Fusion Programme - ENEA - Fusione

1. Magnetic Confinement2007 Progress Reportscenario was obtained at B=1.8 T, I p =1.2 MA, witha high triangularity shape optimised for high poloidalbeta. To study q-profile effects on stability andconfinement at high beta, the current profile shapewas varied by using the start time of the main NBIpulse. The normalised beta was controlled in realtime by feedback on the NBI power. β N valuesabove the no-wall stability limit as given by resonantfield amplification were obtained. The beta valueswere limited by the onset of MHD activity with n=1toroidal number, which resulted in a significant lossof confinement: a slight reduction of the betarequest resulted in stable discharges, as shown bythe blue dots in fig. 1.25. The sensitivity of this limitto the q-profile shape was systematicallyinvestigated; a clear correlation was observed, withhigher β N reached at lower q min (fig. 1.26). Theconfinement regime was H-mode with type-I ELMs.Except in a few cases, no ITBs were present. Theconfinement enhancement factor H ITER89L-P rangedfrom 1.7 to 2.1, systematically increasing with NBIstart time, i.e., with decreasing q min . It is estimatedthat about one third of the plasma current wasprovided by the bootstrap mechanism and, with theon-axis NBI current, 50-60% of the total current wasdriven non-inductively. The important role of theMHD n=1 mode in achieving the highest β N isoutlined in figure 1.27, where the traces related todischarges #69920 and 69921 are shown. In thefigure, the β N from equilibrium reconstruction isshown by the red curves (EFIT/BTNM). Theβ N43210H 890 0.5 1.0 1.5 2.0 2.5Target q-minimum2.21.81.4no MHDn=1 MHDFig. 1.25 - β N obtained vs minimum q, showing that themax β N is limited by MHD1.00 0.5 1.0 1.5 2.0 2.5Target q-minimumFig. 1.26 - H 89 improvement factor vs minimum q4269920/ EFIT/BTNMSeq=19 (0)69920 DA/C1-G10169920 PROC/LI4Fig. 1.27 – Effect of n=1 MHD mode in preventingthe achievement of maximum β N04269921/ EFIT/BTNMSeq=16 (0)69921 DA/C1-G10169921 PROC/LI41040 42 44 46 48 50 52(s)Physics (Budapest 2000), ECA Vol. 24B, 245-248(2000)31