Generation and transport of fast electrons under a nonuniform magnetic field produced by a coil target

We investigate with particle-in-cell simulation the transport of fast electrons of MeV under a kT-level magnetic field produced by a circular coil target, where the fast electrons are generated by a picosecond laser pulse with typical intensity of 5×10^{19}W/cm^{2} adopted in fast ignition. The fast electrons can be well collimated when the coil diameter is about 200µm in the fast ignition or double-cone ignition scheme of laser fusion when the current in the coil is taken as around 200 kA according to typical experimental results. The optimized coil diameter results from the competition between the strength enhancement and the nonuniformity growth of the magnetic field as the reduction of the coil diameter. Here the nonuniformity growth can induce the magnetic mirror effect, which causes the reflection of some transporting fast electrons. In particular, when the coil diameter is larger than 500µm, the fast-electron transport under the produced magnetic field is very close to the case with a uniform magnetic field. We also find shifting the coil longitudinal position along the laser progapagation can change the electron transport range within the convergent or divergent magnetic field lines, and placing the coil tens of micrometers away from the electron generation position is favorable for the electron transport. This work could be applied not only in fast ignition and double-cone ignition schemes, but also to ion acceleration and radiation sources based on laser-driven fast electrons.