Magnetic Islands Rotation in JET
The propagation of magnetic islands in tokamaks is a relevant issue both for tearing mode stability and for diagnostic applications. The latter consist in the determination of rational-q locations from the comparison between magnetic signal frequencies and plasma rotation profiles. The former issue arises from the fact that ion inertial effects (the polarization current) can be stabilizing or destabilizing depending on the ratio between the island propagation frequency in the frame with zero radial electric field (w­wE) and the ion diamagnetic frequency w*i. Here w is the island frequency in the laboratory frame and wE is the Doppler shift associated with the E¥B velocity. The linear tearing mode stability theory predicts propagation at the electron diamagnetic frequency, w­wE w*e (note that w*i is positive by definition while w*e is negative). Non-linear theoretical studies give frequencies that increase from w*e to w*i for increasing ratio between the island size (w) and the ion sound gyroradius (rs). Early experiments in ohmic plasmas apparently confirmed the linear theory prediction, but, since the possibility of substantial rotation in ohmic plasmas was not known at that time, the Doppler shift was neglected. More recent investigations in H-mode plasmas heated by co-injected neutral beams found frequencies between 0.5w*i and 1.5w*i. A few (8 cases in all) islands with different periodicity numbers (m poloidal and n toroidal) were studied in . The main difficulty of this kind of comparison is that large error bars arise from the evaluation of wE and w*i at the location where q = m/n. In the present study, the precision of the comparison was improved by averaging over a large (149 data points) database of islands with the same m/n.