Lu W, Nicoul M, Shymanovich U, Tarasevitch A, Zhou P, Sokolowski-Tinten K, von der Linde D, Masek M, Gibbon P, Teubner U
Institut für Experimentelle Physik, Universität Duisburg-Essen, D-47048 Duisburg, Germany.
Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Aug;80(2 Pt 2):026404. doi: 10.1103/PhysRevE.80.026404. Epub 2009 Aug 27.
We investigate the generation of ultrashort Kalpha pulses from plasmas produced by intense femtosecond p-polarized laser pulses on Copper and Titanium targets. Particular attention is given to the interplay between the angle of incidence of the laser beam on the target and a controlled prepulse. It is observed experimentally that the Kalpha yield can be optimized for correspondingly different prepulse and plasma scale-length conditions. For steep electron-density gradients, maximum yields can be achieved at larger angles. For somewhat expanded plasmas expected in the case of laser pulses with a relatively poor contrast, the Kalpha yield can be enhanced by using a near-normal-incidence geometry. For a certain scale-length range (between 0.1 and 1 times a laser wavelength) the optimized yield is scale-length independent. Physically this situation arises because of the strong dependence of collisionless absorption mechanisms-in particular resonance absorption-on the angle of incidence and the plasma scale length, giving scope to optimize absorption and hence the Kalpha yield. This qualitative description is supported by calculations based on the classical resonance absorption mechanism and by particle-in-cell simulations. Finally, the latter simulations also show that even for initially steep gradients, a rapid profile expansion occurs at oblique angles in which ions are pulled back toward the laser by hot electrons circulating at the front of the target. The corresponding enhancement in Kalpha yield under these conditions seen in the present experiment represents strong evidence for this suprathermal shelf formation effect.
我们研究了由强飞秒p偏振激光脉冲作用于铜靶和钛靶产生的等离子体中产生超短Kα脉冲的情况。特别关注激光束在靶上的入射角与受控预脉冲之间的相互作用。实验观察到，在相应不同的预脉冲和等离子体标长条件下，Kα产额可以得到优化。对于陡峭的电子密度梯度，在较大角度可实现最大产额。对于对比度相对较差的激光脉冲情况下预期的某种程度扩展的等离子体，通过使用近正入射几何结构可以提高Kα产额。在一定的标长范围（0.1至1倍激光波长之间），优化产额与标长无关。从物理角度来看，这种情况的出现是因为无碰撞吸收机制（特别是共振吸收）强烈依赖于入射角和等离子体标长，这为优化吸收从而优化Kα产额提供了空间。基于经典共振吸收机制的计算和粒子模拟支持了这种定性描述。最后，后者的模拟还表明，即使对于初始陡峭的梯度，在倾斜角度会发生快速轮廓扩展，其中离子被在靶前端循环的热电子拉回朝向激光。在本实验中看到的这些条件下Kα产额的相应增强代表了这种超热平台形成效应的有力证据。
