激光驱动的冲击波自生磁场以及外加磁场的冲击波放大研究

Shock wave amplification by shock wave self-generated magnetic field driven by laser and the external magnetic field

冲击波是天体物理观测中常见的现象,其对粒子的加速被认为是高能宇宙射线的来源.宇宙中冲击波周围往往存在很强的磁场,但人们对于此类强磁场的产生放大过程的理解并不充分.本文利用二维粒子模拟程序研究了激光与磁化或者非磁化等离子体相互作用产生的冲击波现象,给出了冲击波波前处磁场的产生放大特性.研究发现,作用过程中的自生磁场可以储存能量,从而进一步加速电子;当存在外加磁场时,由冲击波加速的电子和离子的能量都比同条件下非磁化等离子体的能量高;而且外加磁场藉由冲击波放大倍数则与其值有极大关系.与天文观测中推断的磁场与背景磁场相比放大千倍这一研究结果的比较可以看出,天体冲击波周围磁场放大主要是由局域内生磁场导致的.

Shock wave is a common phenomenon in astrophysics. Shock wave acceleration has been regarded as a source of high-energy cosmic rays. Very strong magnetic field exists in the surrounding of the shock wave at the edge of the supernova remnants. But the mechanisms of generation and amplification of such a strong magnetic field are not clear yet. In this paper, the properties of shock wave driven by the laser irradiating on un-magnetized and magnetized plasmas are investigated using two-dimensional particle-in-cell （PIC） simulations. It is found that very strong spontaneous magnetic field can be generated around the laser-driven shock front in the un-magnetized plasma. The spontaneous magnetic field can store energy and accelerate electrons further. When an external magnetic field is introduced, the electrons and ions are accelerated more efficiently by the shock wave than in the un-magnetized plasma. The external magnetic field can transfer its energy to electrons and ions, and strengthen the shock wave. In simulations, the introduced external magnetic field has three different strengths： 1072 MG, 107.2 MG and 10.72 MG, which determine the shock structures through the driven currents. There are two single-polar magnetic arcs that constitute the shock structure when the external magnetic field is 1072 MG, i.e., one is the shock itself and the other is actually the reverse shock, whereas only one magnetic arc is produced but with a bipolar structure in the direction perpendicular to the shock propagation when the externally added magnetic fields are much lower （107.2 MG and 10.72 MG）. The two bipolar magnetic structures will evolve into a single-polar arc when the externally added magnetic field is 107.2 MG, but they are kept for all the time when the external magnetic field is 10.72 MG. It can be explained by taking the Larmor radius into the consideration. That the amplification ratio of the magnetic field decreases as the introduced external magnetic field increases implies that the magnetic amplification in the space is possibly due to the local field generation rather than the field compression. An amplification ratio of tens of the external magnetic field is achieved due to the pseudo Rayleigh-Taylor instability, but still much smaller than that around the astrophysical shock front, indicating that other efficient mechanisms are responsible for the observed magnetic amplification around shocks in the supernova remnants.