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Conditions under which ballooning modes are expected to be excited were recently obtained in two different types of discharges in JET. In the first type, extremely large pressure gradients were produced in the plasma core through pellet injection in the current rise phase, followed by strong additional heating. In the second type, the total pressure of the discharge approached the Troyon limit. The stability of these discharges with respect to the ideal MHD ballooning modes was studied with the HBT stability code. The equilibria were reconstructed with the IDENTC code, using the external magnetic measurements and the experimental pressure profile. The results show that the evaluated high beta discharge is unstable in the central region of the plasma. This is related to the low shear and not to a large pressure gradient, as expected at the Troyon limit. In the pellet discharges the regions with the large pressure gradients are unstable to ballooning modes at the time of the beta decay, which ends the period of enhanced performance. The maximum pressure gradient in these discharges is limited by the boundary of the first region of stability. The observed phenomena at the beta decay are similar to those observed at the beta limit in DIII-D and TFTR.

Additional information

Authors: LAZZARO E, JET Joint Undertaking, Abingdon (GB);SMEULDERS P, JET Joint Undertaking, Abingdon (GB);HUYSMANS G T A, FOM-Institut voor Plasmafysica, Nieuwegein (NL);GOEDBLOED J P, FOM-Institut voor Plasmafysica, Nieuwegein (NL);GALVAO R M A, Instituto e Pesquisas Espaciais, Sao Jose dos Campos (BR)
Bibliographic Reference: Report: JET-P(89)34 EN (1989) 13 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
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