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(EXP3/01) MHD Studies in FTU Plasmas with Low and Negative Magnetic Shear

   
P. Buratti , F. Alladio , G. Bracco , S. Cirant ¹, F. Crisanti , D. Frigione , E. Giovannozzi , L. Panaccione , O. Tudisco , L. Acitelli , B. Angelini , M. L. Apicella , G. Apruzzese , E. Barbato , L. Bertalot , A. Bertocchi , M. Borra , A. Bruschi ¹, G. Buceti , A. Cardinali , C. Centioli , R. Cesario , C. Cianfarani ³, S. Ciattaglia , V. Cocilovo , R. de Angelis , F. de Marco , B. Esposito , L. Gabellieri , G. Gatti , C. Gourlan , G. Granucci ¹, M. Grolli , A. Imparato , H. Kroegler , M. Leigheb , L. Lovisetto , G. Maddaluno , G. Maffia , S. Mancuso , M. Marinucci , G. Mazzitelli , P. Micozzi , F. Mirizzi , S. Nowak ¹, F. P. Orsitto , D. Pacella , M. Panella , V. Pericoli-Ridolfini , L. Pieroni , S. Podda , G. B. Righetti , F. Romanelli , F. Santini , M. Sassi , S. E. Segre ², A. Simonetto ¹, C. Sozzi ¹, S. Sternini , N. Tartoni , A. A. Tuccillo , V. Vitale , G. Vlad , V. Zanza , M. Zerbini , F. Zonca 
 
Associazione Euratom-ENEA sulla Fusione, CRE Frascati, CP 65 00044, Frascati, Roma, Italy
¹ Associazione Euratom-ENEA-CNR, Istituto di Fisica del Plasma, Milano, Italy
² INFM and Dipartimento di Fisica, Il Universitá di Roma `Tor Vergata', Roma, Italy
³ ENEA fellow

Abstract
Fast current ramps with central ECRH and pellet injection  in ohmic plasmas allowed to study several kinds of MHD instabilities in plasmas with peaked pressure profile and low or negative magnetic shear. Double tearing modes  were systematically observed when the minimum q value crossed the $\rm q_{min} = 2$ value; such modes either ended into full core reconnection, or saturated at a large amplitude, with a displacement involving more than one third of the plasma radius. Macroscopic fluctuations clamp the peak temperature during central ECRH; the cause of such fluctuations is attributed to MHD modes that are excited when $\rm q_{min}$ is close to a low order rational value; gaps in the distribution of such values can explain the effective transport barriers  observed near interger $\rm q_{min}$ values. When $\rm q_{min} \approx 1$ either sawteeth  or saturated internal kink modes are observed.

         

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