Reduction of ripple fields in DC magnet using an auxiliary winding

This paper presents a simple method to achieve 10−6 level of ripple fields in a large current DC magnet which is excited by a power supply of moderate ripple current content (10−4–10−5). A theory of the ripple field in a magnet with an auxiliary winding is derived and the conditions for ripple field reduction are obtained. When the resistance of the auxiliary circuit is much smaller than the reactance of that circuit, the ripple field bucking ratio is proportional to the quality factor of the auxiliary winding. To compare the theory with the experiment, a dipole magnet with an auxiliary winding was used. The auxiliary winding was either short-circuited or loaded with a resistor and the ripple fields were measured with a pickup coil placed in the magnet gap. When the auxiliary winding was short-circuited, a ripple field bucking ratio greater than 40 was achieved with a high output impedance power supply. This experimental result agrees well with the theory, and indicates that an auxiliary winding of a large cross-sectional area, instead of a large turn-number, is desired for an efficient ripple field reduction.