强激光产生的强磁场及其对弓激波的影响

Strong magnetic fields generated with a metal wire irradiated by high power laser pulses and its effect on bow shock

强激光照射金属线圈后,会在打靶点附近的背景等离子体中诱发冷电子的回流,在金属丝内形成强电流源,从而产生强磁场.本文利用神光II高功率激光器产生的强激光照射金属丝靶,产生了围绕金属丝的环形强磁场.利用B-dot对局域磁感应强度进行了测量,根据测量结果,结合三维模拟程序,反演得到磁场的空间分布.再利用强激光与CH平面靶相互作用产生的超音速等离子体撞击该金属丝,产生了弓激波.通过光学成像手段研究了磁场对冲击波的影响,发现磁场使得弓激波的轮廓变得不明显并且张角变大.同时,通过实验室天体物理定标率,将金属丝表面等离子参数变换到相应的天体参数中,结果证明利用该实验方法可以在实验室中产生类似太阳风的磁化等离子体.

Laboratory astrophysics is a rapid developing field studying astrophysical or astronomical processes on a high-power pulsed facility in laboratory. It has been proved that with the similarity criteria, the parameters in astrophysical processes can be transformed into those under laboratory conditions. With appropriate experimental designs the astrophysical processes can be simulated in laboratory in a detailed and controlled way. Magnetic fields play an important role in many astrophysical processes. Recently, the generation of strong magnetic fields and their effects on relevant astrophysics have attracted much interest. According to our previous work, a strong magnetic field can be induced by a huge current formed by the background cold electron flow around the laser spot when high power laser pulses irradiate a metal wire. In this paper we use this scheme to produce a strong magnetic field and observe its effect on a bow shock on the Shenguang II （SG II） laser facility. The strength of the magnetic field is measured by B-dot detectors. With the measured results, the magnetic field distribution is calculated by using a three-dimension code. Another bunch of lasers irradiates a CH planar target to generate a high-speed plasma. A bow shock is formed in the interaction of the high-speed plasma with the metal wire under the strong magnetic condition. The effects of the strong magnetic field on the bow shock are observed by shadowgraphy and interferometry. It is shown that the Mach number of the plasma flow is reduced by the magnetic field, leading to an increase of opening angle of the bow shock and a decrease of the density ratio between downstream and upstream. In addition, according to the similarity criteria, the experimental parameters of plasma are scaled to those in space. The transformed results show that the magnetized plasma around the wire, produced by X-ray emitted from the laser-irradiated planar target in the experiment, is suitable for simulating solar wind in astrophysics. In this paper, we provide another method to produce strong magnetic field, apply it to a bow shock laboratory astrophysical study, and also generate the magnetized plasma which can be used to simulate solar wind in the future experiments.