(EX/P1-1) JET Hybrid Scenarios with Improved Core Confinement

F. Crisanti1), A. Becoulet2), P. Buratti1), E. Giovannozzi1), C. Gormezano1), E. Joffrin2), A. Sips3), C. Bourdelle2), A. Cardinali1), C. Challis4), N. Hawkes4), J. Hobirk3), X. Litaudon2), G. Regnoli1), M. Romanelli1), A. Thyagaraja4), A. Tuccillo1), JET EFDA Contributors5)
1) Euratom-ENEA Fusion Association, Frascati, Italy
2) Association Euratom-CEA, DSM/DRFC, Cadarache F-13108, France
3) Max-Planck-Institut fur Plasmaphysik, Euratom Association, 85748 Garching, Germany
4) Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK
5) App. of M. L.Watkins et al., Fusion Energy 2006 (Proc. 21st Int. Conf. Chengdu, 2006) IAEA, (2006)

Abstract.  The data from the Hybrid scenario experiments has been incorporated into a database that comprises 100 discharges and 280 time points, and contains more than 30 plasma and scenario parameters. Analysis of the database has revealed that there is a spectrum of temperature gradient scale lengths that exceed the critical level considered at JET to indicate the presence of an ITB. In the best example ( BT = 2.5 T, Ip = 2.1 MA, PNBI = 13.4 MW, PICRH = 2.5 MW, 1 < q(0) < 1.5) it was R/LTi∼14, R/LTe∼11) on a wide region ( r∼0.2 m) of the plasma core ( r/a∼0.4). These steep gradients were steady for almost 5 seconds. The central ion and electron temperature were Ti(0)∼17 keV and Te(0)∼8 keV. In the initial phase of the heating pulse the edge temperatures had been quite low ( Ti≈Te∼2 keV), but suddenly they increased to 4keV while, at the same time, the edge density decreased providing a more or less constant edge pressure. The discharge was essentially without ELMs, although the additional heating power was well above both the usual H-mode power threshold. The global performance was comparable to, or slightly better than, that of an equivalent standard Hybrid discharge with type I ELMs ( βN∼2, H89∼2.2). The plasma energy associated with the pedestal was Wped∼37% in this discharge, whilst the part within the steep gradient region was Wcore∼23%. Despite the relatively low density the toroidal rotation was only about half that of a typical JET ITB plasma. Nevertheless, the E×B shearing rate is calculated to be very large. Analysis using the gyrokinetic code, KINEZERO shows that, in the improved confinement region, ITG-TEM wavelength instabilities should be stable. An important factor in the plasmas with improved core confinement appears to be the density, which was higher in the case of similar Hybrid plasmas without core confinement improvement. Transport and stability analysis of discharges with and without core confinement improvement will be presented and compared. So far plasmas with improved core confinement in the Hybrid domain of JET represent a small minority of the database and occur only at low density and plasma collisionality. However, the possibility to obtain good global confinement in plasmas without significant ELM activity would be a useful extension of the Hybrid scenario operational domain and very attractive for ITER operations.

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