[en] We examine helical fields possessing a mean magnetic well and circumstances in which they may exhibit the negative-V' property. The stream function formation which provides a simple picture of the field geometry allows one to connect the topology of magnetic surfaces with positions of the stationary points of the stream function. The shape of flux surfaces for a new configuration which provides a mean magnetic well and the value of V' on the magnetic well at an arbitrary magnetic axis are calculated. It is pointed out that the maximum value of the magnetic well is connected with a position of external helical conductors