Experimental characterization of M-Mode in JET tokamak

Experimental observations in JET tokamak have revealed the existence of a confinement regime with a clear increase of the density close to what observed in H-mode but with a weak temperature pedestal. This regime, dubbed M-Mode has been reported in [1] and it is associated to a low frequency magnetic oscillations around 1-2 kHz. These oscillations exhibit an inverse density dependence whereas no evidence on cs was revealed, excluding GAMs (observed around 10 kHz)) as a possible source. The magnetic structure exhibits a periodicity (n;m) = (0;1). Experimental observations of similar magnetic activity, although at different frequency has been observed in EAST [2] and HL-2A [3], associated to the transition for L-I-H mode. Actually the M-Mode has been observed Ohmic, NBI and RF discharges but in the case of NBI scenario only at power close to the L-H power threshold. The present contribution will report on the more recent experimental observations and interpretation concerning the M-Mode as observed in JET tokamak. We will provide observations on the relation between high frequency (above 60 kHz) magnetic broad-band activity and coherent low frequency mode and we will try to correlate the high frequency activity with fluctuations on density or temperature as obtained from Li-Beam and ECE. Further correlation between density oscillations associated to the M-Mode and IR cameras will be provided. To verify Alfvén waves interpretation as suggested in [1], the isotope effect on the scaling of the mode frequency with the Alfvén pedestal velocity will be reported.