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Analysis of MHD activity in Pellet Enhanced Performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n) = (1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events is usually n = 3, whereas the mode following them is dominantly n = 2. Toroidal linear MHD stability calculations, assuming a non-monotonic q-profile with an off-axis minimum decreasing from above to below 1, describe this sequence of modes (n = 3, 1, 2) but always give a larger growth rate for the n = 1 mode than n = 2, due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n) = (1,1) mode is observed as a hot spot with a ballooning character on the low field side. This has some of the properties of a hot island, consistent with the presence of a region of negative shear.

Additional information

Authors: HUGON M ET AL., JET Joint Undertaking, Abingdon, Oxon. (GB);VAN MILLIGEN B P, FOM Instituut voor Plasmafysica, Rijnhuizen, Nieuwegein (NL);HUYSMANS G, FOM Instituut voor Plasmafysica, Rijnhuizen, Nieuwegein (NL);APPEL L, AEA Fusion, Culham Laboratory, Abingdon, Oxon. (GB);HENDER T C, AEA Fusion, Culham Laboratory, Abingdon, Oxon. (GB);PORTE L, University of Strathclyde, Glasgow (GB)
Bibliographic Reference: Report: JET-P(91)29 EN (1991) 26 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199111587 / Last updated on: 1994-12-02
Original language: en
Available languages: en