Swain Kalyani, Mahalik Sagar Sekhar, Kundu Mrityunjay
Institute for Plasma Research, Bhat, Gandhinagar, 382 428, India.
Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai, 400094, India.
Sci Rep. 2022 Jul 4;12(1):11256. doi: 10.1038/s41598-022-14816-4.
In the few-cycle pulse regime of laser-cluster interaction (intensity [Formula: see text], wavelength [Formula: see text] nm), laser absorption is mostly collisionless and may happen via anharmonic resonance (AHR) process in the overdense (cluster) plasma potential. Many experiments, theory and simulation show average absorbed energy per cluster-electron ([Formula: see text]) close to the electron's ponderomotive energy ([Formula: see text]) in the collisionless regime. In this work, by simple rigid sphere model (RSM) and detailed particle-in-cell (PIC) simulation, we show enhanced [Formula: see text] 30-70[Formula: see text]-a 15-30 fold increase-with an external (crossed) magnetic field near the electron-cyclotron resonance (ECR). Due to relativistic mass increase, electrons quickly deviate from the standard (non-relativistic) ECR, but time-dependent relativistic-ECR (RECR) happens which also contributes to enhanced [Formula: see text]. Here laser is coupled to electrons in two stages, i.e, AHR and ECR/RECR. To probe further we retrieve the phase-difference [Formula: see text] between the driving electric field and corresponding velocity component for each electron (in PIC and RSM). We find absorption by electron via AHR happens in a very short interval [Formula: see text] for less than half a laser period where [Formula: see text] remains close to [Formula: see text] (necessary condition for maximum laser absorption) and then [Formula: see text] drops to its initial [Formula: see text] (meaning no absorption) after such short-lived AHR. On the contrary, auxiliary magnetic field near the ECR modifies AHR scenario inside the cluster and also helps maintaining the required phase [Formula: see text] for the liberated cluster-electron accompanied by frequency matching for ECR/RECR for a prolonged [Formula: see text] (which covers 50-60% of the laser pulse through pulse maxima) even after AHR-leading to jump in [Formula: see text] 30-70[Formula: see text]. We note that to realize the second stage of enhanced energy coupling via ECR/RECR, the first stage via AHR is necessary.
在激光与团簇相互作用的少周期脉冲 regime 中（强度[公式：见正文]，波长[公式：见正文]nm），激光吸收大多是无碰撞的，并且可能通过过密（团簇）等离子体势中的非谐共振（AHR）过程发生。许多实验、理论和模拟表明，在无碰撞 regime 中，每个团簇电子的平均吸收能量（[公式：见正文]）接近电子的有质动力能量（[公式：见正文]）。在这项工作中，通过简单的刚性球体模型（RSM）和详细的粒子模拟（PIC），我们表明在电子回旋共振（ECR）附近施加外部（交叉）磁场时，[公式：见正文]增强了 30 - 70[公式：见正文]——增加了 15 - 30 倍。由于相对论质量增加，电子迅速偏离标准（非相对论）ECR，但会发生随时间变化的相对论 ECR（RECR），这也有助于增强[公式：见正文]。这里激光分两个阶段与电子耦合，即 AHR 和 ECR/RECR。为了进一步探究，我们获取了每个电子（在 PIC 和 RSM 中）的驱动电场与相应速度分量之间的相位差[公式：见正文]。我们发现电子通过 AHR 的吸收发生在非常短的时间间隔[公式：见正文]内，不到半个激光周期，在此期间[公式：见正文]保持接近[公式：见正文]（激光最大吸收的必要条件），然后在这种短暂的 AHR 之后，[公式：见正文]降至其初始[公式：见正文]（意味着无吸收）。相反，ECR 附近的辅助磁场改变了团簇内部的 AHR 情况，并且即使在 AHR 之后，也有助于为释放的团簇电子维持所需的相位[公式：见正文]，同时伴随着 ECR/RECR 的频率匹配，持续时间更长的[公式：见正文]（覆盖激光脉冲最大值的 50 - 60%）——导致[公式：见正文]跃升 30 - 70[公式：见正文]。我们注意到，要通过 ECR/RECR 实现能量耦合增强的第二阶段，通过 AHR 的第一阶段是必要的。
