Localized electron heating in merging spherical tokamaks T. Yamada 1 , M. Gryaznevich 2 , H. Tanabe 1 , R. Scannell 2 , C. Michael 2 , S. Kamio 1 , T. Ii 1 , Y. Hayashi 1 , R. Imazawa 1 , M. Inomoto 1 , Y. Ono 1 , and the MAST team 2 1 Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan 2 EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, UK Spherical tokamak (ST) is a low aspect ratio tokamak, which means that the ratio of the major radius to minor radius is less than two. ST could lead to a compact and economical fusion reactor if the central solenoid (CS) can be removed. Therefore, CS-less start-up and non-inductive plasma sustaining method are required for the ST research development. CS-less ST start-up is studied all over the world, such as, by using electron cyclotron waves [1], radio-frequency wave [2], coaxial helicity injection [3], and point-source dc helicity injection [4]. Here, we discuss one promising CS-less ST start-up method, that is, plasma merging start-up. When two initially created plasmas merge together to form a single plasma, magnetic field lines reconnect, and their energies are converted to the plasma kinetic or thermal energies during a very short period. On TS-3, two STs were merged to form a single ST having beta up to 50% in the cohelicity merging, and an oblate field-reversed configuration plasma formed by two spheromaks in the counter-helicity merging was transformed to an ST having ultra-high-beta up to 80% [5–7]. It has been reported that electrons are heated inside the current sheet during magnetic reconnection, while ions are heated at around the two downstream areas [8]. However, compared to the ion heating, the electron heating has not been clarified in detail, e.g., when and where in the current sheet electrons are heated. This work investigated and determined that electrons are locally heated at the merging X-point by using two ST devices; one is the Mega Ampere Spherical Tokamak (MAST) device, which is the world’s largest plasma merging device, and the other is the University of Tokyo Spherical Tokamak (UTST) device, which uniquely demonstrates double null merging (DNM) by using out-vessel poloidal field coils. MAST has the highest magnetic field (~0.6 T) and Lundquist number (10 6 –10 8 ) among the merging laboratory plasma devices [9]. The plasma merging start-up method in MAST is called merging compression, which creates two initial plasmas around a pair of in-vessel poloidal field coils by its current ramping down and merges them at the mid-plane. In MAST, initial currents up to 500 kA and initial electron heating up to 1.2 keV were measured during the merging-compression plasma formation. Figure 1 shows the schematic cross view of the 39 th EPS Conference & 16 th Int. Congress on Plasma Physics P1.013