In a closed electron drift thruster, a magnetic circuit for creating a magnetic field in a main annular channel comprises at least one axial magnetic core surrounded by a first coil and an inner upstream pole piece forming a body of revolution, together with a plurality of outer magnetic cores surrounded by outer coils. The magnetic circuit further comprises an essentially radial outer first pole piece defining a concave inner peripheral surface and an essentially radial second pole piece defining a convex outer peripheral surface. The concave inner peripheral surface and the convex outer peripheral surface present respective adjusted profiles that are distinct from circular cylindrical surfaces so as to form between them a gap of varying width presenting zones of maximum value in register with the outer coils and zones of minimum value in between the outer coils so as to create a uniform radial magnetic field.