(EX/P1-15) Internal Transport Barriers in FTU at ITER relevant plasma density with pure electron heating and current drive

V. Pericoli Ridolfini1), P. Buratti1), G. Calabṛ1), M. De Benedetti1), B. Esposito1), L. Gabellieri1), G. Granucci2), M. Leigheb1), M. Marinucci1), D. Marocco1), C. Mazzotta1), F. Mirizzi1), S. Nowak2), L. Panaccione1), G. Regnoli1), M. Romanelli1), P. Smeulders1), C. Sozzi2), O. Tudisco1), A.A. Tuccillo1)
 
1) ENEA - C.R. Frascati, Frascati (Roma), Italy
2) Associazione EURATOM-ENEA sulla Fusione, IFP-CNR, Milano, Italy

Abstract.  The advanced tokamak scenario eligible for the steady state scenario in ITER foresees to built and sustain an internal transport barrier (ITB) at high plasma density (ne up to 1020 m-3), with largely dominant electron heating and negligible momentum input. Contrarily to most present tokamaks, FTU can simultaneously satisfy these two requests: high plasma densities are easily accessed, while electrons are heated by two radiofrequency systems: electron cyclotron (EC, up to 1.5 MW at 140 GHz) and lower hybrid (LH, up to 1.9 MW at 8 GHz). LH also forms and sustains the current profile j(r) suitable for the ITB, by driving off-axis a large fraction of the plasma current Ip: an almost flat or slightly reversed profile q(r) (safety factor) with qmin∼1.3 is built inside the ITB radius, rITB. Successful methods to vary rITB through j(r) have recently allowed obtaining steadily rITB/a up to 0.67. Peripheral LH absorption and current drive (CD) is favored primarily by low q, and, at a bit lower extent, by broader Te(r) that can in turn be affected by off-axis EC heating. Off-axis EC-CD and central counter ECCD proved tools for direct finer shaping of j(r). Full CD conditions and τE (energy confinement time) improved up to 1.6τE, ITER97-L are maintained in plasmas with peak densities and electron temperatures up to $ \overline{{n}}_{{\rm e0}}^{}$≥1.3×1020 m-3 and Te0∼5.5 keV, for the whole duration of the heating pulse, longer than 35τE, and about one τR/L (ohmic current relaxation time). Good alignment of the bootstrap current is always obtained, with Ib/Ip up to 30%. At ne0∼0.8×1020 m-3 Te0 can be larger than 11 keV. The ITB strength is instead controlled by the level of the EC heating inside the barrier. The significant collisional ion heating inside the ITB ( ΔTi0/Ti0∼35%) indicates that e--i+ collisions do not affect the barrier dynamics. The ion heat diffusivity is lowered to the neoclassical value but the long e--i+ equipartition time ( ∼4 - 5τE) still prevents ion thermal equilibrium. Reflectometry shows a clear change in the turbulence close to the ITB radius, consistent with the reduced electron transport. An anomalous inward particle pinch persists also at high density, as the quite peaked profiles, ne0 ∼ 1.7ne(averaged ), in full CD plasmas show, despite the absence of the neoclassical Ware pinch.

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