Towards a Steady-State Scenario with ITER Dimensionless Parameters in JET
To demonstrate ITER Steady-State scenario, total bN 3, thermal bN 2.4, H98 1.4-1.5 and bootstrap current fraction (fBS) 750% need to be demonstrated in existing devices, ideally at dimensionless parameters such as the normalised Larmor radius (r*), collisionality (n*), ratio of ion to electron temperature (Ti/Te), qprofile and Mach number as close as possible to those in ITER since they affect transport and/or current drive. This motivated experiments in JET where the performance of high triangularity NBI-only plasmas developed to study stability and confinement at high bN and H98 was extended to higher BT, IP (2.7T, 1.8MA, q95 = 4.7) and power (electron heating ICRH, LHCD in addition to NBI). Good ICRF coupling is obtained with gas dosing, with good edge confinement (H98 ~1) and type I ELMs. With the ICRF ELM resilient systems, up to 8MW is coupled. LH coupling is affected by the nearest ICRH antenna and only PLH 2.5MW was available. The following steady (>10¥tE) and peak performances are obtained: H98 1.2 (up to 1.35), bN 2.7 (up to 3.1). The thermal fraction is up to 80% and <Ti>/<Te> 1.05, with Te = Ti at the pedestal. The Mach number at midradius 0.3, and r*/r*ITER 2, n*/n*ITER 4. In the range of target q-profiles used (1.8 < qmin< 2.9), only plasmas with an internal transport barrier, in addition to an edge transport barrier, have H98 >1.1. In some shots, n = 1 kink modes are seen, often limiting the performance and in a few cases causing disruptions. In all shots the q-profile is evolving, indicating a shortage of Non-Inductive (NI) current. Interpretative modelling show that fBS 40-45% where the highest fBS is for plasmas with highest q0 and bN. Predictive modelling shows that fBS goes from 45% to 50% when scaling to ITER n*. NBI provides ~20% of externally driven NI current and LHCD contributes <10%. An ECRH system was recently proposed for JET. Predictive modelling shows that ECCD can provide the narrow off-axis current required to maintain the q-profile at the time of high performance.