Investigation of magnetic field properties in the helical axis stellarator

Experimental measurements of the magnetic field properties of a helical axis stellarator were carried out. The results, together with the numerical calculation, are described herein. The toroidal device, Asperator NP-4, has a helical magnetic axis with a pitch angle of 45deg and a multi-field period of n = 8. The magnetic field of NP-4 is produced by the currents of helical solenoids and L = +1, -1 helical windings. By an electron beam mapping of field lines, the existence of a high rotational transform angle and magnetic surfaces are confirmed and the position of the helical magnetic axis is obtained. The other fundamental magnetic properties such as the shear and the specific volume of the configuration are also deduced from the measurements. Numerical calculations lead to a good agreement with the results of the experiment. (author)