High beta experiments on JET in preparation of JT60SA

High beta discharges with dimensionless parameters collisionality (ν*), normalized toroidal Larmor radius (ρ*), and normalized beta (βN) relatively close to the JT-60SA scenarios hybrid and advanced were realized on JET: the JET ρ*=0.04 and bootstrap fraction fBS=0.4 were close to JT60SA values. So the discharges realized on JET are expected having comparable confinement properties as in JT60SA[1]. Since the maximum normalized beta βN_MAX≈ A-1/2 , is slowly dependent on the aspect ratio A, the equilibrium properties at high beta on JET (A=3.1) would not differ in JT60SA(A=2.5). Using the similarity scaling laws [1], JET pulses at BT/Ip (magnetic field/current)=2.4T/1.4MA are ‘similar’, i.e. share confinement and beta behaviour with the JT60SA scenario 5-1 with parameters BT=1.62T, Ip=1.4MA, and auxiliary heating power PAUX_JT60SA >10MW. The JET60SA high beta program can take profit of the high beta JET experiments in particular: i) in the phase of program development at BT=1.6-1.7T at low power to optimize the JT60SA current drive capabilities to get the full current drive , and ii) as a database for the validation of transport models. Strategy of JET experiments was to explore high normalized beta (βN) values, MHD effects at different Ip/BT and find parameters for discharges with mild MHD. Deuterium plasmas were realized in a variant of the hybrid-advanced scenario at toroidal magnetic field BT = 1.7, 2, 2.4 T, plasma current Ip = 1.4 MA, elongation k = 1.6, and high triangularity δ ≈ 0.4, q95=3.5-4.5, and central safety factor q0>1.2 at NBI start, with NBI power PNBI = 16-25 MW, no ICRH. Shots at BT=2.4T were realized in third Deuterium-Tritium (DTE3) campaign. The deuterium dataset is new. Pulses at BT=1.7/Ip=1.4MA are similar to 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[3]. are executed at higher q95 with respect to the yr 2014 advanced pulses. Two scans were executed : i) a NBI power scan, affecting βN and ii) a NBI start time scan, affecting the central safety factor q0, which is a key ingredient for MHD stability of the high beta phase. Results of the experiments were: i) Good confinement properties and relatively high βN values; the betaN achievable increases with input power, βN>3.5 for BT/Ip=1.7T/ 1.4MA ; ii) Good control of q0 at start of main heating phase with NBI starting time t0_NBI depending on the toroidal magnetic field value, as investigated in JET hybrid and advanced scenarios; iii) Maximum βN≈ 2.5-2.7 with mild MHD at BT = 2.4 T and q0 > 1 (pulse#103116). Preliminary transport analysis has been done using Bohm-gyroBohm , QuaLiKiz , CDBM codes . The measured ion and electron temperature profiles and neutron fluxes are reproduced by CDBM code at all the magnetic fields. 1.Michele Romanelli and Francesco Paolo Orsitto PPCF 63(2021)125004 2.C Challis et al., Nucl. Fusion 55(2015) 053031 3.J Mailloux et al , 41st EPS Conf. Plasma Physics Berlin 2014 , O4.127. (*) see the author list of ‘Overview of T and D-T results JET with ITER-like wall’ by C Maggi et al. to be published in Nuclear Fusion Special Issue for IAEA FEC23 London 19-21 oct.2023.