激光与近相对论临界密度薄层相互作用产生大电量高能电子束

研究了激光与近相对论临界密度等离子体薄层相互作用时所产生的高能电子束的主要特征,包括平均有效温度以及截止能量等.实验结果表明,电子束的电量超过nC量级,平均有效温度可达8 MeV以上.PIC数值模拟证明,近相对论临界密度等离子体内,相对论自透明效应和激光钻孔效应共同形成一条磁化等离子体通道,电子与激光将在角向磁场的协助下发生Betatron共振.激光可将电子直接加速到很高能量,因此电子束平均有效温度("斜坡温度")远远超过Wilks定标率预计的平均温度.该研究为产生高亮度X射线源提供了一种新的可能途径.

High energetic electron bunches from laser–near critical density layer interaction

In this paper, we report our results from interactions between sub-picosecond laser and relativistic near-critical density plasma layer. To create the near-critical density plasma layer, low density foam targets are utilized in our experiments. The foam is comprised of tri-cellulose acetate. Their average densities vary from 1 mg/cm³ to 5 mg/cm³, corresponding to full ionization densities ranging from 0.6n_c to 3n_c. When laser pulse is incident on the near-critical density plasma, some energetic bunches with a large quantity of charges are measured in most of the shots. The maximum charge quantity reaches to 6.1 nC/sr. Furthermore, the observed electron energy spectrum is Boltzmann-like with a wide plateau at the tail of the energy spectrum, rather than a Maxwell-like. The concept of average temperature is not available any more, and we define average effective temperature instead, namely the slope temperature. Fitting the Boltzmann-like spectrum exponentially, we find that the average effective temperature even exceeds 8 MeV at 7.5×10¹⁹ W/cm², far beyond the ponderomotive limit. Aiming at analyzing the implication of physics, several two-dimensional particle-in-cell (PIC) simulations are performed. The PIC simulations indicate that the hole-boring effect and relativistic self-transparency play an important role in the electrons heating process. At the earlier stage of heating process, a short plasma channel is created by the hole-boring effect and relativistic self-transparency. The length and the width of the plasma channel are about tens of micrometers and several micrometers respectively. Around the plasma channel, there is an intensive azimuthal magnetic field. The magnitude of the azimuthal magnetic field is 100 MGs. However, the radical electrostatic field is not seen. The possible reason is that the plasma channel would be cavitated by the hole-boring effect. As a result, the electrons will experience Betatron resonance in the magnetized plasma channel. The traverse momentum of the electron would be converted into forward momentum. Assisted by the Betatron resonance, the electrons gain energies from the laser directly and efficiently. Thus, the average effective temperatures of the electron bunches are much higher than predicted by the ponderomotive scaling law. Besides, we also conducte another simulation to instigate the differences by adopting different laser polarizations. Within our expectation, the electron spectrum of the P-polarization accords well with the experimental result, while the electron spectrum of the S-polarization obviously deviates from the experimental result. It also demonstrates that the Betatron resonance heating dominates the electron acceleration process. This research paves the way to generating the highly energetic bunches with a large quantity of charges, and wound also be helpful for producing the high-bright laser bremsstrahlung sources in future.