High BetaN Regimes at JET: Progress Towards Steady-State Operation

High bN plasmas, with or without an ITBs, have been studied in JET AT scenarios at high triangularity and high normalised density at Ip = 1.2-1.8MA / BT = 1.8-2.7T. The current profile is tailored via a fast current ramp, ohmic or with Lower Hybrid Current Drive (LHCD), and early application of Neutral Beam (NBI) or NBI + Ion Cyclotron Resonance Heating (ICRH) power. The initial q profile has low or weakly reversed shear in the core. bN approaching 3 has been achieved with HIPB98(y,2) ~ 1.0­1.2. The development of an ITB contributes by 20-25 % to­bN, the best ITB performance being obtained when a barrier forms in both ion and electron temperature channels. In the absence of ITBs the global confinement and the core pressure decrease with increasing qmin. The total non-inductive current INI reaches transiently 75% of Ip, at the maximum values of bN and > 60% Ip in a more stationary phase. Unlike previous JET experiments, where strong pressure peaking led to MHD limitations at relatively low values of bN, the new data indicate that a viable route towards sustainable high bN operation may exist, with different q profiles, with or without ITBs, as long as strong pressure gradients are avoided. Of particular importance for JET is the preparation of the AT scenario in view of the upgrade of the JET heating power, NBI and ICRH: modelling suggests that operation at high b will be achievable at higher toroidal field which, in turn, may improve the LH wave accessibility and increase the capability to tailor and sustain a non-monotonic the current profile for times comparable to the resistive diffusion time.
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EFDC080513 169.52 Kb