  Magnetic reconnection has long been known to be the most important mechanism
not only for mixing the plasmas by changing the magnetic field topology but
also for releasing the magnetic field energy into the plasma kinetic energy.
During magnetic energy release, it is possible for some of the heated plasma to
be accelerated to energies much higher than the thermal energy. Recently, the
energy partitioning of the thermal and the nonthermal energy has been studied
by using particle-in-cell (PIC) simulations, and it has been shown that the
acceleration efficiency of nonthermal particles increases with increasing the
plasma temperature, and the nonthermal energy density occupies more than 90% in
the total heated plasma when the Alfven velocity is close to the speed of light
c. However, the acceleration efficiency decreases as the guide magnetic field
increases. So far the acceleration efficiency has been mainly studied in
two-dimensional systems, but it is interesting to study three-dimensional
effects where the patchy and turbulent reconnection can dynamically occur. This
study explores the effects of three-dimensional relativistic reconnection on a
pair plasma with the guide magnetic field, utilizing three-dimensional (3D) PIC
simulations. The results indicate that the decrease in nonthermal particle
production is smaller in 3D guide-field reconnection compared to 2D. More
importantly, contrary to general expectation, 3D reconnection is capable of
maintaining a hard nonthermal energy spectrum even in the presence of a strong
guide magnetic field.
