Investigation of fine structure formation of guide field reconnection during merging plasma startup of spherical tokamak in TS-3U

Abstract

Ion heating/transport and its fine structure formation process through magnetic reconnection have been investigated by high guide field tokamak merging experiments in TS-3 and TS-3U. In addition to the previously reported demonstration of high-temperature plasma startup without center solenoid, the detailed fine structure formation process of reconnection heating has been revealed using new 96CH/320CH ultra-high-resolution 2D ion Doppler tomography diagnostics. By identifying the double-axis field configuration with the X-point on the midplane using in situ magnetic probe diagnostics, the detailed measurement successfully revealed that the ion temperature profile forms two types of characteristic heating structure, both around the X-point and downstream. The former is affected by the Hall effect to form a tilted heating profile, while the latter is affected by the transport process which a forms a poloidal double-ring-like structure. The achieved ion heating mostly depends on the reconnecting component of the magnetic field, and the contribution of the guide field to decrease the heating efficiency tends to be saturated in the high guide field regime. Under the influence of better toroidal confinement with higher guide field, the downstream ion heating is transported vertically, mostly by parallel heat conduction, and finally forms a poloidal ring-like hollow distribution aligned with the closed flux surface at the end of merging.

Onset of fast magnetic reconnection and particle energization in laboratory and space plasmas

“…They observed the dissipation region adjusts to produce impulsive behaviour. Recent studies on energy transfer in different guide field configurations have been carried out with MRX, supported by Kinetic simulations (Fox et al 2018;Pucci et al 2018), as well as with the Spheromak merging experiments in Tokyo Tanabe et al (2019), and compared with observations from MMS (Eriksson et al 2016;Wilder et al 2017). In Yoo et al (2014) asymmetric (plasma density and magnetic field strength) current sheets have been reproduced with the MRX apparatus, confirming simulation studies (see, e.g., Pritchett 2008).…”

“…2018), as well as with the Spheromak merging experiments in Tokyo Tanabe et al. (2019), and compared with observations from MMS (Eriksson et al. 2016; Wilder et al.…”

27th IAEA Fusion Energy Conference: summary of sessions EX/C, EX/S and PPC

“…Establishing techniques for solenoid-free start-up is a critical issue for the further development of the ST concept towards burning plasma operation. Progress in this area was discussed in several papers which also considered the prospects for future R&D. Experiments carried out at the UTST, TS-3 and TS-3U (TS-6) STs on plasma formation by axial merging of 2 start-up plasmas and the implications for the expansion of ST experiments towards thermonuclear conditions were discussed in a series of papers [169,170,171]. The merging process involves magnetic reconnection of the two initial plasmas in the presence of a guide (toroidal) field, B t , which is perpendicular to the reconnection (poloidal) field, B rec .…”

Control of electron acceleration process during merging start-up of spherical tokamak

“…Axial plasma merging is one of the ST plasma start-up schemes without use of a CS coil and is adopted in several ST experiments to achieve high performance discharge. The merging method is capable of reducing CS flux and forming high ion temperature ST plasma [17,18] due to the magnetic reconnection process that converts magnetic energy to plasma kinetic/thermal energy. Since the duration of plasma heating and current drive provided by the merging start-up scheme is limited within a short period (<1 ms), the formed ST plasma must be sustained by external heating/current drive methods, such as neutral beam and high frequency waves.…”