Magnetic compensation of gravity forces in liquid/gas mixtures: surpassing intrinsic limitations of a superconducting magnet by using ferromagnetic inserts

Magnetic compensation of gravity forces, similar to the space conditions of "microgravity", needs the production of a uniform magnetic force field. We derive here a basic mathematical result that shows the impossibility to establish exact gravity compensation in a finite volume. The imperfection of compensation can be, however, quantified and a relation is derived between homogeneity accuracy and compensation volume in a cylindrical symmetry. We study how the use of inserts made of saturated ferromagnetic materials can modify the homogeneity of magnetic force field. In order to illustrate this result, an iron insert has been numerically calculated for the particular case of gravity compensation of H2 in a 10 T superconducting coil. An experimental test has been carried out on a H2 vapour bubble very close to its gas-liquid critical point. Near the critical point the gas-liquid interfacial tension is vanishing, then any bubble deformation from the ideal spherical shape reveals the non-homogeneities in the magnetic compensation force.