High beta experiments on JET in preparation of JT60SA

High beta discharges with dimensionless parameters collisionality (ν*) and normalized Larmor radius (ρ*), and also normalized beta (βN) relatively close to the JT-60SA scenarios hybrid and advanced were realized on JET. In particular the JET poloidal Larmor radius ρP*=0.036 and bootstrap fraction fBS=0.6 were close to JT60-SA values. So the discharges realized on JET are expected having similar confinement properties as in JT60SA[1]. Since the maximum normalized beta βNMAX≈ A-1/2, is dependent on the aspect ratio A, the equilibrium properties at high beta on JET (A=3.2) would be similar as in JT-60SA(A=2.7). Strategy of the experiments was to explore high normalized beta (βN) values, MHD effects at different BT ( Toroidal Magnetic field on axis) and find an optimal set of parameters for discharges with mild MHD. Deuterium plasmas were realized in a variant of the hybrid-advanced scenario at BT = 1.7, 2, 2.4 T, Ip = 1.4 MA, elongation k = 1.6, and high triangularity δ ≈ 0.4, q95=4-6, and central safety factor q0>1.2 at NBI ( Neutral Beam Injection) start, with NBI power PNBI = 16-25 MW, no ICRH ( Ion Cyclotron Resonant Heating). Shots at BT=2.4T were realized in third Deuterium-Tritium (DTE3) campaign. The deuterium dataset is new: the pulses at BT=1.7/Ip=1.4MA have the same characteristics of 2014 Hybrid power scan at high δ[2], but there is an extension in the range of NBI power to PNBI =25MW. While pulses at BT=2.4T are executed at higher q95 (q95>5) with respect to the yr 2014 advanced pulses[3]. Two scans were carried out : i) a NBI power scan, affecting 50th EPS Conference on Contr. Fusion and Plasma Phys, 8-12 July 2024 F. P. Orsitto et al. : P1.051 (2024) 2 βN and ii) a NBI start time scan, affecting the central safety factor q0 at the time of NBI onset, which is a key ingredient for MHD stability of the high beta phase. Main results of the experiments are: i) Good confinement properties and relatively high βN values; the normalized beta achievable increases with input power, βN≈3.7-4 for BT/Ip=1.7T/1.4MA(JPN 102422) ; ii) Good control of q0 at the beginning of the main heating phase with NBI starting time t0_NBI depending on the toroidal magnetic field value, as investigated in JET hybrid[2] and advanced scenarios; iii) Maximum βN≈2.5-2.7 with relatively mild/stable MHD in the hybrid-advanced scenario at BT = 2.4 T and q0 > 1 (pulse#103116 and 103117). Preliminary transport analysis has been done using Bohm/gyroBohm , QuaLiKiz , CDBM. The measured ion and electron temperature profiles and neutron fluxes are reproduced by CDBM code at all the magnetic fields. The paper is organized as follows : in sec.2 the experiment is described and the usefulness of the JET experiment for preparing the high beta scenarios in JT-60SA is discussed ; in sec.3 The main experimental results are presented : i) the max βN versus the NBI heating power ii) the MHD characteristics ; iii) the confinement improvement factor and the plasma energy content versus βN ; in sec .4 the conclusions are drawn.