背景气体对激光等离子体和外磁场界面上槽纹不稳定性的影响

Influence of background gas on flute instability produced at interface between laser plasma and external magnetic field

等离子体在外磁场中膨胀产生的抗磁腔和不稳定性是空间物理和聚变物理中的重要现象.本文实验研究了激光产生的等离子体在外磁场中膨胀时在抗磁腔表面产生的槽纹不稳定性,数据分析显示实验中观察到的不稳定性属于大拉莫尔半径槽纹不稳定性.实验发现充入稀薄背景气体能够显著抑制槽纹不稳定性的发展,背景气体气压超过50 Pa时(约为抗磁腔表面等离子体密度的1%),槽纹不稳定性几乎被完全抑制.动理学分析表明离子-离子碰撞是抑制不稳定性发展的主要因素.这些结果对磁场辅助激光聚变和爆炸空间物理现象等领域有重要参考价值.

e Lni Ln Diamagnetic cavity and flute instability generated by plasma expansion in an external magnetic field are important phenomena in space and fusion physics.We use a nanosecond laser irradiated carbon planar target to generate plasma,and the plasma expands in a 7 T transverse pulsed magnetic field to produce diamagnetic cavity.The flute instabilities formed on the surface of the diamagnetic cavity are explored experimentally.Data analysis shows that,under our experimental parameters,the gyroradius of electron()is much smaller than the density gradient scale length of the diamagnetic cavity(),while the ion’s gyroradius()is much larger than,indicating that the electrons are magnetized while the ions are not.The relative drift between electrons and ions provides free energy for developing the flute instability,which is composed of gravity drift and diamagnetic drift.The calculation shows that the gravity drift velocity is much larger than the diamagnetic drift velocity in our experiment,so the instability belongs to the large Larmor radius instability.By filling the target chamber with rarefied helium ambient gas,we find that the flute instabilities are inhibited significantly.When the ambient gas pressure exceeds 50 Pa(about 1%of the interface plasma density of diamagnetic cavity),the flute instabilities are almost completely suppressed.Kinetic analyses show that ion-ion collision and electron-ion collision,especially the former,are the main effects that inhibit the development of instability.Our results are of benefit to laser fusion and address the fundamental question of explored space phenomena.