The Transport Of Super-thermal Electrons Generated By Ultra-strong Lasers In Micro-nano-structured Targets

In the interaction of an ultra-intense laser with solid target,a large fraction of laser energy is transferred to forward-going fast electrons.The generation and transport of fast electrons have attracted much attention for a fast electron beam has potential applications in fast ignition of inertial confinement fusion,ion acceleration,compact x-ray source and relativistic positron generation.In general,these applications demand for well collimated and narrow electron beams.However,recent experimental and simulation results found that a fast electron beam has a large divergence angle when it is generated by irradiating laser on solid target.To date,the angular spread of fast electrons in laser-plasma interaction region is not fully understood.However,it is clear that the beam divergence,mainly caused by the Weibel instability near target front surface,is inevitable.To confine fast electron beams,many guiding and collimating schemes have been proposed.Recent experiments have demonstrated that nano-array structures,such as metal nanowires or carbon nanotubes,are helpful for guiding electrons.By using this kind of targets,one can artificially construct some aligned micro/nano wires along a certain direction,which could be used to control fast electrons to transport along the wires.However,the previous works are accomplished under femtosecond laser conditions.Generally speaking,pulse domain of picoseconds is suggested to greatly improve the brightness of X-ray source,the population of positrons and the energy deposition in fast ignition.And high laser energy means a rigorous demand on target structure.Based on the above background,in this paper we will research on the transport of picosecond fast electron beam in nanostructure targets and perform four works as follows.First,we give an analysis of the factors which cause the angular dispersion of fast electrons.The Weibel instability is detailedly discussed and the influence of laser wavelength and intensity in the angular dispersion of hot electrons is investigated.Both our theoretical model and numerical simulations show that the angular dispersion would increase rapidly with the laser frequency,while the laser intensity has little effect on it.The reason for the dispersion independent of laser intensity is:when the transverse components of ponderomotive force and Weibel magnetic pressure increase with the laser intensity,the longitudinal momentum of fast electrons is also increased,leading to the fixedness of dispersion v?/v||.Therefore,the change of divergence angle with laser intensity will be inconsiderable in the subsequent research.Second,the transport of picosecond laser generated fast electrons in a nanowire array is studied.Our two-dimensional collisional particle-in-cell simulations show that the electric field on the wire surface can be neglected in the picosecond scale transport.The fast electron beam is initially guided and collimated by strong magnetic filaments in the array.Subsequently,after the decomposition of the structure of nanowire array due to plasma expansion,the beam is still collimated by the resistive magnetic field.An analytical model is established to give a criterion for long-term beam collimation in a nanowire array;it indicates that the nanowire cell should be wide enough to keep the beam collimated in picosecond scale.According to the criterion,we designed a nanowire target and performed experiments.The experimental results show a good collimation and tight focus of fast electron beam in the tailored nanowire target.Third,nanopore arrays are proposed to manipulate fast electron beams produced by ultraintense lasers.We demonstrate in experiments that fast electron beams with controllable direction and small spots can be obtained.More interestingly,electrons can form annular structures in the target.The implied physics is investigated with two-dimensional particle-in-cell simulations.Investigation shows that the beam manipulation is due to the micro-structured magnetic fields generated in the nanopore arrays.By the longitudinal reduction of magnetic field,the angular dispersion of fast electron is gradually decreased along the nanopores.Electrons with energy below a certain threshold form a collimated beam in the central region while residual electrons will form annular beams in the fringe regions.The energy threshold is related to the nanopore distance,therefore the beam structure can be artificially manipulated via changing the nanopore distance.The collimation could increase the beam compactness and energy concentration while the annular structure is broadly believed to improve the current limit of ultraintense beams.Finally,the transport efficiency of fast electrons in the nano-array target is investigated.Via comparing the backside Ka yield with the foreside Ka yield,we measure the penetration length in a nanopore array target and a planar target,respectively.It is found that the penetration length for MeV electrons is around 100?m.When the transport distance equals to the penetration length,the beam energy will be reduced to 25%.Through experimental results,we found the penetration length in the nanopore array is three times which in the planar target.Meanwhile,the ratio of transport efficiency in nanopore arrays compared to which in planar target increase with the target thickness.In the thick target,the transport efficiency of fast electrons in nano-array target may be one order higher than which in planar target.Thus,the nano-array target can be used to enhance the fast-electron transport efficiency.