Deliberate seeding of impurities has been applied on several fusion devices in order to create a strongly radiating boundary, distributing the power transported from the plasma core on a larger wall area, and to reduce head loads on limiters and divertor target plates. Under conditions of the Low (L) confinement mode simultaneous reduction of the anomalous transport of particles and energy and transition to the so called Radiative Improved (RI) mode has been achieved on TEXTOR. By puffing of argon impurity a noticeable confinement improvement has been also seen in the High (H) confinement mode in the divertor tokamak JT-60U. However, earlier analogous experiments on JET did not lead to the desirable result and suffer, when argon concentration exceeded some critical level, from unwanted and uncontrollable impurity accumulation in the plasma core with strongly peaked core radiation and flat temperature profile.