73b to the electrode 122a within the laminate structure 80 the laminate structure 80 as shown in fig8 and 9 includes a plurality of interleaved composite plate 114 and return conductors 130 as shown in fig8 the composite plate 114 includes a pair of side elements 116a and 116b disposed along the sides of the duct module 69 and made of a magnetic material to facilitate in conjunction with the pole pieces 78 the flow of the magnetic field through the air gap formed by the duct modules 69 the composite plate 114 further includes top and bottom elements 118a and 118b made of a non magnetic material of increased strength such as a non magnetic steel to resist internal pressures within the ducts 70 and 72 without 34 shunting 34 the magnetic field about the duct modules 69 the return conductors 130 may in one illustrative embodiment of this invention as shown in fig9 and 11 be an integrally formed hollow rectangularly shaped member having side portions 130a and 130b disposed along the long sides of the duct modules and a top portion 130c disposed at the top of the duct module as indicated in fig1 it being understood that a bottom element 130d not shown would be disposed at the other end of the duct module 69.