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Summary form only given. Centre d'Etudes de Gramat (CEG) is now studying long implosion time (about 1/spl mu/s) plasma radiation sources which are of great interest because they allow to reduce the complexity and cost of generators. Single or double shell Z-pinch sources are routinely used by other laboratories in the 100 ns regime with the well known experimental result that an about 2 cm initial radius should not be exceded to preserve the stability of the implosion. In a many hundreds ns implosion time scale, the question is raised if bigger radius could be shot. This paper discusses a numerical and theoretical approach which shows that a 6 to 7 cm radius is acceptable in the 1 /spl mu/s regime. It is based on a shell like implosion approximation and on an analysis of Rayleigh-Taylor instabilities in the r-z plane. It will be demonstrated that all the spatial features of the implosion are determined by the magnetic skin depth and so essentially depends on the square root of the implosion time. Some critics of this result will finally be done taking into account the 3D array structure. Some experiments carried out with the 4MA SPHINX generator at CEG confirm that a 7 cm initial radius can be used with 800 ns implosion time, especially with double nested shells. With aluminum wire arrays, this allows to obtain more than 10 kJ of energy radiated above 1 keV, with pulse widths of 30-50 ns, for a total radiation yield around 100 kJ.