The invention relates to a magnetic coil structure for a magnetic field generator for heating magnetic or magnetizable materials or solids in biological tissues, which has a coolable yoke (2) on which two mutually opposed , pole pieces (7, 8) separated by a field forming gap (12), wherein in order to generate an alternating magnetic field, two magnetic coils (22) respectively assigned to a pole piece (7, 8) are designed to have The disc coils of the helically advancing coil winding and the pole shoe ends of the respective pole shoes (7, 8) are annularly surrounded by a magnetic coil/pole shoe gap (57) running therebetween. According to the present invention, the magnetic coils (22) are installed respectively in a coil box annularly surrounded by a corresponding pole shoe (7, 8), and the coil box (9, 10) has at least one cooling air supply for connecting to a cooling air pump. - inlet openings (51, 52, 53) and at least one cold air-outflow opening (57, 66).