Total suppression of superconductivity by high magnetic fields in YBa2 Cu3O6.6

We have studied in fields up to 60T the variation of the transverse magnetoresistance (MR) of underdoped YBCO6.6 crystals either pure or with Tc reduced down to 3.5K by electron irradiation. We evidence that the normal state MR is restored above a threshold field H'c(T), which is found to vanish at T'c>>Tc. In the pure YBCO6.6 sample a 50 Tesla field is already required to completely suppress the superconducting fluctuations at Tc. While disorder does not depress the pseudogap temperature, it reduces drastically the phase coherence established at Tc and weakly H'c(0), T'c and the onset Tnu of the Nernst signal which are more characteristic of the 2D local pairing.