A series of two flux loop (tokamak) merging experiments: TS-3, TS-4 and UTST revealed high-power reconnection heating of plasma ions up to 30MW [1,2]. Two dimensional

An advanced ISO-FLUX equilibrium control scheme is developed and implemented in the JT-60SA equilibrium controller. For future fusion reactor, the VDE control by the active superconducting coils is essential due to the absence of in-vessel coils and stabilization plates, and the presence of large gap between plasmas and vacuum vessels with the tritium breeding blanket. Under the circumstances, the equilibrium controller should achieve plasma position/shape control and plasma current control by superconducting coils within limited power supply voltages, where plasma current control affects plasma position/shape control, and vice versa. The newly developed ISO-FLUX control scheme enhances the controllability under the above hybrid control of position/shape and plasma current circumstances. Furthermore, the new scheme also provides good controllability under large eddy currents conditions, which will also be expected in future superconducting tokamaks, such as JT-60SA, ITER, and DEMO due to its large inductance and high target plasma current. The JT-60SA equilibrium controller with the developed scheme is well verified by a magnetohydrodynamic equilibrium control simulator ‘MECS’ and ready for the integrated commissioning of JT-60SA.

We report the experimental finding of n=1 helical cores (HCs) accompanied by saturated m/n=2/1 tearing modes (TMs) with low mode frequencies in JT-60U. The HCs accompanied by TMs were observed after an increase in the mode amplitude and a decrease in the mode frequency of m/n=2/1 precursors with tearing parity. The decreased mode frequency is typically lower than 20 Hz. With various diagnostics, the coupling of n=1 HCs and m/n=2/ 1 TMs has been clearly observed. Because the coherent oscillations in the ion temperature are observed in both the core region and the edge region, the flux surfaces including the m/n=2/1 magnetic island appear to have m=1 helical deformation. It has also been suggested that the m/n=2/1 TM and the HC rotate in the electron diamagnetic direction keeping f m/n=1/1(HC) =2f m/n=2/1(TM) in several plasmas. Here, f m/n=1/1(HC) is the mode frequency of HCs and f m/n=2/1(TM) is the mode frequency of TMs. In addition, the core seems to be shifted to the high-field side when the O-points of the m/n=2/1 magnetic island line up in the midplane, which is confirmed by reconstructions of magnetohydrodynamic equilibria with motional Stark effect measurement and the MEUDAS code. Our observation of m/n=2/1 TMs having HCs contributes to the understanding of the excitation mechanism of HCs in tokamak plasmas.

The first two-dimensional particle in cell simulation of the reconnection region of two merging torus plasmas lead us to quantitative studies on the energy conversion mechanism under a high out-of-plane (guide) magnetic field condition. Even with the existence of the strong guide field, magnetic reconnection causes the efficient conversion of in-plane (poloidal) magnetic field energy. The ratio of the plasma kinetic energy flux of ions to that of electrons is roughly two to one, in agreement with the recent experimental results. Due to the suppression of nonlinear dynamics of ions motions in the vicinity of the reconnection region with guide field, the major energy flux of ions is changed to the flow energy flux. For electrons, a field-aligned acceleration caused by parallel electric field generates the non-thermal electrons through trapping (bouncing) effect, which is exhausted as the anisotropic energy flux of electrons. The inventory of the converted magnetic energy in the case with the guide field is quantitatively revealed.

The key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of in-plane bipolar converging electrostatic electric field (E→es) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar E→es around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole out-of-plane magnetic field, which causes the parallel electric field (E→||) from E→ind to accelerate particles along the magnetic field. We explain the electron and ion dynamics and their velocity distributions and flow structures during the time-dependent driven reconnection as they move from the upstream to the downstream. In particular, we address the following key physics issues: (1) the decoupling of electron and ion dynamics due to meandering orbits around the field reversal region and the generation of a pair of converging bipolar electrostatic electric field (E→es) around the reconnection region; (2) the slowdown of electron and ion inflow velocities due to acceleration/deceleration of electrons and ions by E→es as they move across the neutral sheet; (3) how the reconnection current layer is enhanced and how the orbit meandering particles are accelerated inside the reconnection region by E→ind; (4) why the electron outflow velocity from the reconnection region reaches super-Alfvenic speed and the ion outflow velocity reaches Alfvenic speed; (5) how the quadrupole magnetic field is produced and how E→|| is produced; (6) how electrons and ions are accelerated by E→|| around the separatrix region; (7) why electrons have a flat-top parallel velocity distribution in the upstream just outside the reconnection region as observed in the magnetotail; (8) how electron and ion dynamics decouple and how the monopolar electrostatic electric field is produced around the separatrix region; (9) how ions gain energy as they move across the separatrix region into the downstream and how the ion velocity distribution is thermalized in the far downstream; and (10) how electrons move across the separatrix region and in the downstream and how the electron velocity distribution is thermalized in the far downstream. Finally, the main energy source for driving magnetic reconnection and particle acceleration/heating is the inductive electric field, which accelerates both electrons and ions around the reconnection current layer and separatrix regions.