field could not be ignored. The last closed magnetic surface agreed relatively well with the numerically calculated one. The rotational transform at the magnetic axis was found to be slightly less than 0.5. Results showed that Heliotron E has rotational transform profiles sensitive to the low-mode-numbered perturbation field.