A study of the transition from collisional to collisionless plasma flows has been carried out at the National Ignition Facility using high Mach number (M>4) counterstreaming plasmas. In these experiments, CD-CD and CD-CH planar foils separated by 6–10 mm are irradiated with laser energies of 250 kJ per foil, generating ∼1000 km/s plasma flows. Varying the foil separation distance scales the ion density and average bulk velocity and, therefore, the ion-ion Coulomb mean free path, at the interaction region at the midplane. The characteristics of the flow interaction have been inferred from the neutrons and protons generated by deuteron-deuteron interactions and by x-ray emission from the hot, interpenetrating, and interacting plasmas. A localized burst of neutrons and bright x-ray emission near the midpoint of the counterstreaming flows was observed, suggesting strong heating and the initial stages of shock formation. As the separation of the CD-CH foils increases we observe enhanced neutron production compared to particle-in-cell simulations that include Coulomb collisions, but do not include collective collisionless plasma instabilities. The observed plasma heating and enhanced neutron production is consistent with the initial stages of collisionless shock formation, mediated by the Weibel filamentation instability.
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机译：在国家点火设施中，已经使用高马赫数（M> 4）逆流等离子体进行了从碰撞到无碰撞血浆流的转换研究。在这些实验中，用250kJ /箔的激光能量照射间隔6-10mm的CD-CD和CD-CH平面箔，产生约1000 km / s的等离子流。改变箔片分离距离可缩放离子密度和平均体积速度，因此可缩放中平面相互作用区域处的离子-离子库仑平均自由程。流动相互作用的特性是由氘核-氘核相互作用产生的中子和质子，以及来自热的，互穿的和相互作用的等离子体的x射线发射所推断的。在逆流的中点附近观察到局部中子爆发和明亮的X射线发射，这表明强烈的加热和激波形成的初始阶段。随着CD-CH箔间距的增加，与包括库仑碰撞但不包括集体无碰撞等离子体不稳定性的单元中粒子模拟相比，我们观察到中子产生的增加。观察到的等离子体加热和增强的中子产生与无碰撞激波形成的初始阶段相一致，这是由Weibel丝状失稳所介导的。
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