Description
High beta discharges with normalized Larmor radius (ρ*), and normalized beta (βN) relatively close to JT-60SA scenarios hybrid and advanced were realized in a new series of experiments on JET-ILW[1]. The strategy of these experiments was to explore high normalized beta (βN) values, MHD effects at different BT ( Toroidal Magnetic field on axis) and find discharges with mild MHD. JET/JT-60SA ‘similarity experiments‘ can be done with plasma parameters on JT-60SA, at BT/Ip=1.45T/1.4MA and heating power less than 10MW. Deuterium plasmas were realized in hybrid scenario at BT = 1.7, 2, 2.4 T, Ip = 1.4 MA, elongation k = 1.7, triangularity δ ≈ 0.4, q95=4-6, and central safety factor q0>1 at NBI ( Neutral Beam Injection) start, with NBI power PNBI = 16-25 MW. The preliminary results of the JET high beta experiments were presented in [1]. Here an overview is presented on the following topics : i) confinement properties of high beta discharges versus input power and scaling of confinement improvement with beta; ii) dynamics of formation of the Internal Transport Barriers(ITB) while approaching high beta; iii) pedestal structure and its evolution at high beta ; iv) current profile evolution. It is found that the maximum normalized beta ( βN) obtainable, increases with input power and depends ( decreases with ) on the magnetic field . These effects are compatible with a small dependence of the confinement time on the input power. Looking into the electron kinetic spatial profiles , the progress toward the high beta is characterized by the formation of an ITB. The confinement dynamics can be related to the critical gradient scale length of the ITG turbulence, which is depending on the ratio Ti/Te and magnetic shear. The confinement properties of these discharges indicate that the H98Y ( the improvement confinement factor with respect to the ITER confinement scaling ) has a linear dependence on the βN, changing with the magnetic field. TRANSP analysis done for the pulse 103116 (B=2.4T) shows that the spatial profile of the diamagnetic current increases from negative values toward zero while βN increases toward the maximum.
- F P Orsitto et al , High beta experiments on JET in preparation of JT-60SA , Proc. 50th EPS Conf on Plasma Physics , 8-12 july 2024, Salamanca , Spain , paper P1-151