美国普林斯顿大学Abraham Chien和Hantao Ji团队的一项最新研究提出了实验室等离子体中磁驱动重联的非热电子加速。2023年1月16日出版的《自然—物理学》杂志发表了这项成果。
本文报道了利用激光电容线圈平台在实验中直接测量低β磁驱动重联加速非热电子。研究人员使用千焦耳激光驱动平行电流，在准轴对称几何中重新连接兆高斯级磁场。在粒子模拟支持下，测量电子能谱和由此产生的加速能量角依赖性表明，面外重联电场的直接电场加速机制正在起作用。使用这种机制的尺度能量显示出了与天体物理学观测的直接相关性。
据了解，磁重联将磁能迅速转化为等离子体流能、热能和非热能粒子的某种组合。在不同的无碰撞和低β环境下，各种重联加速机制已经从理论上提出并进行了数值研究，其中β是指等离子体-磁压力比。这些机制包括费米加速、betatron加速、沿磁场平行电场加速和重联电场直接加速。然而，由于原位测量的德拜长度较短，非原位测量的平均自由程较短，在实验室实验中很难直接观测到非热粒子加速。
附：英文原文
Title: Non-thermal electron acceleration from magnetically driven reconnection in a laboratory plasma
Author: Chien, Abraham, Gao, Lan, Zhang, Shu, Ji, Hantao, Blackman, Eric G., Daughton, William, Stanier, Adam, Le, Ari, Guo, Fan, Follett, Russ, Chen, Hui, Fiksel, Gennady, Bleotu, Gabriel, Cauble, Robert C., Chen, Sophia N., Fazzini, Alice, Flippo, Kirk, French, Omar, Froula, Dustin H., Fuchs, Julien, Fujioka, Shinsuke, Hill, Kenneth, Klein, Sallee, Kuranz, Carolyn, Nilson, Philip, Rasmus, Alexander, Takizawa, Ryunosuke
Issue&Volume: 2023-01-16
Abstract: Magnetic reconnection rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy and non-thermal energetic particles. Various reconnection acceleration mechanisms have been theoretically proposed and numerically studied in different collisionless and low-β environments, where β refers to the plasma-to-magnetic pressure ratio. These mechanisms include Fermi acceleration, betatron acceleration, parallel electric field acceleration along magnetic fields and direct acceleration by the reconnection electric field. However, none of them have been experimentally confirmed, as the direct observation of non-thermal particle acceleration in laboratory experiments has been difficult due to short Debye lengths for in situ measurements and short mean free paths for ex situ measurements. Here we report the direct measurement of accelerated non-thermal electrons from magnetically driven reconnection at low β in experiments using a laser-powered capacitor coil platform. We use kilojoule lasers to drive parallel currents to reconnect megagauss-level magnetic fields in a quasi-axisymmetric geometry. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that the mechanism of direct electric field acceleration by the out-of-plane reconnection electric field is at work. Scaled energies using this mechanism show direct relevance to astrophysical observations.
DOI: 10.1038/s41567-022-01839-x
Source: https://www.nature.com/articles/s41567-022-01839-x