Influence of a Guide Field on Collisionless Driven Reconnection

Abstract

The influence of a guide field on collisionless driven reconnection is investigated by means of twodimensional electromagnetic particle simulation in an open system. In a quasi-steady state when reconnection electric field evolves fully, a current layer evolves locally in a narrow kinetic region and its scale decreases in proportion to an electron meandering scale as the guide field is intensified. Here, the meandering scale stands for an average spatial scale of nongyrotropic motions in the vicinity of the reconnection point. Force terms associated with off-diagonal components of electron and ion pressure tensors, which are originating from nongyrotropic motions of charged particles, becomes dominant at the reconnection point and sustain the reconnection electric field even when the guide field is strong. It is also found that thermalization of both ions and electrons is suppressed by the guide field. For the weak guide field, an electron nonthermal component is significantly created through a fast outburst from the kinetic region, while for the strong guide field, an ion nonthermal component is generated through the acceleration by an in-plane electric field near the magnetic separatrix.

Energy transfer and electron energization in collisionless magnetic reconnection for different guide-field intensities

“…We carry out two dimensional particle-in-cell simulations of driven magnetic reconnection using the PASMO code 1,[18][19][20] . The system is subject to an external driving flow, obtained by imposing an electric field at the two upstream boundaries (y = ±y b ), perpendicular to the magnetic field, which pushes particles into the simulation domain via the E × B drift.…”

Electron and Ion Heating Characteristics during Magnetic Reconnection in the MAST Spherical Tokamak

“…Figure 5 shows the effect of guide field for electron and ion heating with B rec ∼ 0.08T. The localized X point electron heating becomes more steep and increases under high guide field condition probably because the higher guide field suppresses cross-field collisional transport, so that the electrons remain in the region of high toroidal electric field for longer or the enhancement of steep sheet current profile for smaller amplitude of meandering motions by higher guide field [38]. However bulk ion heating downstream does not change as demonstrated in the push ST merging experiment with intermittent plasmoid ejection in TS-3 [39] and PIC simulation [40].…”

Global Ion Heating during ST Merging Driven by High Guide Field Reconnection

“…The corresponding E × B drift velocity is about 11 km/s which is equivalent to 0.63 eV per hydrogen ion -much smaller than energy required to compensate for the increment of thermal energy (∼ 10 eV). The most probable interpretation of this global ion heating profile is that upstream nonadiabatic ions move across the separatrix and are accelerated by a large in-plane electric field near it (potential drop across separatrix is more than 10 V) [18]. The ions accelerated through this in-plane electric field are significantly heated by viscous damping and affected by the magnetic field further in the downstream, increasing the chance to get thermalized by collisions with the plasma inside the thick closed (reconnected) field lines surrounding the hot spot [10].…”