ELMy H-mode pulses have been obtained with different hydrogenicisotopes (H and D) but having the same profiles of thedimensionless parameters ρ*,β,ν* and q, to testwhether the confinement scale invariance principle is valid in atokamak. The fact that the confinement times, the ELM andsawtooth frequencies in the two pulses all scale as expectedsuggests that the invariance principle is satisfied through theplasma radial extent, in spite of the differing physicalprocesses taking place in the plasma centre, core and edgeregions. An application of this `isotope windtunnel technique'to predicting the D-T performance of the next step devices isdiscussed.