Helical magnetic configuration formed by using shaped ferromagnetic material

Formation of a straight helical magnetic field is attempted by using ferromagnetic material. A shaped ferromagnetic cylinder, which modifies an external uniform magnetic field, is demonstrated to realize an l = 3 helical magnetic field that is suitable for plasma experiments. The measured values for the angle of rotational transform and shear length agree with the predictions within a factor of 2, which is calculated without taking account of the self-demagnetization. This indicates that a rough configuration of magnetic field can be obtained even if the self-demagnetization is not known accurately. It is also suggested that this method is widely applicable to the formation of other complicated configurations of magnetic field