Laboratory observation of electron energy distribution near three-dimensional magnetic nulls

The acceleration of electrons near three-dimensional (3D) magnetic nulls is crucially to the energy conversion mechanism in the 3D magnetic reconnection process. To explore electron acceleration in a 3D magnetic null topology, we constructed a pair of 3D magnetic nulls in Peking university Plasma Test device (PPT) and observed acceleration of electrons near magnetic nulls. This study measured the plasma floating potential and ion density profiles ambient the 3D magnetic null. The potential wells near nulls may related to the energy variations of electrons, so we measured the electron distribution functions (EDF) at different spatial positions. The axial variation of EDF shows that the electrons are deviated from the Maxwell distribution near magnetic nulls. With scanning probes that could measure directionally and theoretical analysis based on curves fitting, the variation of EDF are linked to the changes of plasma potential under 3D magnetic nulls’ topology. The kinetic energy of electrons accelerated by the electric field is 6eV (v_e~7v_(e-Alfven)) and the scale of the region where accelerating electrons exist is in the order of serval electron skin depths.